Abstract:
An ignition feedback regenerative amplifier consists of an injector, a linear accelerator with energy recovery, and a high-gain free electron laser amplifier. A fraction of the free electron laser output is coupled to the input to operate the free electron laser in the regenerative mode. A mode filter in this loop prevents run away instability. Another fraction of the output, after suitable frequency up conversion, is used to drive the photocathode. An external laser is provided to start up both the amplifier and the injector, thus igniting the system.

Description:
The United States Government has rights in this invention pursuant to Contract No. DE-AC03-76SF00098 between the U.S. Department of Energy and the University of California, for the operation of Lawrence Berkeley National Laboratory and Agreement BG97-100(00) between Lawrence Berkeley National Laboratory and the City of Ridgecrest, Calif. 
    
    
     BACKGROUND OF THE INVENTION 
     Electrical power has always been a limiting factor for satellites, and it restricts the services that they can perform. The need for additional transponders to satisfy the demand for satellite-supplied television, e-mail, worldwide web, long distance telephones, rapid computer data transfer, and many other types of telecommunication is increasing. The number of transponders has risen from about 24 active transponders per satellite in the late 1980&#39;s to 94 active transponders on the latest Hughes satellite launched in late 1997. The demand for additional power for transponders over the last few years fits an exponential curve and the end is not in sight. The only practical limitation for generation of the additional power for transponders is the availability of electricity from the solar panels carried on the satellite. At present the size of satellite solar panels is effectively at a maximum. Additional satellites in the same “space slot” can be deployed to increase the total solar panel area, and this is the direction that many satellite companies are going. A major drawback of this approach is that the output signals of the various satellites are not in phase, so interference between satellite transmissions can be a problem. A bigger drawback, however, is that the multiple satellite approach is very expensive. 
     One way to power the satellites is laser power beaming, (LPB). Laser beams can increase the power level an order of magnitude above that available from the sun. The wavelength 840 nm is within one of the transmission windows of the atmosphere, and at the same time near the peak of the photo-voltaic conversion efficiency of Si the most commonly used material for the solar panels. Beaming from the earth&#39;s surface requires the laser beam to travel through our planet&#39;s atmosphere. The atmosphere causes various problems such as scattering, absorption and distortion. The development of adaptive optics has helped solve this problems. 
     Free electron lasers, FELs, are capable of generating high power optical radiation without using a material medium. Unlike other lasers, which all utilize changes of electronic energy levels in a material, the light in a free electron laser is generated in a vacuum and should have no distortion. This characteristic of free electron lasers makes them ideal for generation of high power light with a diffraction limited light beam. This high quality beam can propagate through the atmosphere to great distances. This ability is due to a distortion-free initial wave front which allows all of the required corrections to result only from the atmospheric imperfections. This correction technique is now well known. 
     The light in an FEL is emitted from bunches of electrons traveling at very nearly the velocity of light. They are deflected by a series of magnetic poles. When the electrons are deflected, an electro-magnetic wave is radiated. The apparatus causing this deflection contains small magnets oriented somewhat like the teeth of two interlocking combs and consists of magnets with alternate north and south poles. This system is called an undulator, or in more vernacular terms a “wiggler” since it wiggles the electron bunches, which then emit light. If there is a light beam of an appropriate frequency in the vicinity of these electrons, the phenomenon of stimulated emission occurs. The electrons emit light in phase and at the same frequency as the initial light, creating Light Amplification by Stimulated Emission of Radiation or a LASER. Current state of the art for FEL generating visible light is an average power level 1-10 W. Main problems to be solved before FELs can produce hundreds of kW of optical power include (1) production of a high average current electron beam with low emittance, (2) high thermal loading in mirrors, and (3) radiation hazards from a high average power high energy electron beam. 
     As was mentioned above, one of the most attractive features of FELs is the possibility of generating fully transverse coherent light, having high average power. On the other hand, the efficiency of the conversion of the electron beam power to the light power is rather small in an FEL, being typically not more than a few percent. For high light power application, therefore, it is necessary to use an intense average electron beam current. In the FEL producing visible light, this beam must have high quality, i.e. it must have a low transverse and longitudinal emittance. Radio frequency (RF) photocathode guns are, in principle, capable of production of an electron beam of adequate quality, but they need a laser driver which supplies the photocathode with photons. Existing lasers generate too little average flux of photons, much less than is needed for production of an intense average electron beam current. Thus, there is an obvious problem. One can get either a high intense average electron beam current, but of poor quality, for example the electron beam current from a thermionic electron gun, or an electron beam of a good quality from the RF photocathode gun, but with low average intensity. 
