it might be thought that the ‘ virtual bullet ’ formed by the coalescing particles will be less dense than a normal solid of the same material , e . g . if the particles are perfect uniform spheres then at least by the ratio of the spherical packing fraction ˜ 0 . 74 , and less than that if allowance is made for inevitable inaccuracies in their shape , position and size . however as the back layers of the bullet are travelling faster than the forward layers , the bullet will compress to many times the normal density of the raw material as ( or just before or after ) it strikes the target . in each of fig5 - 9 , parts ( a ), ( b ), ( c ) and ( d ) show particle positions ( black circles ) at consecutive timesteps en route to striking the target ( hatched ). for clarity the drawings are necessarily not to scale , with particle size vastly exaggerated , and particle number vastly reduced , compared to a likely implementation . fig5 shows a basic configuration : particles come together to form a precompressed bullet before striking the target . fig6 shows a variant in which additional particles , fired first at lower speed , pre - compress the target before the bullet strikes it . fig7 shows a variant in which there is no pre - positioned target : rather additional particles , fired earlier at lower speeds , actually constitute the target . fig8 shows a variant in which the prepositioned target is not flat but conical , so that it is precompressed sideways as well as vertically before the bullet strikes it . fig9 shows a bullet whose central part travels fastest , so first ignition occurs at the centre . it will be appreciated that almost infinite variations on the theme are possible . for instance the particles may be of differing sizes , shapes and made of different materials , likewise the target ( if a prepositioned target is provided ) may consist of layers of different materials . for example , the ensemble may provide back and front portions of dense material constituting a ‘ hammer ’ and ‘ anvil ’, between which the material to be fused is sandwiched . diamond dust ; chopped graphite whiskers ; chopped carbon fibres ; ceramic microspheres ; frozen deuterium ; frozen deuterium - tritium compound ; frozen methane in which the hydrogen atoms have been replaced by deuterium and / or tritium ; an engineering plastic in which the hydrogen atoms have been replaced by deuterium and / or tritium ; tungsten ; uranium : are all examples of constituents which could be used in the particles and / or target . the particles can comprise outer shells of one material containing a second material within , e . g . the second material might be helium - 3 . during their transit , the particles may have their charge neutralised ( e . g . by spraying electrons on to them ) so that they do not repel one another as they come together . electron sprays might combine the functions of charge neutralisation , electron microscopy for precise position measurement , and adjustment of particle trajectory by momentum exchange . in a terrestrial system , particles may equally conveniently be measured and adjusted at any point in flight , not just near the accelerator or target ends of the system . note that relative ( rather than absolute ) accuracy of the particle positions is required . this is easy to obtain , as all particles can pass through the same sensors . if a prepositioned target is provided , the target may be adjusted to precisely the correct position for impact as the particle cluster approaches , or vice versa . note that the guidance system avoids any disruptive premature collisions between particles occurring . the tetris method may be suitable for separating compression and ignition , which is difficult to do by either laser or with a single physical bullet , for two reasons : a tetris accelerator can , at no extra cost , fire many slower pellets immediately preceding the higher energy ignition pulse , with all arriving near - simultaneously . the extra pellets can supply energy and / or momentum for target precompression . as the ignition bullet is made by a train of pellets of differing speed coming together , it can easily be both precompressed to high density , and given a very high mass per frontal area , like an armour - piercing bullet . thus it can punch through and deposit most of its energy at the heart of even a dense target plasma . allowance must be made for inefficiencies . in laser compression hydrodynamic inefficiency — evaporating material from the outside of the pellet to compress the central fuel by rocket - like reactive force — reduces the useful kinetic energy to 10 % of radiative energy supplied . pellet impact is potentially more efficient . however for ignition the available energy must be calculated in the centre - of - mass frame of the bullet and an equal mass of target , reducing the available energy by half . also , if the bullet has mean atomic mass equal to that of the target , half the energy will be wasted as internal heating of the bullet itself ; if on the other hand its atomic mass is higher than the target &# 39 ; s , energy is lost to rayleigh - taylor instabilities . further energy is wasted as the bullet punches through low - density plasma en route : not all its kinetic energy is donated at the ideal point . for compression the impact method could in principle be extremely efficient . however pellets provided at near - ignition speed are moving much too fast to impart momentum directly to the target , and can probably not be spread sufficiently uniformly . a solution is to make them