**1. Introduction**

Free electron lasers (FELs) are widely acknowledged as the most versatile generators of coherent electromagnetic radiation. Since the first studies in the late 1960s [1] and their "official" invention in 1977 [2], it has been clear that FELs are capable of bridging the gap between conventional electron-based sources (such as klystrons, magnetrons, travelling wave tubes) that are limited to the high frequency direction, and the lasers that, with some exceptions, generate single frequency radiation with a power that is limited by the nature of the active medium. FELs can be designed, in principle, to operate at any frequency and with a time structure and related power suitable for any kind of experiment. This kind of flexibility comes from the fact that many of the parameters involved, such as the electron energy and the magnetic field, can be adjusted with continuity. Moreover, many dynamical regimes can be exploited for FEL design; from the low-gain regime, suitable for oscillator devices [3], to the high gain regime ideal for the self-amplified spontaneous emission (SASE) scheme [4]. Furthermore, FEL offers significant potential options to combine different schemes in sequence in order to increase the performances of some specific features without overly stressing the project parameters. One of the most relevant FEL structures arrangements, among others, is a modulator at a synchronous frequency followed by a frequency multiplier that exploits the harmonic content in the modulated beam [5,6]. Such an arrangement may be reproduced several times, creating a kind of cascade [7] that results in final beam degradation.

This study presents one of the possible arrangements based on a Cerenkov FEL oscillator as a modulator and a magnetic undulator as a radiator: this solution can be considered a hybrid FEL scheme device [8]. The reason why the term hybrid can be ascribed to such a scheme is not only due to the fact that the modulator and radiator present two different mechanisms of radiation generation, but mainly because in Cerenkovlike FEL devices [9] (as is the case for Smith–Purcell gratings [10]), the coupling between electrons and field is longitudinal, while the magnetic undulator induces a transverse motion to the electrons and accordingly, to the coupling. This circumstance, combined with the fact that Cerenkov (and Smith–Purcell) FELs operate with slow-wave guiding structures, has a relevant effect on the differences, for "synchronism condition" [9,11],

**Citation:** Doria, A. Hybrid (Oscillator-Amplifier) Free Electron Laser and New Proposals. *Appl. Sci.* **2021**, *11*, 5948. https://doi.org/ 10.3390/app11135948

Academic Editors: Giuseppe Dattoli, Alessandro Curcio and DaniloGiulietti

Received: 25 May 2021 Accepted: 22 June 2021 Published: 26 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that links the spontaneous emission wavelength with the most relevant parameters of the experiment, between Cerenkov and undulator FELs.

The paper is arranged in four sections: after the present Introduction, Section 2 is dedicated to the study of the Cerenkov-based modulator. Section 3 will introduce the hybrid system as a whole, highlighting the value of the main parameters. Section 4 will describe the effects of the modulation on the emission from a magnetic undulator exploited as a radiator; this analysis includes the ballistic effects of a variable drift space. Section 5 is devoted to final conclusions.

#### **2. The Hybrid System**

The hybrid FEL system can be assumed to be composed of three main elements: (1) an electron beam accelerator; (2) a Cerenkov slow-wave guiding structure, with an adequate radiation resonator for the saturation regime accomplishment; and lastly, (3) a magnetic undulator acting as a radiator after the velocity modulation induced by an electric field associated with the radiation stored in the Cerenkov oscillator (see Figure 1).

The first problem to face is related to the energy of the electron beam that will be common to both generating structures. As discussed in the introduction, the synchronism conditions for Cerenkov and undulator FELs are rather different and can be summarised as follows (see Refs. [9,11]), where the subscript *U* and *C* indicate the undulator and Cerenkov, respectively:

$$
\lambda\_{ll} = \frac{\lambda\_{\text{uv}}}{2\gamma^2} \left( 1 + K^2 \right); \lambda\_{\text{C}} = 2\pi\gamma d \left( \frac{\varepsilon - 1}{\varepsilon} \right) \tag{1}
$$

where *λw* indicates the undulator period, *K* is the undulator parameter, *d* is the dielectric thickness of the Cerenkov guiding structure, *ε* is its dielectric constant and *γ* is the Lorentz parameter of the electrons. All these parameters will be more deeply described in the following sections.

In order to obtain an efficient modulation of the electron beam, it is required that the radiation generated from the Cerenkov oscillator and from the Undulator radiator be "correlated"; a way to achieve this target is to obtain the condition: *λU* = *λC/n* where *n* = 1, 2, ... specifies the harmonic number. Such a relation, with the use of Equation (1), becomes:

$$\frac{\lambda\_w}{2\gamma^2} \left(1 + K^2\right) \;=\; \frac{2\pi\gamma}{n} d\left(\frac{\varepsilon - 1}{\varepsilon}\right) \Leftrightarrow \gamma^3 \;=\; \frac{\lambda\_w}{4\pi} n \left(1 + K^2\right) \frac{\varepsilon}{d(\varepsilon - 1)}\tag{2}$$

Equation (2) gives the value of the electron energy as a function of all the parameters involved in both the Cerenkov and undulator FELs. In order to have an idea about the numbers, we can assign some reasonable values for the involved parameters in Equation (2) such as: *λw* = 2.5 cm, *K* = 1, *d* = 5 μm, *n* = 1, *ε* = 5 (dielectric constant of quartz [12]) from

which we obtain a result, for the electron energy, *γ*~10, that corresponds to a moderate relativistic energy. By inserting these values into Equation (1), we conclude that such a source would operate in the so-called far-infrared (FIR) spectral region (also called TeraHertz (THz)-region) because *λU*~*λC*~250 μm.

Another approach can be obtained by rearranging Equation (2) as follows:

$$d\left(\frac{\varepsilon - 1}{\varepsilon}\right) = \frac{\lambda\_w}{4\pi\gamma^3} \left(1 + K^2\right) n \tag{3}$$

from which it is possible to obtain the Cerenkov parameters values starting from the undulator FEL ones. Let us suppose we have an FEL source with *λw* = 2.5 cm, *K* = 1, *γ* = 20 and again working on the fundamental harmonic *n* = 1, Equation (3) gives: [*d*(*ε* − 1)/*ε*]~5·10−7 μm. Such a value may be obtained in several ways since, for instance, *ε* = 2 and *d* = 1 μm. Using all these values for the hybrid FEL parameters, we eventually obtain from Equation (1) the value for the emitted wavelength from both the elements of the hybrid FEL: *λU*~*λC*~60 μm.

#### **3. The Cerenkov FEL Oscillator**

A well-known class of waveguides is characterised by having open boundaries and the possible presence of an electromagnetic (e.m.) field outside the boundary; these guides support propagating modes called "surface waves" [13]. Dielectric films deposited over conducting plates are devices that fall into the aforementioned category; the surface waves, in this case, present an exponential behaviour, normal to the dielectric surface, that decays when moving away. Such a waveguide category is the relevant one for the realisation of the Cerenkov FEL device because the field–particle interaction occurs in the region of the space where the "evanescent" e.m. field is present.

