fig1 a shows a surface discharge assembly suitable for use as a pump source for lasers . a planar , dielectric substrate 1 is connected via electrodes 3 , 5 to an energy discharge source ( not shown ). electrode 5 is connected to an earth plane 7 . one surface of the substrate is in contact with a gas 9 . upon initiation of a high voltage pulse from electrode 3 electrical breakdown of the gas 9 occurs in the region between the electrodes 3 , 5 as shown in fig1 b . a surface plasma is then formed . fig2 shows a pump source comprising a substrate / cover gas arrangement and a resonant circuit discharge . the figure shows an ale 402l + 50 kv power supply 21 connected to a 100 nf energy storage capacitor 23 . the supply 21 and capacitor 23 are located in a high inductance (˜ 2 . 0 μh ) section of the circuit . the inductance is generated by a loop of wire 25 and a switch 27 . this switch 27 isolates the capacitor 23 from the rest of the circuit and comprises a gas filled , in line , trigatron spark gap , capable of holding off 30 kv when filled with nitrogen at a pressure of 1 atmosphere . the circuit also comprises a 100 nf low inductance (˜ 4 nh ) capacitor 29 , manufactured from a 0 . 5 mm thick mylar ® sheet having dimensions of 3 m × 0 . 45 m and covered with 76 μm copper foil . this sheet capacitor is connected in series with the surface discharge load via a high aspect ratio low inductance (˜ 5 nh ) stripline ( also of mylar and copper construction ). ( note : the stripline comprises two planar electrical conductors with a dielectric insulator between them . the aspect ratio refers to the width of the conductors compared with the insulator thickness i . e . a high aspect ratio is high conductor width and a small insulator thickness ) the surface discharge is provided by a dielectric substrate manufactured from 1 mm thick ptfe 31 . a copper earth plane ( indicated by dotted lines ), 20 mm wide , is connected to the underside of the ptfe substrate 31 to minimise the surface discharge inductance . the earth plate also forms part of the stripline and the current return for the surface discharge . two copper electrodes 33 , 10 mm wide , 0 . 45 mm thick , ˜ 2 mm radius of curvature corners ( see fig1 b where the length of the arrow on the top corner of electrode 3 represents the radius of curvature of the electrode corner ) are located on the substrate 31 above the earth plane . they are 45 mm apart . the substrate 31 is enclosed in an air tight box 35 which is filled with a mixture of xe or kr with a small amount of sf 6 ( partial pressures are 800 mbar for xe / kr and 150 mbar for sf 6 ). the discharge circuit was charged to 22 kv . upon switching of the spark gap approximately 75 % of this energy was transferred to the mylar capacitor 29 and was therefore available for the surface discharge . the arrangement at fig2 was connected to a laser head and the laser energy was measured . fig3 shows the optical pulse shape for the discharge and laser emission for cases where the cover gas is either xe / sf 6 or kr / sf 6 . the plots in fig3 have been normalised to show the pulse length and shape . plot lines 41 and 42 show the laser emission with kr / sf 6 and xe / sf 6 respectively . plot lines 43 and 44 show the discharge emission from kr / sf 6 and xe / sf 6 respectively . fig3 shows how laser emission responded to the optical excitation pulse and shows the laser pulse length was ˜ 400 ns . fig4 shows the spectral energy emission of the surface discharge over a ptfe substrate for the kr / sf 6 ( plot 45 ) and xe / sf 6 ( plot 46 ). as can be seen there is considerable emission in the optical region of the spectrum thus making this surface discharge arrangement appropriate as a source for an optical dye laser . fig5 a and 5 b show two laser head arrangements suitable for use with a surface discharge pump source . fig5 a shows a bethune cell arrangement . here the pump source ( substrate 51 and electrodes 53 , 55 ) is located below one face of a triangular cross - sectioned herasil ® silica dye cell 57 . most of the light emitted by the surface discharge is reflected by the walls of the cell into a dye filled channel 59 in the silica . laser output is shown by the arrow 61 . fig5 b shows an alternative laser head arrangement , the