Prof. Avraham Gover
Faculty of Electrical Engineering,
Tel Aviv University
Free Electron Lasers (FEL) are the high power lasers which were the prime candidates to produce the intense laser beams intended to destroy attacking ballistic missiles in the "star wars" program. This veteran of the cold war has made a vocational conversion in the post cold war era. Presently it is being used mostly as a very lexible tuneable source of coherent radiation for scientific, technological and medical research which is conducted in big user facilities. This application does not really take full advantage of the high operating power capabilities of the device. Quite natural applications for it lie in the industrial and energy related related areas. At the present time there are more than 50 FEL facilities in operation and development all over the world – primarily in the US, Europe and Japan. The Israeli FEL is one out of three projects in the world which are based on electrostatic accelerators. Only such devices can operate with a continuous wave (cw) or in a quasi-cw (long pulse) mode. All other FELS are based on pulsed accelerators and can produce, therefore, only a pulsed waveform. Electrostatic Accelerator FELs (EA-FEL) are also characterized by potential for high average power, high energy conversion efficiency and high spectral purity
(narrow linewidth). These special features are summarized:
Wavelength range: IR to mm waves
Average power: approx 1 MWatt
Energy Conversion Efficiency: > 50%
Linewidth: < 10(-5)
Wave Form: pSec to continuous wave
THE PRINCIPLE OF EA-FEL OPERATION
Contrary to conventional lasers where the active medium which provides the optical gain is matter, the active medium and source of energy in FEL are accelerated electrons transported in vacuum. The FEL operates in principle as a DC to optical frequency power converter, converting efficiently a big fraction of the large kinetic energy of the electrons into radiated power. Operation in vacuum eliminates the power limitations which in conventional lasers are due to material damage, heating and non-linear optical effects. This explains the high operating power capabilities of the device.
Because the radiation wavelength is determined only by the elector velocity and the wiggler period, any wavelength of operation can be attained by proper choice of FEL parameters, and tuning can be carried out by varying the e-beam energy (velocity). This is a fundamental difference from conventional lasers where the emission frequencies are usually discrete and are determined by the naturally occurring quantum energy level of the active medium material. Thus the FEL may provide radiation energy in wide ranges of wavelengths where no other coherent radiation sources exist, predominantly in the IR mm wave region. The virtual power of the electron beam at the HV terminal is very high.
The beam is collected externally by a multi-stage collector with little heat generation. The scheme called "depressed collector" or "energy retrieval" scheme enables the FEL to operate with very high energy conversion efficiency. In fact, if the entire electron beam is transported along the accelerator without interception then the dominant consumer of electric power in the FEL system is the collector power supply. The power it provides is transferred almost entirely into radiative power. This explains the high total (wall plug) conversion efficiency of EA-FEL as opposed to conventional FEL.
POTENTIAL ENERGY – RELATED APPLICATIONS
The outstanding characteristics of EA-FEL and, on the other hand, its large dimensions and cost, determine the range of applications that can be ascribed to this unique radiation source. The high average power capability of the device makes it naturally fitting for energy related applications. A number of attractive future applications were identified. Most of them require long range development programs and international cooperation.
A. HEATING MAGNETICALLY CONFINED PLASMA IN THERMONUCLEAR FUSION REACTORS
Thermonuclear power reactors are the "environmentally clean" future power reactors, which like "little suns" use hydrogen isotopes as the fuel for power production instead of the radioactive heavy elements used in nuclear reactors. Recently a scientific break-even demonstration took place in the JET (Joint European Tokomak) Labs in England and gave a great impetus to the pursuit of this energy source of the future. Nevertheless, despite this success, a continuous engineering, technological and scientific development effort for tens of years will be required to achieve commercial realization of this concept. To ignite the plasma in the fusion reactor, it has to be heated to high temperatures which only exist in the sun. One of the schemes suggested for heating the plasma and controlling its density profile, requires radiation sources with total power of 20 MWatt which operate quasi-CW at wavelength l = 1-2 mm and are tuneable.