These propagating devices present field distributions associable with transverse electric (TE) or transverse magnetic (TM) modes. The case of TM modes is more favourable for the FEL use due to the presence of a longitudinal electric field component related to them. The vector potential for the TM modes of a single slab geometry waveguide is expressed by [9,13]

$$\begin{cases} A\_X = A\_0 \frac{c k}{\omega \eta} e^{-qx} \sin(kz) e^{-i\omega t} \\\ A\_y = 0 \\\ A\_z = A\_0 \frac{c}{\omega \nu} e^{-qx} \sin(kz) e^{-i\omega t} \end{cases} \tag{4}$$

where *k* indicates the longitudinal momentum, while *q* is the transverse momentum in vacuum and *p* is the transverse momentum in the dielectric of thickness *d*. All these parameters are linked by two coupled "dispersion relations" that can be deduced by the field continuity along the vacuum–dielectric interface and dielectric–conductor interface:

$$\begin{cases} qd = \frac{pd}{\varepsilon} \tan(pd) \\ \left( (pd)^2 + (qd)^2 \right) = d^2 \left( \frac{\omega}{c} \right)^2 (\varepsilon - 1) \end{cases} \text{and} \begin{cases} p = \sqrt{\varepsilon \left( \frac{\omega}{c} \right)^2 - k^2} \\ q = \sqrt{k^2 - \left( \frac{\omega}{c} \right)^2} \end{cases} \tag{5}$$

In order to obtain the electric and magnetic fields from Equation (4), we can adopt the Coulomb gauge (**E** = −*∂***A**/*∂t*; **B** = ∇ × **A**) which is useful for evaluating the fields in absence of charges and currents, as for the free modes in a guiding structure. Neglecting the coupling between the electrons and the transverse electric field *Ex* (due to the small values of the electrons' velocities in the *x* direction), we obtain for the longitudinal field:

$$E\_z = -cA\_0 e^{-qx} \sin(kz) e^{-i\omega t} \tag{6}$$

In order to achieve the maximum field intensity, it is necessary to reach the saturation regime inside the Cerenkov waveguide resonator. The dynamics of the emission process in Cerenkov FEL has been established in the past as far as the spontaneous emission process [9,14] is concerned and with regard to the stimulated emission mechanism and

⎪⎪⎪⎨

the gain coefficient calculation [14,15]. Let us, therefore, consider the case discussed in Section 2 while discussing Equation (2); the relevant parameters are summarised in Table 1.

Table 1 reports the parameters for both a single slab geometry device [9] and for the double slab one [16] adequate for an operation around *λC =* 250 μm. The behaviour of the spontaneous emission for a single-electron beam with a transverse size of *σx* = 1 mm (r.m.s.), as a function of frequency and beam centroid distance from the dielectric surface, is reported in Figure 2a). The same analysis for the double slab geometry and a beam size of *σx* = 1 cm (r.m.s.) is reported in Figure 2b):

**Figure 2.** Spontaneous emission spectra as a function of the beam centroid distance from the dielectric surface: (**a**) single slab waveguide geometry; and (**b**) double slab waveguide geometry.

The one-dimensional stimulated emission process, until saturation, can be studied by means of semi-analytical techniques [17] (three-dimensional theory can be found in [18]). The saturation mechanism in free electron devices is quite similar to that occurring in conventional lasers, as has been derived by Rigrod approximately sixty years ago [19] and is also present in the theory of passive saturable absorber for laser mode-locking [20]. Two different but similar approaches can be applied; the first technique starts from the application of the Ginzburg–Landau equation to FEL systems [21] and leads to the *GI*(*I*) expression in Equation (7). The second method is based on the logistic equation approach [22] and ends up the *GII*(*I*) expression in Equation (7):

$$\begin{cases} \begin{array}{c} \text{G}\_{I}(I\_{n}) = 0.849 \cdot \text{g}\_{\text{MAXX}} \frac{1 - \exp\left(-\frac{\pi}{2} \frac{I\_{n}}{I\_{\text{SAT}}}\right)}{\frac{\pi}{2} \frac{I\_{n}}{I\_{\text{SAT}}}}\\ I\_{\text{SAT}} = \frac{\gamma\_{\text{I}} I\_{\text{IF}}}{4\_{\text{Gmax}} N\_{\text{eq}} \varepsilon} \end{array}; \begin{cases} \begin{array}{c} \text{G}\_{II}(n) = \frac{I(n) - I(n-1)}{I(n-1)}\\ I(n) = 1\_{\text{Sp}} \frac{\exp\left[\left(\frac{\pi}{2} \text{MAX}\left(1 - \Gamma\right) - \Gamma \right)\right]}{1 + \left(I\_{\text{S}} / I\_{\text{cj}}\right) \left[\exp\left[\left(\frac{\pi}{2} \text{MAX}\left(1 - \Gamma\right) - \Gamma \right) - 1\right]\right]}\\ I\_{\text{cj}} = \frac{2}{\pi} \frac{1 - \Gamma}{\Gamma} \text{g}\_{\text{MAXX}} \left\{ 1 - \exp\left[ -\frac{1.8}{1 + \text{g}\_{\text{MAX}} \left(\frac{\Gamma}{1 - \Gamma}\right)} \right] \end{array} \end{cases} \tag{7}$$

where *Neq = L/*(*2γ2λC*) *~* 7.3 represents the equivalent value of the number of undulator periods in Cerenkov FELs; *ISAT*~7.4·106 [*W*] expresses the saturation power value related to the gain of the device (conventionally, the intensity is used instead of power, however, in this case, the radiation transverse cross-section is a constant due to the presence of a waveguide, making power and intensity interchangeable); *n* indicates the round-trip number; *IAV*~17,000 *A* indicates the Alfven current; Γ represent the total losses in the radiation resonator; *ISP* expresses the spontaneous emission power of all the electrons in the bunch and integrated over the bandwidth; *Ie,i* refers to the intracavity equilibrium power, and finally, *g*0*MAX* indicates the maximum value of the small-gain, small-signal gain coefficient as a function of frequency [15]. The relevant parameters for the gain calculation are reported in Table 2.