     Another severe problem concerns the optical resonator of the FEL. Mirrors that form optical resonators become vulnerable to damage as the power level of the FEL increases. 
     SUMMARY OF THE INVENTION 
     The Ignition Feedback Regenerative Free Electron Laser Amplifier(IFRA FEL) is made from a RF photocathode gun, a RF initial accelerator, a main linear accelerator, a bunch compressor, a bunch decompressor, a regenerative FEL amplifier, and a beam dump. A feed-back loop from the FEL undulator output to the RF photocathode gun provides the photon flux necessary to produce a high average electron beam current. A frequency up converter is used to change the frequency of light from the FEL undulator output to a frequency which maximizes electron current from the FR photocathrode. Another loop in the light beam provides the input power for the regenerative FEL amplifier. A mode filter controls the power levels which are fed back, thus preventing developing positive feedback loops and electron beam instabilities associated with them. A conventional laser is used to start up the operation of the RF photocathode gun and the regenerative FEL amplifier. The main linear accelerator is used for (1) acceleration of electrons before radiation and (2) deceleration of spent electrons after the radiation. The linear accelerator may be built from room temperature normal conducting cavities. The linear accelerator may also be built using superconducting cavities. Superconducting cavities greatly reduce the demand for RF power needed for operation. A deceleration of the electrons before they can be safely sent to a dump is also needed to reduce the radiation hazards. 
     The problem of obtaining a good quality high intense electron beam is solved as follows: In a steady state operation of the FEL, a small fraction of the output light is diverted and converted to the ultraviolet. This light is sent to the photocathode where it creates new electrons. These electrons will radiate in the FEL and a fraction of their radiation can be taken to create new electrons and so on. For the purpose of illustration, assume that ten thousand visible wavelength photons are converted to one thousand ultraviolet wavelength photons and produce a single electron in the photocathode. This single electron radiates one million visible wavelength photons in the FEL, so just 1% of this radiation will supply enough photons to create a new electron. Now, the system is closed and self-supported, but it needs a start up (ignition). For ignition, a conventional laser is used. This laser has to be able to support the operation for a short time until first light from the output radiation reaches the photocathode. The start up laser cannot produce the electron beam intensity from the RF photocathode desired but it only has to start operations as the FEL will produce the greater optical input to produce the good quality high intense electron beam needed from the RF photocathode. In this example, a relatively low conversion efficiency of photons to electrons of 0.1% was used. This is the efficiency that one can currently obtain on rugged metallic photocathodes having a long lifetime. Existing photocathodes built from semiconductor materials have better conversion efficiency, but possess much poorer lifetime especially in a high average electron beam current environment. This drawback makes it impractical to use them for industry application with a continuous run time. 
     By operating the FEL in the regenerative amplifier mode, the problems with cavity mirrors are avoided. Since there is no optical resonator, there are no cavity mirrors. The idea again is to use a fraction of the output radiation from the FEL undulator and feed it back to the beginning of the FEL undulator. This light must appear there simultaneously with the following electron bunch. Then both the electrons and the light propagate through the FEL undulator together. A subsequent interaction results in a fast microbunching of the electron beam (modulation of its longitudinal density) and a coherent radiation of electrons. Tapering the FEL undulator after the microbunching has been achieved compensates the effect of electron energy losses and improves efficiency of the regenerative amplifier. 
     The above configuration will permit optical power of 200 kW and an optical wavelength of 840 nm, what is currently sufficient for LPB, while various applications at various power levels and wavelengths are possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic drawing of the present invention. An ignition laser  10 , which can be any commercially available short pulse laser of predetermined wavelength, for example 840 nm, emits an optical pulse which can be divided into two optical pulses  12  and  14 . Optical pulse  12  is fed via a mirror  16  through a frequency up converter  18  into an injector  20 . In practice, optical pulse  12  may be directed in any number of ways. The present configuration is shown only for illustrative purposes. Frequency up converter  18  changes the wavelength of the original optical pulse  12  to a preselected wavelength, for example 210 nm. Injector  20  provides a predetermined electron pulse when photocathode gun  21  is illuminated with optical pulse  12  at a desired wavelength and preset power level. This electron pulse is a bunch of electrons traveling at high speed close to the speed of light. Injector  20  has two portions, a photocathode gun  21  and an accelerator  22 . Electrons released by photocathode gun  21  are accelerated by accelerator  22  to a high energy, and exit injector  20  as electron bunch  23 . To preserve the small emittance of electron bunch  23 , a high accelerating gradient is needed at photocathode gun  21 . One option for photocathode gun  21  is one that utilizes the emittance compensation technique. This permits injector  20  to provide a high peak electron current with the appropriate emittance and energy spread. Providing a good quality electron bunch is significant when they will be regenerate. 