collide with a strip of foil which acts as a hohlraum . radiation from the hohlraum evaporates a backing material to accelerate dt fuel , perhaps into a conical pit within a surround of dense metal , in much the same way that a spherical pellet is compressed by laser . as long as the gap between hohlraum and dt backing material is at least a few times greater than any inhomogeneities in the pellet wavefront or the foil itself , very uniform compression will be achieved . the accelerator operates on the same principle as a fundamental particle accelerator . however because the pellets travel at less than 1 % of light speed , the drive frequency can be much lower . tiny cheap mosfet transistors thus can be used in place of klystrons for power conditioning , with small air - core ( or vacuum - core ) transformers to boost their output voltage . the electrodes do not form rf resonant cavities , and can be simple disks of metal mounted within a teflon tube . three - phase electrodes can be used ( or even more phases ) so that individual pellets ride a ‘ wave ’ of near - constant electric field . this minimizes vibration and induced surface current , so the pellets can be made from any reasonably strong material , e . g . plastic microspheres . at the start of the accelerator , pellets could be injected from a ‘ waterfall ’ which falls between a pair of electrode plates . individual pellets are ‘ zapped ’ with electrons from a steerable cathode ray tube beam ( or positive ions from an ion gun ) to adjust their time and speed of entry into the accelerator . as the pellets approach the target shuffling may be necessary because while the direction of every pellet can be adjusted , its longitudinal position cannot . it will therefore be necessary to reallocate pellet positions dynamically in order to form a close - packed array as required . any surplus pellets may be discarded by turning them away to the side . it is this dynamic allocation which makes ‘ tetris ’ an appropriate system name . the ideal fusion reactor requires no rare or radioactive isotopes as input , and produces no radioactive isotopes as output . basic dt fusion as above does not achieve this goal . although the blast chamber could in principle be surrounded with pure elements which do not produce unwanted isotopes under neutron bombardment , for continued operation it is necessary to breed tritium from lithium . the dt reaction itself does not yield sufficient neutrons for this , so it is normally assumed that fissionable isotopes must be included in the blast chamber surround , which intentionally multiply up the number of neutrons available to react with the lithium . however a tetris reactor can easily be scaled up to much higher energies . at a temperature 10 × higher than that for dt burn , the reaction cross - section becomes large enough to support dd direct burn , with the reactions dd → helium - 3 + n and dd → tritium + p taking place at approximately equal rates . so by raising the reaction temperature and including a higher proportion of d to t , the tetris reactor can manufacture sufficient tritium for its own use . ( helium - 3 is not radioactive .) more ambitiously , the reaction boron - 11 + p → helium - 4 might be achievable . this is the ideal aneutronic reaction , but very challenging first because it would require enormous densities , and second because energy output ratio is relatively modest : indeed unless an initial degenerate compressed plasma can be produced , the electrons take up too much energy to make self - propagating fusion burn possible . for a tetris system the outlook is much better . the temperature of 550 kev at which the reaction cross - section is maximized is achievable , vastly lowering the density requirement . the efficiency with which tetris turns electrical into kinetic energy allows net energy generation with a very modest output ratio . and because a tetris machine can produce pellets with any speed distribution required , varied over the pulse , gentle piston - like precompression with minimum heating ( as required to produce a degenerate plasma ) is possible . ( if very much slower pellets are required for compression than ignition , for example to perform without a hohlraum - type membrane , either by direct momentum transfer by impact with the target or by colliding pellets with one another to heat them and so produce radiant energy to power compression , separate accelerators can be used for the purpose . since these secondary accelerators can piggyback on the buildings , vacuum tubes and trajectory adjustment systems of the primaries , cost will be modest .) a pair of opposed tetris accelerators firing boron pellets into a central hydrogen target may well be optimal . boron - a pellets are strong and can be given a high charge - mass ratio . all mass used is actually involved in the fusion , and the centre of mass is stationary with respect to the frame of the accelerators , so hydrodynamic efficiency is close to 100 %. the accelerators used do not necessarily have to face one another exactly , so problems with curvature of the earth &# 39 ; s surface are minimized . it may be possible to fit multiple parallel accelerator tubes within the same building . tubes need only be separated by only a few times their electrode spacing to minimize mutual interference . spare tubes can be provided . a worst - case pellet disintegration damages only one tube ; the system can continue functioning while it is replaced . the accelerators can be aimed so that the beams gradually converge , initially entering small individual vacuum tubes which converge to become a single tube nearer the target . an angled mirror can be used so that a single camera can capture a stereo image . the unit can measure particle position in all dimensions to ˜ 0 . 