**Table 2.** Cerenkov FEL Gain Parameters @ λ = 250 μm.

The behaviour of Equation (7) is reported in Figure 3a) for the gain expressions, *GI* and *GII*, as a function of the round trip number *n*, and in Figure 3b) for the ongoing power radiation, *In* and *I*(*n*), again with respect to the round trip. The main result, as can be deduced from Figure 3, is that saturation is reached after about *n* = 100 round trips considering both methods reported in Equation (7). Considering the resonator length, as indicated in Table 1, we can deduce that saturation is gained in about *TSAT*~200 ns, which is quite a short time with respect to the conventional macro-bunch duration of electron pulses generated by RF accelerators in the S-band (~3 GHz), which of the order of several microseconds. This will be relevant for the analysis of the velocity modulation induced on the remaining part of the macro-bunch itself.

**Figure 3.** Saturation behaviour: (**a**) gain as a function of the round-trip number; and (**b**) intracavity power as a function of the round-trip number.

The peak intra-cavity peak power, as deduced by Figure 3b), is about *PSAT*~4.2 × 107 *W*; considering an average transverse radiation dimensions, obtained from the data reported in Table 1, of about Σ*L* = 1 cm2, the electric field associated to the intracavity radiation at saturation is:

$$E\_{SAT} = \sqrt{2I\_{SAT}/(\varepsilon\_0 c \Sigma\_L)} \approx 1.782 \cdot 10^7 \text{[V/m]}$$

Before analysing the dynamical process, it is worth briefly addressing the question of the longitudinal modes in the Cerenkov FEL resonator. The wavenumber mode separation, as evident from Equation (6), is (Δ*ω/c*) = *π/L*. The relative gain bandwidth is [11,15]:

$$\frac{\Delta\omega\_{\rm G}}{\omega} \approx \frac{1}{4N\_{\rm eq}} = \frac{\gamma^2 \lambda\_{\rm C}}{2L} \Rightarrow \frac{\Delta\omega\_{\rm G}}{c} = \frac{\omega\_{\rm C}}{c} \frac{\gamma^2 \lambda\_{\rm C}}{2L} \approx \frac{\pi \gamma^2}{L}$$

The number of modes contained in the gain bandwidth is therefore: Δ*ωG*/Δ*ω* = *γ*2. It is well known [11] that any FEL based on the RF accelerator locks all the longitudinal modes in a natural way. The result of such mode-locking is that a single short pulse of radiation travels back and forth inside the tuned resonator with a group velocity that is slightly smaller than that of the electron beam, causing a kind of anti-lethargic effect [23], unlike what happens for undulator FELs.

#### **4. Electron Beam Modulation and Undulator Emission**

At the end of the previous section, we saw how an intense electromagnetic pulse, that can be generated inside an optical resonator in about 200 nanoseconds, needs to be saturated. For electron macro-bunches, as long as few microseconds we will have thousands of micro-bunches available interacting with the electromagnetic field. If we analyse each of these electron micro-bunches, we recognise that, at the resonator entrance, each of them superimposes with a correspondent radiation micro-pulse as illustrated in Figure 4a). In such a situation, the electrons of the bunch, generally equally spaced in phase, will experience the oscillating longitudinal electric field *Ez* (Equation (6)) with a spatial periodicity of *λC*, as showed in Figure 4b). The situation represented in Figure 4 is quite similar to that of an electron beam accelerated by a series of RF cavities such as those of a standing-wave Linac [11,24]. The number of cavities can be estimated by calculating the number of the electric field oscillations "contained" within the electron micro-bunch length (*δz~τeβec*) divided by the wavelength *λC*: *Nc*~(*δz/λC*). The most widely used accelerators for FELs operate in the so-called S-band (*νRF*~3 GHz) and generate micro-bunches of a typical duration of *τe*~15 ps, which corresponds to a bunch length of about *δz*~5 mm. Considering a radiation wavelength of *λC*~250 μm, as for the example discussed in the previous sessions, we obtain *Nc*~20.

It is possible, therefore, to imagine any optical cycle acting on the electrons in a way similar to that of the accelerating cavities of a Linac. We know, in fact, that electrons undergo to an energy variation that is connected to the particle-radiation relative phase. Such an energy variation will be positive for electrons close to the so-called accelerating phase and negative for those with an opposite phase. The result is a bunching of the electrons into *Nc* "slices".

**Figure 4.** *Cont.*

**Figure 4.** Electron bunch and radiation pulse interaction: (**a**) phase locking; and (**b**) electrons and radiation electric field interaction.

To properly evaluate the energy variation of a single electron, let us consider that the energy exchange occurs along the time needed for the electron, to cross the resonator length *L*. The particle, during this time *T = L/*(*cβe*), experiences a quasi-static electric field due to the fact that, even with small differences, its speed is almost synchronous with the electromagnetic pulse that travels with its group velocity *βg* = *d*(*ω*/*c*)/*dk* (see Equation (5)), and therefore, *βg~βe*. The energy variation, for each electron, is consequently:

$$
\Delta\gamma = \frac{\Delta\mathbf{E}}{\mathbf{m}\_0 \mathbf{c}^2} = \frac{\mathbf{F} \cdot \mathbf{s}}{\mathbf{m}\_0 \mathbf{c}^2} = \int\_0^\mathrm{T} \frac{\mathbf{e} \mathbf{E}(\mathbf{t})}{\mathbf{m}\_0 \mathbf{c}^2} \cdot \mathbf{v}\_\mathbf{e} \mathrm{d}\mathbf{t} = \frac{\mathbf{e} \mathbf{E}\_{0\mathbf{z}}}{\mathbf{m}\_0 \mathbf{c}^2} \mathrm{c}\beta\_\mathbf{e} \int\_0^\mathrm{T} \cos(\omega \mathbf{t} + \phi) \mathrm{d}\mathbf{t} \tag{8}
$$

where the longitudinal electric field has been derived from Equation (6) and the constant term *E*0z contains the transverse coupling between the electron and the evanescent field in the x direction. Before getting an explicit form for Equation (8), it is worth underlining that the phase *ϕ* usually refers to the RF field, at an angular frequency *ω*RF, that accelerates the electron beam in the accelerator (Linac, for instance). In the present case, the phase *ϕ* should be referred to as the Cerenkov radiation field in the optical resonator, and therefore, ought to be "normalised" by means of the frequency ratio *ωC*/*ωRF*: *φ* ⇒ *φ*(*ωC*/*ωRF*) = *φ*(*λRF*/*λC*). After some algebra, we obtain:

$$\Delta\gamma = \frac{eE\_{0z}}{m\_0 c^2} L \left[ \frac{\sin\left(\frac{\omega}{c} \frac{L}{\beta\_r}\right)}{\left(\frac{\omega'}{c} \frac{L}{\beta\_r}\right)} \cos\left(\phi \frac{\lambda\_{RF}}{\lambda\_\odot}\right) + \frac{\cos\left(\frac{\omega}{c} \frac{L}{\beta\_\odot}\right)}{\left(\frac{\omega}{c} \frac{L}{\beta\_\odot}\right)} \sin\left(\phi \frac{\lambda\_{RF}}{\lambda\_\odot}\right) \right] \tag{9}$$

The result of what has been discussed is that the energy modulation expressed by Equation (9) can be exploited to generate powerful radiation inside a magnetic undulator utilised as a radiator. The discussion after Equation (2) in Section 2 leads to a set of parameters, summarised in Table 3, for an undulator synchronous with the fundamental Cerenkov emission.