transverse flow dye laser . in this configuration the dye flows above the surface of the substrate ( as indicated by arrow 63 ). laser output is in a direction perpendicular to the dye flow , see arrow 65 ( in the longitudinal view of fig5 b the dye will be flowing either into or out of the page ). in fig5 b like numerals are used to identify identical elements of the device to those shown in fig5 a . in both fig5 a and 5 b the dye solution consisted of 86 μm rhodamine 6 g in a methanol solvent . the bethune cell was observed to operate at a higher level of efficiency than the transverse flow cell . fig6 shows three views of a solid state dye laser head arrangement . a planar dielectric polymer slab 70 pyr 597 doped ( modified poly ( methyl methacrylate ) polymer — mpmma ) is connected via electrodes ( 72 , 74 ) to a discharge circuit 76 . the load is separated from the discharge circuit by a nitrogen filled trigatron spark gap 78 . the substrate is covered by a gas 80 ( xenon at 0 . 5 bar ). the stripline and electrodes have a low inductance arrangement , i . e . the return from the earth electrode 74 to the discharge circuit is on the underside of the substrate ( earth return 84 ). at one end of the slab there is a 100 % reflective mirror 86 and at the other end an output coupler 88 . the distance between the 100 % mirror and the output coupler is minimised by arranging the striplines so they lie at right angles to the surface discharge propagation and laser emission directions 82 a ptfe reflector 90 and ptfe sheet 92 are respectively placed above and below the substrate in order to maximise the coupling of the discharge light into the gain medium . un - modified pmma is used as an insulating sheet either side of the dye slab 94 . the substrate 70 depicted in the figure had a length of 14 cm , a width of 5 cm and a thickness of 4 mm . the laser dye pyrromethene 597 ( pyr 597 ) is incorporated into the substrate at a concentration of 10 − 4 m . ( note : to pump a laser dye effectively , the surface discharge should have intense emission in the laser dye absorption bands . because the dye is doped into mpmma which only transmits above ˜ 290 nm , only the visible absorption band of pyr 597 is pumped ( 470 - 554 nm ).) the electrodes are 11 cm apart and 2 . 5 cm wide with ˜ 2 mm radiussed corners . the electrode separation limits the discharge length to 11 cm . the discharge circuit is designed to deliver 100 j of electrical energy to the surface discharge load in a ˜ 80 mw pulse . a surface discharge with this energy loading , power loading and discharge length produces a discharge impedance of approximately 0 . 5 ω and so the discharge circuit is designed as a matched 0 . 5 ω lumped element pulse - forming network ( pfn ). the circuit consists of 5 parallel l - c elements with a total storage capacitance of 1 . 14 μf . when using a 0 . 5 ω load the pulse length was designed to be 1 . 25 μs . the circuit was charged to 13 . 3 kv corresponding to a capacitively storage energy of 100 j and hence a peak power of 80 mw for a 0 . 5 ω load . in order to minimise the stray circuit inductance , which distorts the electrical pulse shape , the circuit components are connected using low inductance copper striplines 10 cm wide , 0 . 5 mm apart with a mylar dielectric layer . the earth stripline which is placed under the dye slab ( 84 ) provides the discharge with capacitive coupling via the substrate . fig7 compares the optical pulse shape of the surface discharge 96 and the laser output 98 . the optical pulse length was 1 . 9 μs with a 6 . 5 μs tail . the laser pulse was 950 ns long , shorter than the surface discharge emission pulse as expected . the highest enrgy value for the laser was 69 mj — see fig8 — which was obtained for an output coupler reflectivity of 50 %. for the laser pulse shown in fig4 this gave a power output of 73 kw . the skilled person will readily appreciate that a range of other arrangements for the pump source are understood to be within the scope of this invention . for example , for the arrangement shown in fig6 a dye doped polymer with lower absorption at the lasing wavelength could be used . also , the surface discharge could be modified to ensure there is more emission in the visible absorption band of the laser dye .