1 micron . in practice a cheaper laser giving longer pulses can probably be used , with streak analysis to determine along - axis position . the cameras can also assess the size of each particle ( shape , spin , mass and mass / charge ratio can also be measured independently if desired , e . g . by measuring the charge and also the magnitude of deflection by a known magnetic field ). the relative positions and timings of the particles can be adjusted , and their intended destinations within the final bullet interchanged if necessary , to ensure that the density of the final bullet is almost perfectly uniform despite variations in individual particle size etc . this is analogous to the computer game tetris . a possible advanced fusion target is hydrogen - boron , which produces no neutrons , hence no radionucleides . this reaction has resonance peaks . given the fine - tuning possible for the virtual bullet , which can be precompressed , shape - tailored , and given any desired internal velocity distribution , the peaks may be usable . the pellets are assumed to be boron , an ideal material which is strong and very hard even as a pure element : compounds such as boron carbide are even tougher . the target may be boranes , boron - hydrogen compounds which are stable at low temperature . an effective fusion burn wave can be generated in a proton - boron plasma providing the plasma is degenerate , i . e . not too hot in relation to its density . the intrinsic ability of a tetris accelerator to provide a first wave of slower pellets carrying high momentum relative to their energy , which arrive at the target simultaneously with the faster ignition pellets , is perfectly suited to providing such piston - like compression . note that energy several times greater than the ignition pulse is automatically available for target precompression , by firing lower - speed pellets before the high - speed ignition burst . target compression may optimally be performed by pellets travelling very much slower ( orders of magnitude slower ) than the ignition pellets . the ignition pellet accelerator would be capable of providing particles at the very slow speed appropriate , perhaps a few tens of kilometers per second , by leaving all except the first few electrodes switched off , but the impulse would be modest . a much larger impulse can be provided cheaply using a small ancillary accelerator which shares its expensive components — building , vacuum tube , insulation and course correction systems — with the main accelerator . up to say 100 millisecond &# 39 ; s worth of this machine &# 39 ; s output can arrive at the target as a pulse : an output of just 20 mw can provide a 2 mj compression pulse . to keep within the limits of the tweaking system , the pellet arrival rate should be no greater than that of the main pulse . the compression particles may therefore be of order one million times as massive as the ignition particles : e . g . 100 × the diameter , 50 μm . the ancillary accelerator will be around one - tenth the length of the main one , but a negligible fraction of its switch cost . although the compression pellets each carry several thousand times the momentum of the ignition pellets , they also carry 100 × the charge and remain within the effective vicinity of correcting fields for several hundred times as long , so the existing course tweakers can easily handle them . the speed ratio is so high that all compression pellets have cleared the final tweaker before the first ignition pellets arrive . a magnetic field is provided at a few points along the vacuum tube connecting accelerator and target to bend the compression pellet trajectories slightly upward , to prevent them falling too far under gravity . as an alternative method of target compression , fast pellets may hit a front layer of material on the target which heats up and evaporates , pressing and compressing deeper layers . this ‘ rocket - like thrust ’ method whereby energy in the form of very fast particles heats material to cause propulsion at much lower speeds is analogous to laser - driven inertial confinement fusion , wherein the fast particles are however photons . compression may be performed by pellets , and heating at a point to cause ignition provided by other means , such as laser ( s ) or beam ( s ) of fundamental particles , atoms , etc . or vice versa . fast and / or slow pellets may approach the target zone from opposite directions , or indeed from many directions . they may be fired from separate accelerators and / or have trajectories bent by passage through electric or magnetic fields . in this case there may or may not be a separate target : the pellets themselves may comprise all of the material to be fused . successive fusion reactions may take place at various points within a pipe , which may be long with a narrow internal diameter . if fusion is ignited in pellets which continue to travel rapidly ( for example by colliding pellets of markedly differing speeds ) the rapid motion of the fusion source may help dispense the energy release ( whether in the form of photons , neutrons , or other particles ) along the length of the pipe so that no one portion of the pipe walls or equipment beyond is subject to damaging levels of heat , radiation , etc . development is greatly facilitated because optimisation of compression and