**Table 3.** Undulator–Radiator FEL Parameters @ λU = 250 μm.

The emission of radiation from a radiator by means of an ensemble of charged particles distributed in the phase space has been faced in the recent past [25,26]. To summarise some concepts, we can say that the electron beam, after the interaction, carries a modulation of the current → *J* that can be expressed as a Fourier expansion in terms of the harmonics *l* of the RF. Moreover, the electromagnetic field can be expanded in terms of the transverse modes, expressed by the aggregate index *λ*, of the waveguide that is needed to confine the radiation in the far infrared spectral range, and expressing the expansion coefficients as *Al*,*λ*. Finally, the power *Pl*,*λ*, emitted due to the energy exchange between the electron beam and the radiation field in the magnetic field of the undulator, can be evaluated by means of the flux of the Poynting vector. Equation (10) summarises all the above in an extensive demonstration which can be found in [25] and the references therein:

$$\begin{aligned} \stackrel{\rightarrow}{J} &= \sum\_{l=1}^{\infty} \stackrel{\rightarrow}{J}\_{l} \exp(-i\omega\_{l}t) \; ; \; \omega\_{l} = 2\pi \frac{l}{T\_{RF}}\\ A\_{l,\lambda} &= -\frac{Z\_{0}}{2\tilde{p}\_{\xi}} \frac{\varepsilon}{T\_{RF}} \frac{KL}{\sqrt{\omega\_{l}}} \sum\_{j=1}^{N\_{\xi}} \frac{1}{\tilde{p}\_{z\_{j}}\gamma\_{j}} \frac{\sin\left(\theta\_{l,\lambda j}/2\right)}{\theta\_{l,\lambda j}/2} i \exp\left(\frac{\theta\_{l,\lambda j}}{2} + l\psi\_{j}\right); \; \theta\_{l,\lambda j} = \left(\frac{\omega\_{l}}{\varepsilon \tilde{p}\_{z\_{j}}} - \frac{2\pi}{\lambda\_{w}} - k\_{\lambda}\right)\\ P\_{l,\lambda} &= \frac{\theta\_{\xi}}{\Delta t\_{0}} \big|\_{} A\_{l,\lambda} \Big|^{2} \end{aligned} \tag{10}$$

where *e* is the electron charge, *Z*0 is the vacuum impedance, Σ*λ* is the radiation transverse mode size, *Ne* is the number of electrons contained in the bunch, *βz,j* is the longitudinal velocity of the j-th electron, *βg* is the radiation group velocity and *kλ* is the transverse mode momentum.

A brief analysis of Equation (10) tells us that the expansion coefficient *Al*,*λ* plays a significant function in the determination of the radiation formation. It is, in fact, the role of each electron in the bunch is relevant because it carries a specific energy *γj* and a specific phase *ψj*. In particular, the electron phase *ψj*, together with the interaction phase *θl*,*λ*,*j* enter in a more general phase term in *Al*,*λ* that, when summing up all of the contributions of the *Ne* electrons of the bunch, may generate constructive or destructive interactions. The maximum radiated power in the undulator is obtained when all the electrons contribute to the radiation formation with the same phase term (only depending on the harmonic number *l*), therefore, with an adequate distribution of the electrons in the phase-space.

What has been discussed, so far, about Equation (10), refers to the longitudinal properties of the particle–wave interaction, which concur with the undulator parameter *K* and period *λw* and the electron longitudinal velocities *βz,j*. The transverse characteristics of the radiation emission are summarised by the mode aggregate index *λ*. It is evident, however, that the transverse space can play a role when the electron beam emittance values could be relevant for the radiation wavelength to be generated. A deeper and exhaustive analysis can be found in [27].

With such a mathematical background, an appropriate computational code was set up for the evaluation of the coherent generation of radiation in the undulator–radiator. The code is capable of evaluating the impact of an electron bunch, with a specific distribution in the phase-space, for the radiation generation, calculating the contribution of any electron, integrated along the interaction length. The code, at the moment, does not take into account saturation effects. The effect of the energy modulation introduced by the electron– field interaction inside the Cerenkov FEL optical resonator can be assessed by comparing the emission from an electron distribution *Dγj*, *ψj* and the same distribution to which an energy modulation is added *Dγj* + Δ*γψjj*, *ψj*, where the contribution Δ*γψjj* is calculated from Equation (9) for each electron. To this aim, an ad hoc electron distribution was generated in the phase-space having a uniform partitioning in energy with a total Δ*γ* = 0.2 around *γ* = 10 (see Table 3) corresponding to a *σγ* ≈ 5 · 10−3 r.m.s. The phase partitioning is again uniform, with an added Poissonian component, with a total Δ*ψ* = 20◦ corresponding to the actual micro-bunch duration expressed as phase interval for Linac [11]. In Figure 5, we report the aforementioned electron distribution (Figure 5a) together with

the corresponding power spectrum (Figure 5b) calculated with Equation (10). The total power, integrated over the whole bandwidth, results in *PTOT*~55 W.

**Figure 5.** Emission by undulator–radiator: not the modulated case: (**a**) phase-space; and (**b**) power spectrum.

As anticipated, a similar calculation can be performed for the electron distribution of Figure 5a), to which the energy modulation Δ*γj* of Equation (9) is added for each electron of the distribution with its actual phase *ψj* and energy (through the velocity *βe,j*).The amplitude of the electric field is the one evaluated at the end of Section 3 from the saturated intracavity radiation power *E*0*z*~1.8 × 107 [V/m]. In Figure 6, we report the energy modulated electron distribution (Figure 6a) together with the corresponding power spectrum (Figure 6b), again calculated by means of Equation (10). As can be clearly seen,the emission is strongly increased by orders of magnitude due to the modulation introduced which, moreover, is not clearly distinguishable when looking at Figures 5a and 6a. It is further worth underlining how the total bandwidth is slightly reduced with respect to the unmodulated case. The total power, integrated over the whole bandwidth, now results as *PTOT*~1.28 × 107 W.