     An example of suitable design parameters for photocathode gun  21  are: 
     RF frequency 476 MHz 
     Peak electric field at the cathode 200 kV/cm 
     Electron beam peak current 50 A 
     Electron bunch charge 1 nC 
     Normalized electron beam emittance 10 mm mrad 
     Electron beam energy spread 5×10 −3    
     Photocathode material copper 
     Quantum yield (at a wavelength 210 nm) 10 −3    
     For purposes of example only, injector  20  provides the electrons with a 14 MeV energy. Injector  20  has an output electron bunch  23  which is fed into linear accelerator  24  by steering magnets, not shown. The use of steering magnets is well known in the art. 
     When electron bunch  23  passes through linear accelerator  24  energy is added to the electrons. If for example electron bunch  23  is at a 14 MeV energy level coming out of injector  20 , upon exiting linear accelerator  24  the energy has increased to a higher level, say 90 MeV. Thus, when electron bunch  23  exits linear accelerator  24  it is now an enhanced energy or enhanced electron bunch  26 . Both accelerator  22  in injector  20  and linear accelerator  24  operate with RF cavities at a rather low RF frequency, say, 476 MHz for the above example. This low frequency keeps beam instabilities and emittance dilution at a minimum. For the same reason special care should be taken for the suppression of high order modes (HOM) excited in the RF cavities of injector  20  and linear accelerator  24  by electron bunch  23 . As an example of an RF cavity suitable for this purpose, is a single mode RF cavity developed at Lawrence Berkeley National Laboratory. 
     After exiting linear accelerator  24  enhanced electron bunch  26  enters a bunch compressor  28 . Bunch compressor  28  may be built from a set of drift spaces, bending magnets, quadrupoles, sextupoles, and possibly octupoles (not shown). The use of drift spaces, bending magnets, quadrupoles, sextupoles and octupoles is well known in the art. Bunch compressor  28  also directs enhanced electron bunch  26  as desired. For the above example enhanced electron bunch  26  has to have a bunch-length of about 5 mm. Upon exiting bunch compressor  28  enhanced electron bunch  26  becomes a compressed electron bunch  29  with a bunch-length reduced to approximately 1 mm in this example. This bunch compression allows an increase in the electron peak current in this example from 50 A in the linear accelerator  24  to 200 A after bunch compressor  28 . To perform this function, the arc angle shown does not have to be 180° but is a matter of design. 
     Compressed electron bunch  29  is now steered into undulator  30 , which may also be described as an FEL amplifier. Compressed electron bunch  29  may enter undulator  30  simultaneously with an optical pulse  14 , which is steered and focused into undulator  30  by a set of optical elements shown as mirrors  52 . In practice, optical pulse  14  may be directed in any number of ways and the present configuration is shown for only illustrative purposes. Optical pulse  14  passing through undulator  30  together with compressed electron bunch  29  gains energy in this example from 30 microjoule at entry of undulator  30  to 1700 microjoule at the exit of undulator  30 . This energy is actually radiated by electrons of compressed electron bunch  29 . By repeating the above described process with a repetition frequency of, say, 119 MHz (in this illustrative example it means sending electron bunch  23  in every fourth cycle of the RF frequency), a high radiation power, say, 200 kW is achieved. 
     Undulator  30  may be either a linear undulator or circular undulator. To date the preferred embodiment is for compressed electron bunch  29  to first enter a uniform section of undulator  30  and then pass through a tapered section of undulator before exiting. Thus, undulator  30  may have two sections: a uniform section extending in this example from the entrance of undulator  30  to a distance of approximately two meters into undulator  30 . After two meters undulator  30  has a tapered section extending in this example from the end of the linear section to the end of undulator  30 , an additional seven meters. The total length of nine meters is a design option. The interaction of the electrons of compressed electron bunch  29  with optical pulse  14  in the uniform section of undulator  30  results first in the modulation of the electron energy at an optical frequency and second in the electron microbunching (modulation of the compressed electron bunch  29  longitudinal density). In the tapered section of undulator  30 , bunched electrons radiate coherently. The structure of the taper should be designed to compensate for the effect of electron energy losses on the radiation in order to maximize the efficiency of the bunched electron radiation. 
