Coherent light generators – Free electron laser
Reexamination Certificate
1999-07-27
2001-09-04
Cuchlinski, Jr., William A. (Department: 3662)
Coherent light generators
Free electron laser
C359S341430, C359S335000
Reexamination Certificate
active
06285690
ABSTRACT:
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'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's surface requires the laser beam to travel through our planet'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,
Kim Kwang-Je
Zholents Alexander
Zolotorev Max
Bennett Optical Research, Inc.
Cuchlinski Jr. William A.
Hughes Deandra
Pritchard Kenneth G.
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