**Figure 6.** Emission by undulator–radiator: modulated case: (**a**) phase-space; and (**b**) power spectrum.

We may now ask whether there is any optimisation procedure to further increase the power radiated by the undulator. The most relevant tool is the distance between the Cerenkov electromagnetic resonator and the entrance of the magnetic undulator. This is because the energy modulation acquired by the electrons in the cavity transforms into a velocity modulation, and consequently bunches before entering the undulator interaction region. This "drift space" allows a ballistic process that correlates the velocity variation of each electron, with its position, and therefore phase, in the bunch. In Equation (11), we report the phase term that should be inserted in the set of Equation (10) that takes into account the phase variation, after the velocity modulation expressed by (v*z*)*j* and for a specific drift space *LDrift*. The index "*j*" indicates the specific electron and v*ref* is the velocity of the "reference electron":

$$\left(\Psi\_{Drift}\right)\_{\dot{j}} = \omega\_{RF} L\_{Drift} \left(\frac{1}{\left(\mathbf{v}\_z\right)\_{\dot{j}}} - \frac{1}{\mathbf{v}\_{ref}}\right) + \psi\_{\dot{j}} \tag{11}$$

Considering a series of values for *LDrift* ranging from 0 to 3.5 metres, the power spectrum was calculated and reported in Figure 7. As can be appreciated, the peak power, starting from a specific value, tends to reduce, while increasing *LDrift*, and the spectrum itself tends to broaden with a couple of peaks appearing. A further increase in the modulator radiator distance produces a spectral band reduction and a peak power increase up to the final value.

**Figure 7.** *Cont.*

**Figure 7.** Power spectrum for different drift-space values: (**a**) *LDrift* = 0.18 m; (**b**) *LDrift* = 0.90 m; (**c**) *LDrift* = 1.52 m; (**d**) *LDrift* = 2.15 m; (**e**) *LDrift* = 2.87 m; and (**f**) *LDrift* = 3.50 m.

## **5. Conclusions**

The example reported to date indicates how a hybrid scheme for free electron devices can be effective for the power enhancement of the electromagnetic radiation generated. This two-elements scheme, based on an oscillator, in which intracavity radiation acts as an energy modulator, and a magnetic undulator as a radiator, is a versatile arrangement because the oscillator section can be chosen among a series of different devices. As anticipated, together with the Cerenkov device, other sources can be taken into account; among which the Smith–Purcell is an interesting choice because it offers characteristics that lay between that of a Cerenkov FEL (being a slow-wave device) and an undulator FEL (wavelength scales inversely with the electron energy, as can be seen in Equation (12)). The synchronic condition, in fact, results to be [10,28]:

$$
\lambda\_{SP} = \frac{\Lambda}{|n\_{\rm so}|} \left(\frac{1}{\beta\_c} - \cos \theta\right) \tag{12}
$$

where Λ represents the grating period, *nso* indicates the grating spectral order and *θ* is the angle between the electron beam direction (parallel to the grating surface) and the observer. A new parameter, in the Smith–Purcell-based FEL, is evident from Equation (12) and is the angle *θ* that offers a wide and continuous range of spectral tunability [28] that can be exploited, keeping fixed other parameters like the electron energy and the grating geometry. This characteristic increases the flexibility of the hybrid scheme FEL because it easily allows to obtaining the condition *λU = λSP/n*, where *n* = 1, 2, ... , because the *cos θ* term can be used to obtain the maximum wavelength extension:

$$
\lambda\_{SP}^{(+)} = \frac{\Lambda}{|n\_{\rm so}|} \frac{1}{\beta\_{\rm e}} \Rightarrow \lambda\_{SP}^{(-)} = \frac{\Lambda}{|n\_{\rm so}|} \left(\frac{1-\beta\_{\rm e}}{\beta\_{\rm e}}\right).
$$

The harmonic number *n* is, therefore, a pivotal parameter for extending the spectral emission of such a coherent source [5–7], that with a proper combination of all the parameters involved, for both the oscillator and the radiator, prove to be a promising and versatile device, with the additional feature of compactness.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest.


**Noam Weinbach 1,\*, Eldad Keha 2,3, Hila Leib 2 and Eyal Kalanthro**ff **2,\***

Received: 29 September 2020; Accepted: 20 November 2020; Published: 24 November 2020

**Abstract:** Restrained eaters display difficulties engaging in self-control in the presence of food. Undergoing cognitive training to form associations between palatable food and response inhibition was found to improve self-control and influence eating behaviors. The present study assessed the impact of two such response inhibition trainings on food consumption, food-related anxiety, and implicit attitudes toward food among female restrained eaters (Dutch Eating Behavior Questionnaire-restrained eating subscale ≥ 2.5). In Experiment 1, 64 restrained eaters completed either one of two training procedures in which they were asked to classify food vs. non-food images: a food-response training, in which stop cues were always associated with non-food images, or a balanced food-response/inhibition training, in which participants inhibited motor actions to food and non-food stimuli equally. The results revealed reduced snack consumption following the food-response/inhibition training compared to the food-response training. The food-response training was associated with increased levels of food-related anxiety. In Experiment 2, the same training procedures were administered to 47 restrained eaters, and implicit attitudes toward palatable foods were assessed. The results revealed an increase in positive implicit attitudes toward palatable foods in the food-response/inhibition group but not in the food-response training group. The results suggest that balancing response inhibition and execution across food and non-food stimuli may reduce overeating while retaining positive attitudes toward food among female restrained eaters.

**Keywords:** restrained eating; response inhibition; stop-signal task; implicit associations; cognitive training; inhibitory control


*Edited by R.P. Soundararajan*

Growing population in the world demands increase in the food production and intense health care systems. Use of chemical pesticides is imperative for the management insects in agricultural and disease transmission, weeds and harmful microbes. Monitoring and estimating pesticide residue in crop plants, food, soil, water and other ecosystem has become significant in the recent concern on environment and ecosystem. The book comprises of new innovative trends to detect pesticide residue in crop plants, animal origin food and fishes. Different advanced extraction techniques of sample preparation for residue analysis are elaborately described. Apart from residue assays, metabolism and degradation of pesticide compounds fenamophos, chlorpyrifos, pirimiphos, heptachlor and organic pesticides are also documented. This book volume is of twelve chapters contributed by eminent scientists from eleven countries.

ISBN 978-953-51-0681-4

Pesticides - Recent Trends in Pesticide Residue Assay

Photo by sakhorn38 / iStock

## Pesticides Recent Trends in Pesticide Residue Assay

 **PESTICIDES – RECENT**

**TRENDS IN PESTICIDE**

**RESIDUE ASSAY**


**4. Discussion**

This population-based, cross-sectional study assessed eating habits and lifestyle behaviors among residences of the UAE, via an online survey during the COVID-19 pandemic between April and May 2020. The results indicate that the COVID-19 pandemic and the subsequent lockdown resulted in weight gain in about one-third of the respondents with changes in important and highly modifiable dietary and lifestyle behaviors that are considered essential for optimal somatic and psychological health. Specifically, participants also reported an increase in the number of meals consumed per day and a reduction in the percentage of skipping meals particularly breakfast during the pandemic. The present study also indicated that dietary habits were distanced from the Mediterranean diet principles and closer to "unhealthy" dietary patterns, characterized as high in energy but with low nutrient density; viewed as a detrimental combination for immune status. Although more homemade meals were prepared, a factor associated with healthy weight status, at the same time more non-nutritious foods were chosen, as well as being more frequently consumed (since an increase was also seen among frequency of meals per day). These data, therefore, are informative on the potential alterations of food prepared and consumed although at home.