ignition can be explored using software changes only to tailor the ‘ bullet ’, for example using a genetic algorithm to home in on effective patterns . the diacrode &# 39 ; s major limitation is that it a fixed frequency device . in the present application , about 20 % launch speed variation is required . using mosfets , this could be applied simply by steadily increasing the frequency during the period between the first and last pellet leaving the accelerator . with diacrodes , the solution is to switch in units at the fast end successively , resulting in sub - trains of pellets each a few meters long travelling at a uniform speed . after exiting the main accelerator , these sub - trains pass through a short run of mosfet - controlled electrodes which are operated to speed up ( or retard ) pellets by differential amounts : e . g . from zero increment for the first pellet in each sub - train to a few meters per second for the last , so that each individual pellet now has a slightly different speed for perfect convergence as required . at the low - speed end of the accelerator tube there will be a minimum acceptable pellet separation due to inter - pellet repulsion . in place of what would be the first few meters of the accelerator , a longer low - power leader section is therefore provided with fixed electrode separation and highly variable drive frequency . this leader section is continuously fed with pellets which are precharged to a moderate voltage and fired in at ( say ) 5 few km / sec . when the leader contains a line of all pellets required , the variable frequency electrodes then accelerate this line en bloc to ( say ) 50 km / sec , at which point it enters the main accelerator . during this process the pellet voltage is raised by offsetting the local electrode voltages to the required level : note that electrons can flow easily from a pellet to the electrode it passes through , though not vice versa , as the pellet acts as a discharge point source while the electrode surface is smooth . to feed the leader at 5 km / sec , pellets are released from a pelletron - type van de graaff generator . this is a well known technology , except that normally pellets emerge from a source container at a chosen average rate rather than at precise intervals . here this must be modified : a container with several orifices is used , but any pellet can be rejected just after its emergence is detected , e . g . by zapping it with an electron gun to remove its charge , or diverting it using a switched electrode . most pellets are thus recycled , leaving just one per cycle to drop into the accelerator tube . ( note that the required rate is lower than the rate at which a modern bubblejet printer ejects ink droplets of comparable diameter .) the precise timing of those pellets which are allowed to fall into the accelerator can be adjusted with a smaller kick from the same electrodes used for rejection . a pair or larger group of pellets which happen to emerge from the same orifice simultaneously are always rejected . there will be occasional cycles when no pellet is available , because any poisson distribution occasionally yields a value of zero , but these gaps can be closed up by in - flight shuffling as described elsewhere . focussing to keep the pellets centered during acceleration can be provided by electric field , e . g . shaping the electrodes as cones to produce an inward radial field for part of each cycle . magnetic lenses , e . g . quadrupole magnets as used in a fundamental particle accelerator , and / or active steering as described elsewhere in this document , can also be used e . g . to damp down lateral oscillations . pellets can be strength - tested by charging them to slightly above operational voltage before firing . in - tube pellet failure is therefore unlikely . any failure which does occur has the potential to become contagious . however worst - case energy release is 2 mj , and the debris cloud kinetic energy reduces exponentially at each consecutive electrode it impacts : each electrode masses more than the entire pellet cloud , and its central hole constitutes & lt ; 1 % of its area . so damage will be very localised . the tube can be wrapped in a kevlar blanket so that external equipment is not affected . chicane traps can be incorporated at multiple points in the system : slight bends with lateral fields which divert pellets of exactly the correct charge / mass ratio into the next section , but allow other material to fly on into open ended ‘ dump tubes ’ in which the plasma from their impact is safely contained . the precise deflection of each pellet which passes successfully allows its exact charge to be monitored , and tweaked with an electron gun if necessary . the tube connecting accelerator and target can contain a relatively soft vacuum by particle accelerator standards : it does not need to be housed in a building , and can comprise a simple pipe mounted on pylons or stilts . it can be given a much larger internal radius , say ˜ 10 cm , than the maximum pellet deviation from the beamline , so that it can tolerate lateral displacements due to wind etc . of several centimeters , and so that its vacuum can be maintained by pumping from a limited number of points along its length . pellet position measurement near the target end is demanding because of the high rate of pellet flow past a given point , increasing to theoretical infinity at the collision point . measurement stations comprise lines of paired cameras , with a pulsed laser providing light via a leaky optical fibre . exposure is controlled