     Upon exiting undulator  30 , compressed electron bunch  29  is directed by a bunch decompressor  38  built from a set of drift spaces, bending magnets, quadrupoles, sextupoles, and possibly octupoles, not shown. Bunch decompressor  38  decompresses compressed electron bunch  29  and upon exiting from bunch decompressor  38  compressed electron bunch  29  becomes a decompressed electron bunch  39  with a bunch-length back to approximately 5 mm in this example. Bunch decompressor  38  also directs decompressed electron bunch  39  to linear accelerator  24  to pass through for a second time. 
     The light emitted by electrons of compressed electron bunch  29  while in undulator  30  becomes an optical beam  32 . Optical beam  32  passes through a dividing means  35  which passes most of optical beam  32  but does direct a small portion, say 3%, as optical beam  36 , back into injector  20  to initiate another cycle. In practice, optical beam  36  may be created and directed in any number of ways. Dividing means  34  may be a pickoff mirror, beam splitter, or any device that achieves the desired function. Optical beam  36  passes through frequency up converter  18  that transforms the wavelength of optical beam  36  to the optimal wavelength for photocathode gun  21 . Optical beam  36  has a significantly higher power level than optical pulse  12 . It was noted above that copper was a preferred choice for the photocathode material. While copper does not produce electrons as readily as many state of the art photocathodes because of its high work function, copper is a far better choice at high power levels. Optical beam  36  grows as the output beam  32  of the FEL grows. Thus, more electrons are emitted as optical beam  36  increases in power. Copper or a similar material will last longer for continuous operation than many materials. As copper erodes there is still copper being exposed. Thus the present invention can increase output power not just because of increasing optical resonance but also because of increased intensity electron bunches from photocathode  21 . 
     Optical beam  32  also passes through a second dividing means  34  which passes most of optical beam  32  but does direct a small portion, say 2%, as an optical beam  48 , back into the beginning of undulator  30  via mirrors  44  and  46  to initiate another cycle of radiation. In practice, optical beam  48  may be picked off and directed in any number of ways. The configurations shown for optical beans  36  and  48  are only for illustrative purposes. Optical beam  48  appears at the beginning of undulator  30  at the same time as another compressed electron bunch  29 . At approximately the same time optical beam  36  reaches photocathode gun  21 . From this moment ignition laser  10  does not send new optical pulses  14  to undulator  30  and new optical pulses  12  to the photocathode gun  21  and can be switched off. The accelerator-FEL system becomes self supported. 
     The light in optical beam  48  may fluctuate from pulse to pulse in optical power, which in turn can cause an uncontrolled power growth and a runaway instability. To prevent a runaway instability, mode filter  50  is inserted in optical beam  48 . It is shown inserted between mirrors  44 , but this is a design choice. Mode filter  50  is a crystal which becomes opaque above a preset optical power level. In effect, mode filter  50  serves as an automatic shutoff valve to prevent self-destruction. An example of such crystals are those used in laser goggles to prevent eye injury. 
     Decompressed electron bunch  39  is fed into linear accelerator  24  by steering magnets, not shown. The time of its arrival there is adjusted so that it enters linear accelerator  24  at the decelerating phase of the RF cycle. When decompressed electron bunch  39  passes through linear accelerator  24 , energy is subtracted from electrons and returned to the electromagnetic field of the RF cavities. If the average energy of electrons in decompressed electron bunch  39  entering linear accelerator  24  is at a 87 MeV energy level, upon exiting linear accelerator  24 , the energy has decreased to a level of about 11 MeV. 
     A return of the energy back to the electromagnetic field of the RF cavities, as it is described above, does reduce the power consumption by the linear accelerator  24 . Basically, the power is needed only to compensate the resistive wall energy losses in the room temperature normal conducting RF cavities. Room temperature normal conducting RF cavities are capable of performing the needed tasks. If RF superconducting cavities are used they will allow further reduction of energy consumption. 
     Upon reaching a low energy, say 11 MeV as above, decompressed electron bunch  39  can be turned to an electron beam dump  40 . This deceleration is critical for a dramatic reduction of the radiation hazards in the electron beam dump  40 . This deceleration brings the energy of electrons below a threshold energy for giant neutron resonance in most materials, thus making them incapable of creating radioactive isotopes. The decelerated electron beam still has significant energy and poses a radiation hazard. To safely terminate it, the electron beam dump  40  may have its own further deceleration stage  42 . Decompression beam  39  has energy reduced in decelerator stage  42  before the electrons are collected by a collector  43  to accept the spent electrons.