In agreement with our study, the results from Kuwait, United States, Italy and France revealed an increase in caloric intake and indicated weight gain during the current COVID-19 home confinement [10,35–37]. Data from Kuwait, a close Gulf country to UAE, showed a significant increase in weight of respondents during the quarantine and the weight gain was 4.5 times higher among those consuming unhealthy diets [38]. The actual weight increase was not assessed in this study considering the short time interval of COVID-19 lockdown, however, the large percentage of the population that reported an increase in weight can be used as a proxy pertaining to changes in eating behavior and activity level. It has been suggested that the negative alterations in eating behaviors could be due to anxiety or boredom [39], lack of motivation to maintain healthy habits [40], or reduced availability of goods and limited access to food due to restricted store opening hours [41]. The prevalence of overweight and obesity in the UAE even before COVID-19 was high and has increased over time [42]. It is estimated that over one third of the population in the UAE is living with obesity with higher rates among females [43]. Thus, extra efforts are needed to reduce the burden of obesity and its risk factors especially during the COVID-19 pandemic.

Over half of the surveyed participants in this study did not consume fruits daily and about one third did not consume vegetables and dairy products on daily basis. Instead, almost half of the same population reported consuming sweets and desserts at least once per day and over one third consumed salty snacks daily. This transition towards a Westernized diet in the UAE was reported in 1998, where the consumption of fresh fruit and vegetables and of milk and dairy products was found low [32]. Moreover, in 2003, 77.5% of males and 75.7% of females in the UAE had less than five servings of fruit and vegetables per day [44]. Likewise, a recent study among Emirati adolescents revealed that only 28% of them met the recommended daily fruit and vegetable intake [45]. This is concerning especially as fruits and vegetables are an important source of fiber, vitamins, minerals, and antioxidants. Diets rich in antioxidants (such as the Mediterranean diet and Dietary Approaches to Stop Hypertension (DASH) diet) are vascular protective. The Mediterranean diet is recognized as an anti-inflammatory dietary pattern, focusing on high consumption of plant foods, low red meat and dairy and moderate consumption of monounsaturated fat sources such as olive oil [46]. Evidence suggests that the Mediterranean diet is associated with better health status, lower risk of chronic disease and inflammation as well as increased immunity [47–49]. The Mediterranean diet is not only a healthy dietary pattern, but is also a sustainable diet that has a lower environmental impact than the typical Western diet [50]. Moreover, mounting evidence indicates that the Mediterranean diet has a favorable effect on diseases related to chronic inflammation, including visceral obesity, type 2 diabetes mellitus and the metabolic syndrome [51–55]. Knowing that the prevalence of cardiovascular disease incidence is high in the UAE (40%) [56] and rates of dyslipidemia are strikingly elevated (72.5%) [57] makes it imperative that diets such as the Mediterranean diet should be encouraged to prevent the potentially negative e ffect of quarantine on dietary habits and overall health [41].

Due to the increase in obesogenic behaviors related to the COVID-19 pandemic, two dietary patterns were revealed among the studied population, named the "Western-type diet" and the "Free Sugars diet". These patterns indicate unhealthy eating behaviors during the period of the pandemic. This is in agreement with previous studies reporting a transformation of the diet in Eastern Mediterranean countries from a traditional Mediterranean diet to a more Westernized diet which is high in energy, saturated fat, cholesterol, salt, and refined carbohydrates, and low in fruits, vegetables, fiber, and polyunsaturated fats [25,58–60]. Therefore, current dietary behaviors in the UAE may not be e ffective against the COVID-19 virus since it can adversely a ffect the immune system response among other health factors. Furthermore, it is unclear whether these dietary patterns were due to the lockdown that followed the COVID-19 outbreak; however, the implications can be detrimental considering an adequate supply of macro- and micro-nutrients are essential for optimal immune function and response [11,61].

Amidst these passive changes in food behavior, some beneficial aspects emerged from this study, such as a significant increase in home-made food preparations, regular breakfast consumption and lower intakes of fast foods. Similarly, a consumer online based survey conducted by Ipsos across the Middle East and North Africa (MENA) region revealed that 57% out of the 5000 consumers who took part in the survey were preparing their own meals, and 79% were eating less often at restaurants [62].

Among the surveyed participants, more than one third reported a non-engagement in any physical activity during coronavirus pandemic lockdown. This was mostly observed among males in this study, with a simultaneously greater likelihood of increased sedentary time, compared to females. The findings of this questionnaire are in accordance with other studies indicating that the current COVID-19 pandemic had a dramatic impact on lifestyle behaviors globally, including diminished engagement in sports and physical activity in general [63–65]. Moreover, the "E ffects of home Confinement on multiple Lifestyle Behaviours during the COVID-19 outbreak (ECLB-COVID-19)" international survey revealed that the COVID-19 pandemic had a negative e ffect on all levels of physical activity (vigorous, moderate, walking and overall) and increased daily sedentary time by more than 28% [14]. Similarly, in the current study the proportion of participants who spent more than five hours per day on screens for entertainment increased by 23.3%. Together with the unhealthy diet,

the reduction of physical activity would not only contribute to weight gain, but also to an increase in cardiovascular risk during quarantine. Thus, awareness about the importance of regular physical activity and its benefits on overall health is necessary during such times [66,67]. It is also important to identify groups at a higher risk of unhealthy lifestyle behaviors during the COVID-19 pandemic to design interventions targeted towards these groups.

During the COVID-19 pandemic higher levels of anxiety, stress and depression have been observed among individuals [68–70]. In this study, the percentage of participants experiencing exhaustion, irritability, and tension more often during the coronavirus pandemic increased significantly. Sleep was mostly a ffected in females and needs to be further evaluated since it is linked with multiple endocrine functions, as well risk for obesity and depression. The risk of obesity is underlined by the significant increase in daily meal frequency among participants over 36 years with the majority being female. Also, despite WHO recommendations to minimize listening to unreliable news that could cause anxiety or distress and to seek information only from trusted sources [71], over two thirds of participants in this survey used social media as a main source for health updates. Studies have shown the negative and harmful e ffect of misinformation overload "infodemic" on the mental health of individuals [72,73]. Moreover, stress and anxiety could disrupt sleep quality during the night and energy levels during the day. Results of the current survey indicated a 10.8% increase in participants reporting poor sleep quality and 26.2% increase in those feeling lazy during the pandemic. Xiao and his co-workers found a significant negative correlation between anxiety levels and sleep quality and suggested the use of telepsychiatry consultation as an important therapeutic strategy [74]. The use of telehealth has been shown to be useful in providing support to patients and is appropriate for the delivery of mental health services [75]. Additionally, the Mediterranean diet does not only have a protective e ffect on the risk of cardiovascular diseases and certain types of cancer [54,76], but also an increased compliance with it could be associated with lesser mental distress, better sleep quality, and higher scoring for self-perceived health status [77–79].