by the laser , not by shuttering : inexpensive ccds with a shutter rate ˜ 50 hz have a similar readout rate , ˜ 10 mpixel / sec , as expensive high capture rate cameras , and are capable of binary processing involving bit shifts , adds and reads ( normally used e . g . for pixel binning ) to avoid confusion from overlaps , pellet trajectories are spread , and timings chosen , so that each pellet imaged appears in a different part of a camera &# 39 ; s field of view : for example a 1000 × 1000 field may be subdivided into 100 squares each of 100 × 100 pixels . the position of each pellet is reduced to an x , y value for readout . given that a readout delay of up to 20 μs is acceptable ( corresponding to a downstream flight distance ˜ 20 m ) each camera pair can track ˜ 100 pellets per pulse . the laser can be a standard laboratory desktop model providing ˜ 10 μj output pulses of duration & lt ; 1 picosecond at repeat rate up to 2 mhz . a pellet moves only ˜ 1 μm in 1 picosecond . at the final correction the pellets can be given slight radial velocities , with dynamic allocation of placement to make the end product , the cylindrical virtual bullet , as neatly packed as possible , allowing for any ‘ missing pellet ’ gaps and also taking detected variations in pellet size into account if an imperfect monodisperse such as diamond dust is used : hence the tetris system name . after the final correction station , a further electron gun reduces the pellet charge to zero . the basic design assumes that all pellets follow the same beamline . by intentionally diverting the pellets into a number of parallel beamlines , the linear separation between pellets may be kept large enough that independent pellet tweaks even close to the impact point are possible . the basic design uses small rapidly switched electrodes to tweak pellet trajectories . an alternative is to have the pellets pass through fixed strength fields , having reduced the charge on each pellet to a precisely controlled value to produce the course adjustment required , using an electron gun . the severity of the flexure and heating encountered by the pellets during acceleration can be greatly reduced by the following strategy . the tetris accelerator , with its large number of independently switchable mosfets , can easily provide multiple phases of drive voltage . in a strategy very similar to that used in coilguns , if consecutive electrodes cycle 120 ° out of phase , the accelerating force becomes approximately constant : the pellet rides a wave of constant gradient . if the pellet is small compared to the inter - electrode distance , it experiences a field which is to a good approximation both spatially and temporally uniform , and suffers virtually no vibration or induced current . to feed pellets into the accelerator at a controlled rate , they could be mechanically preplaced in ( e . g .) a 500 × 500 array on a plate . pellets are charged via pins on which they sit above an electrode . to launch each pellet at the chosen instant , the charge on the electrode is switched from negative to positive . plates can be removed and reloaded in alternation so the pellet supply is continuous . if the pellets are monodisperse microspheres , they can come together into a close - packed array ( e . g . as found in various crystal lattices ). even at maximum packing density there will be voids , however these will rapidly be filled as the spheres crush together at ( or just before ) target impact . to minimise seeding of rayleigh - taylor instabilities , these voids can be filled with smaller microspheres , or even with a quasi - fractal pattern of microspheres of different sizes . if a non - monodisperse such as sieved diamond dust is used , dynamic reallocation of pellet terminal position by in - flight trajectory tweaking can aim for a goal similar to dry - stone walling , to bring objects of arbitrary size and shape together into a mass with minimal internal voids . if the orientation and spin rate of each pellet has been measured , even orientation at the moment of collision can be taken into account . discharging of the pellets can be done by firing electrons at them from an electron gun , or emitting electrons from a hot or cold cathode close to the flightpath . the electron gun can fire electrons substantially parallel to and at the same speed as the pellets , so that a high capture rate results . likewise positive ions could be fired or emitted in place of electrons to make the pellet charge more positive . pellet charge reduction could be done as a continuous or multi - step process . actual pellet charge ( charge / mass ratio ) can be measured at any point by passing the pellet stream through a known electric or magnetic field and measuring the angle of deflection of each pellet . the basic design assumes a single accelerator tube . a vertical stack of parallel tubes , broadly comparable in size to a very long set of bookshelves , could be used . once technical confidence has increased , it would be possible to suspend a set of 10 or more such stacks in parallel beneath the ceiling of the building , occupying most of its width , but with ˜ 2 m headroom below . any stack can be lowered into this space along all or part of its length for easy maintenance access . when the stack is rehoisted , small positioning motors attached to each tube fine - tune the alignment . the sacrificial projectile could be made from supercooled ice ( whose vapour pressure at cryogenic temperatures becomes utterly negligible ) thus generating steam directly within the reaction chamber .