A battery of a few Free Electron Masers (FEMS) seems to be an excellent candidate to perform this function. A development project of a 1 MWatt FEM for fusion research application is under way in FOM Institute for Plasma Physics in Nieuweigen, The Netherlands. The Dutch project relies on wide international collaboration, the Israeli FEL group participates in making contributions to its conceptual design.
B. ENERGY TRANSMISSION IN FREE SPACE
The high average power and high energy conversion efficiency of FEL gives hope to an old dream of energy transmission by means of "radiation beams" propagating freely in the open atmosphere or in outer space. Options of microwave and optical energy beaming from ground to satellite and vice versa were considered for various schemes like solar energy utilization, satellite powering and energy redistribution on land. More easily applicable schemes may be short range wireless energy transmission links to inaccessible energy consumers beyond natural obstacles, of catastrophe zones (eg earthquake or pollution stricken areas). State of the art of large transmitting and receiving mirrors permits transmission of collimated laser beams to distances of kms to hundreds of thousands kms depending on the wavelength. For atmospheric transmission the tunability of FEL is an advantage, making it possible to choose an atmospheric transmission window in the frequency domain.
C. ISOTOPE SEPARATION
The high spectral purity and tunability of EA-FELs are their most important property in this application which relies on the small difference in the IR absorption lines of molecules composed of different isotopes. This, combined with the high average power and energy efficiency of EA-FEL makes it a well fitted source for molecular laser isotope separation schemes for efficient production of nuclear fuels.
D. STUDY, DETECTION AND REMOVAL OF AIR POLLUTANTS
In the combustion process of fossil fuel, various polluting by products
(as SOx and NOx gaseous compounds) are released into the air. The molecules of these gases have characteristic vibrational resonances which determine their IR absorption spectrum. As in the previous application, the high spectral purity and tunability of EA-FEL make it a good tool for investigating and monitoring the combustion products.
THE ISRAELI FEL PROJECT
The Israeli FEL project is based on a 2-6 MeV EN-Tandem van de Graff accelerator which was used before as an ion accelerator for nuclear physics experiments. The Weizmann (Dannie Heinemann Accelerator Lab) is permitting the FEL collaboration to convert the machine into a high current electron accelerator and demonstrate its use in an FEL experiment. The accelerator development work and the subsequent FEL experiment are funded by the Israeli Ministry of Energy and Infrastructure, the Ministry Academy of Science, US-Israel BSF, and the Meyer Foundation. This scheme uses straight geometry for the electron optics and keeps the electron gun and collector power supplies out of the accelerator. It is therefore the only scheme that can be conveniently developed for future high average power applications.
Up to now, the FEL group scientists succeeded to transport through the bare accelerator system 200mAmp at high transport efficiency and 0.5Amp at low transport efficiency. The group is engaged this year in major internal modifications of the accelerator, installing inside the HV terminal magnetic lenses and fiber-optic linked control electronics which will enable focusing and steering of the electron beam through the acceleration tube, wiggler and deceleration tubes without loss of electrons on the way. Beyond the technical demonstration of lasing in a straight geometry Tandem configuration the FEL team has a scientific interest mostly in the study of the limits of coherence (monochronaticity) of the FEL light and the dynamics of coherence establishment. The high coherence properties of FEL is not only of practical importance but also gives rise to scientific research problems of quite fundamental nature in laser physics.
The expected long electron beam pulse duration of the Israeli studying and improving the EA-FEL coherence properties.
The Electrostatic Accelerator FEL project is one of the major fundamental research and technology projects in Israel concerning future energy-related technologies and advanced concepts. In a spirit of international scientific cooperation, the Israeli research team strives to make its contribution to the world-wide development effort of the science and technology of FELS. The unique experimental set-up of the group, based on a converted EN-tandem van de Graff, will help to advance the concept of high coherence property.