It is acknowledged that this study has limitations related to the use of self-reported questionnaire, snowball sampling method and the cross-sectional study design. The study information was acquired after lockdown, and although comparisons are critical to be made in order to draw inferences, no conclusive remarks can be drawn. Results stratified by sex should be interpreted with caution, since the majority of the participants were females. Furthermore, in order to minimize selection bias that may arise with snowball sampling (including interrelated-similar individuals), each individual could refer a maximum of three people who were not family members, and only one individual per age group (young adults, older adults, elderly) was enrolled from a household. Moreover, the change in dietary pattern was not assessed in this study, since data on food frequency were only obtained during COVID-19 pandemic, although these can be used as a reference for further studies performed, in these uncertain times. This was done to reduce the probability of including recall bias, since the participants had to respond to multiple questions on food frequency and quantity during COVID-19 lockdown and for a prolonged period prior to that. Also, the presence of obesity and eating disorders were not determined in the study, nor was information on infection with COVID-19 reported. Such analysis would require a longer questionnaire, hence may have decreased the compliance and response rate, but also would have required a larger sample size based on the prevalence of all factors to acquire adequate study power. Another potential limitation of the study was that respondents were mostly females. Although this is usual in online questionnaires [80], it should be considered when generalizing the results. However, using an online survey facilitated data collection during COVID-19 pandemic from all seven emirates. It also guaranteed the anonymity of the participants, thus reducing the social desirability bias. The strengths of this research include data collection timing one month after lockdown which minimizes memory failure for previous habits. In addition, the survey provided was in multiple languages in a multilingual environment like UAE.

The results of the study indicate that individuals in the UAE experienced negative lifestyle changes, unbalanced food choices, a reduction in physical activity, and psychological problems during the COVID-19 pandemic. Although quarantine is an essential measure to protect public health and control the transmission of the virus, these findings should be taken into consideration for future regulations in the UAE.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-6643/12/11/3314/s1, Eating Habits and Lifestyle during COVID-19 Lockdown in the United Arab Emirates: A Cross-Sectional Study.

**Author Contributions:** Conceptualization, L.C.I., T.M.O. and A.S.A.D.; methodology, L.C.I., T.M.O., A.S.A.D., M.N.M., M.H., and S.T.S.; validation, L.C.I., T.M.O., A.S.A.D., M.N.M. and E.M.; formal analysis, L.C.I., M.N.M., S.T.S., E.M. and H.H.; investigation, L.C.I., T.M.O., M.N.M., A.S.A.D., A.A.M., A.H.J., D.O.A.J., H.I.A., H.A.S., H.H., L.M.R.A., L.S., M.H., R.R.S.O., and S.T.S.; writing—original draft preparation, L.C.I., M.N.M., A.S.A.D. and S.T.S.; writing—review and editing, L.C.I., T.M.O., M.N.M., A.S.A.D., A.A.M., A.H.J., D.O.A.J., E.M.,H.I.A., H.A.S., H.H., L.M.R.A., L.S., M.H., R.R.S.O., and S.T.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.


**1. Introduction**

Today, the number of tunnel projects has increased significantly due to the developing infrastructure projects. Tunnels are used in most large highways, railways, water, metro and mining projects. Depending on the increasing number of infrastructure projects, the total number being constructed is gradually increasing. As a consequence of this increase, tunnel design and construction works are also gradually increasing. Project design studies carried out after the determination of project routes are of great importance. It is extremely important to determine the rock and ground support classes, excavation methodology and support elements within the project design studies. Although there are different tunnel project approaches today, all designs are based on the principles of rock and soil mechanics; for this reason, the correct determination of soil and rock parameters is the main factor in determining tunnel design. In addition, the correct realisation of the geological model of the route through which the tunnel will pass is another factor that will directly affect the tunnel design. In addition to all of these, surface structures also affect the design. While deformation is not allowed in tunnels to be excavated under towns and cities, a certain amount of deformation is allowed in mountain tunnels. The design of the support system is affected by natural structures such as streams on the surface. This design effort can be divided into empirical, analytical and numerical methods for the project work to be done. Although these three methods are used separately most of the time,

they should be evaluated together for a healthy tunnel project. During the tunnel excavations, tunnel projects should be compared with predicted and encountered geological conditions. If necessary, tunnel projects should be revised according to the encountered geological conditions. It should be well known that the predicted geological conditions and the actual geological conditions encountered will never be the same. For this reason, geotechnical measurements such as geological face maps, convergence measurements and deformation measurements should be made during the tunnel excavation in order to make the necessary revisions and updates. If necessary, the support system and rock mass parameters should be revised with a back-analysis. In addition, laboratory tests made from the boreholes taken during the excavation and rock mass parameters should be re-evaluated.


**6. Conclusion**

498 Heat Exchangers – Basics Design Applications

(Wyborn, 2011). The concept of a 25MWe commercial plant is now designed with 3 injection wells and 6 production wells. The ultimate potential is to supply up to 6500 MWe of long-term base-loadpower, equivalent to electrical supply from ~750 MT thermal

Many research teams are currently working on improvement of existing techniques of innovation developments to ensure better production rates and minimized constraints. Among these innovations, the following are particularly promising but of course non

The use of supercritical CO2 as a heat transfer fluid has been first proposed as an alternative to water for both reservoir creation and heat extraction in EGS (Brown, 2000). Numerical simulations have shown that under expected EGS operating conditions, CO2 could achieve more efficient heat extraction performance than water (Magliocco et al., 2011). CO2 has numerous advantages for EGS: greater power output, minimized parasitic losses from pumping and cooling, carbon sequestration and minimized water use. Magliocco et al. (2011) have performed laboratory tests of CO2 injection while Plaskina et al. (2011) made a numerical simulation study of effects of CO2 injection to provide a new method to improve heat recovery from the geopressured aquifers by combining the effects of natural and forced

During stimulation of EGS wells, water is injected in order to open sealed fractures through shear failure. When the fractures are open, the stimulation fluid flows into them and becomes unavailable for stimulation elsewhere. Fluid diversion agents can serve to temporarily plug newly stimulated fractures in order to make the injected water available to stimulate new fractures (e.g. Petty et al., 2011). The diversion agent is subsequently removed to allow flow from those previously sealed fractures. As demonstrated by Ledésert et al. (2009) and Hébert et al., (2010), calcite is found naturally in fractures of EGS reservoirs and prevents the fluid from flowing into fractures. The in situ precipitation of calcium carbonate

A lot of oil and gas reservoirs have been or will be abandoned in petroleum industry. According to Li and Zhang (2008) these oil and gas reservoirs might be transferred into exceptional enhanced geothermal reservoirs with very high temperatures. Air may be injected in these abandoned hydrocarbon reservoirs and in-situ combustion will occur through oxidization. The efficiency of power generation using the fluids from in-situ combustion reservoirs might be much higher than that obtained by using hot fluids

**5. Forthcoming developments and challenges of EGS projects**

**5.2** *In situ* **formation of calcium carbonate as a diversion agent**

was studied by Rose et al. (2010) for use as a diversion agent in EGS.

coproduced from oil and gas reservoirs because of the high temperature.

**5.3 Use of oil and gas reservoirs for EGS purposes**

coal (Wyborn, 2011).

exhaustive.

**5.1 CO2 EGS**

convection.

Enhanced Geothermal Systems experiences at Fenton Hill (USA), Rosemanowes (UK), Hijori (Japan) and Basel (Switzerland) allowed scientists to develop a European thermal pilot-plant producing electricity in Soultz-sous-Forêts (France) since June 2008. This project is the result of 20 years of active research based on geology (petrography, mineralogy, fracture analysis), geochemistry, geophysics (seismic monitoring, well-logging), hydraulics and modelling. Technical improvements were also necessary to allow deep drilling (down to 5000 m) in a hard (granite), highly fractured rock and circulation of water at great depth (between 4500 and 5000 m). The rock behaves as a heat exchanger in which cold water is injected. The water circulates in the re-activated fracture planes where it warms up. It is pumped to the surface and activates a 1.5 MWe geothermal Organic Rankine Cycle power plant that converts the thermal energy into electricity. In such a project challenges are numerous and difficult since the injected water must circulate at great depths between the 3 wells of the triplet with no or little loss and the flow rate and fluid temperature must be and remain high enough to allow production of electricity. Provided careful monitoring of the reservoir during operation, EGS are a sustainable, renewable and clean way to produce electricity. It has been proven that environmental impacts of EGS are lower than those of nuclear or fossil fuel power plants dedicated to the production of electricity. The Soultz EGS pilot plant is the first one in the world to produce electricity and it should be followed in the forthcoming years by industrial units that will produce electricity at a commercial scale. Many other EGS projects have begun all around the world and a lot of scientific and technical targets are in development to improve the production of energy (electricity and central heating through district networks).


**Hao Wu**

**1**

**Chapter 1**

Applications

1. **Introduction**

sensors include:

*Mingqiang Yi and Fenglei Niu*

harsh environment monitoring [1, 2].

Introductory Chapter: Temperature

Temperature measurements are vital to our life and various activities. Temperature

There are many different types of temperature sensors, each with its own advantages and disadvantages [3, 4]. Some of the most common types of temperature

• Thermocouples: Thermocouples are the most common type of temperature sensor. They are relatively inexpensive and easy to use. However, they are not very accurate and can be affected by environmental factors such as humidity.

• Resistance temperature detectors (RTDs): RTDs are more accurate than thermocouples, but they are also more expensive. RTDs are typically made of platinum

• Semiconductor temperature sensors: Semiconductor temperature sensors are the most accurate type of temperature sensor. They are also very small and can be integrated into electronic devices. However, semiconductor temperature sensors

The advanced applications of temperature sensing are constantly evolving. Some

or nickel, and their resistance changes with temperature.

are also the most expensive type of temperature sensor.

of the most exciting new applications include:

sensors are devices that measure the target temperature and convert it into an electrical signal. Many temperature sensors and measurement technologies have been developed and provided in industry and laboratories, including temperature probesused for immersion temperature measurement in liquids, air, gas, or wells in solid material, and surface sensors such as fast-response thermocouples mounted in a polyimide carrier for easily measuring surface temperatures or heat flux. Fiber optic monitors and fiber optic sensors have been designed to work more reliably than traditional sensors due to their resistance to outside influence such as microwaves, electromagnetic interference, and radio frequency interference. There are also non-contact temperature measurement solutions for remote measurement or human body temperature measurement. Recently, the smart temperature sensing has greatly advanced the applications such as pandemic mitigation, point-of-care healthcare, and

Sensing and Advanced

School of Nuclear Science and Engineering, North China Electric Power University, Beijing, China


**Chapter 6**

[66] Liu Z, Liu A, Wang C, Niu Z. Evolving neural network using real coded genetic algorithm (GA) for multispectral image classification. International Journal of Future Generation Computer Systems. 2004;

Land Use Change and Sustainability

20:1119-1129

74


**8.1 Freeze drying**

Freeze drying/lyophilization is a technique in which water is removed from a product after it has been frozen and put under a vacuum, allowing the ice to convert directly from solid to vapor without going through a liquid phase. Probiotic stability is stabilized by freeze-drying (lyophilization), which is typically done with cryoprotectants present and with the water being sublimated under a vacuum [6]. The purpose of the cryoprotectants is to increase microbial survivability and stabilize the product during storage. Probiotics' viability and shelf life are also enhanced by lyophilization, although this does not affect how long they survive in the digestive system. As a result, it is widely used as a second step in microencapsulation processing to increase the shelf life of probiotic bacteria by drying them out after they have been encapsulated in emulsions or gels [6]. Sugars like glucose, mannose, lactose, trehalose are among the most popular low-molecular-weight cryoprotectants. Most of the molecules with large molecular weight are polysaccharides and proteins, such as soy protein, milk, gelatin, and maltodextrin [94]. Wang et al. [95] coated sodium alginate microcapsules containing *L. plantarum* CCTCC M 2014170 cells with an inner layer of inulin and an exterior layer of skim milk before freeze-drying the mixtures. The findings demonstrated that the probiotic's resilience to the GIT's simulated environment was enhanced by encapsulation. In the stomach, *L. plantarum's* viability remained unchanged after 2 hours, but in the 1% bile salt solution, there was a 1.21 log drop in cfu/mL. The microencapsulation of probiotics *L. acidophilus* and *L. casei* by freeze drying was also studied by Bora et al. [96] utilizing whey protein isolate, fructooligosaccharides, with a combination at ratio (1: 1). In contrast to lose cells, which had a low survival rate after 90 minutes of incubation at pH 2, the probiotic that had been encapsulated had a high encapsulation yield (98%) and was not significantly impacted by the simulated stomach juice [96].