H. Kleinman

Efficiency enhancement of free electron Maser oscillator by mode selection with a prebunched electron beam

We present a method for enhancing the efficiency of a Free Electron Laser Maser oscillator by locking it to a preferred resonator mode. This is done by prebunching of the e beam before injection into the wiggler. In a free running oscillator, the longitudinal mode that dominates the mode competition process during the oscillation buildup period is usually the highest gain mode. However, this mode does not extract the highest energy from the e beam. Lower eigenfrequency modes would provide a higher efficiency if they could dominate the mode competition process. By prebunching the e beam at a frequency near any one of the longitudinal eigenfrequencies of the resonator (having a gain>1), we can make that mode dominant at saturation. The eigenfrequency for which the maximum efficiency is obtained is the lowest eigenfrequency of the resonator for which the net small signal gain is greater than 1. Employing an experimental setup of a prebunched beam Free Electron Maser, we demonstrated efficiency enhancement of...

Free electron maser experiment with a prebunched beam

An experimental project aimed at demonstrating Free Electron Maser (FEM) operation with prebunching is under way at Tel-Aviv university. The FEM utilizes a 1.0 A prebunched electron beam obtained from a microwave tube. The electron beam is bunched at 4.87 GHz and is subsequently accelerated to 70 keV. The bunched beam is injected into a planar wiggler (B, = 300 G, A, = 4.4 cm) constructed in a Halbach configuration with 17 periods. The wiggler utilizes a new scheme for horizontal focusing based on the use of two long permanent magnets at the sides of the wiggler. We plan to study FEM gain enhancement and radiation features due to the prebunched (superradiant) mode of operation. In an oscillator configuration the experiment should be enable study of seed injection by prebunching. Simulation of FEL operation shows an expected gain of approximately 100% and an rf output power of 5 kW. In this paper we review the design of the main parts of the experimental set-up, and present recent analytical, numerical, and experimental results.

A proposal for a tandem accelerator FEL experiment

Abstract A proposal for an FEL experiment, based on a tandem electrostatic accelerator, is presented. The proposed FEL is designed to operate at harmonic frequencies in a quasi-cw mode. An internal wiggler scheme is planned to be used, based upon a strong planar wiggler ( a w > 1), positioned in the high voltage terminal of a HVEC model EN tandem accelerator. The electron beam enters the wiggler at the natural injection angle ( a w / γ ). This approach provides a relatively simple and compact FEL system that will be used as a research tool for studying harmonic operation of FELs and investigating post-saturation laser dynamics and efficiency enhancement. The exceptional electron beam quality and long pulse characteristics of electrostatic accelerators give a special advantage to such an experimental scheme as a means for exploring the ultimate performance parameters of FELs.

Radiation measurements in the new tandem accelerator FEL

The Israeli tandem electrostatic accelerator FEL (EA-FEL), which is based on an electrostatic Van der Graaff accelerator was relocated to Ariel 3 years ago, and has now returned to operation under a new configuration. In the present FEL, the millimeter-wave radiation generated in the resonator is separated from the electron beam by means of a perforated Talbot effect reflector. A quasi-optic delivery system transmits the out-coupled power through a window in the pressurized gas accelerator tank into the measurement room (in the previous configuration, radiation was transmitted through the accelerator tubes with 40 dB attenuation). This makes it possible to transmit useful power out of the accelerator and into the user laboratories. After re-configuring the FEL electron gun and the e-beam transport optics and installing a two stage depressed collector, the e-beam current was raised to 2 A. This recently enabled us to measure both spontaneous and stimulated emissions of radiation in the newly configured FEL for the first time. The radiation at the W-band was measured and characterized. The results match the predictions of our earlier theoretical modeling and calculations.

Recent measurements of electron trapping and phase area displacement of nonrelativistic electrons employing an electromagnetic wiggler

Abstract We report the distinct observation of electron trapping and phase area displacement energy transfer mechanisms in a stimulated Compton scattering free electron laser scheme. The synchronous energy exchange occurred between nonrelativistic electrons and the ponderomotive (beat) force of two counter-propagating intense pulsed CO 2 laser beams. The direction of the energy exchange was dependent on the position where synchronism occurred in the interaction region, as predicted by theory for electron trapping and phase area displacement.

Free electron maser oscillations near waveguide cutoff

Abstract In waveguide-based FEMs there are two possible frequency radiation bands corresponding to the two intersections of the beam line with the waveguide dispersion curve. The low-frequency intersection point occurring near the waveguide cutoff frequency was studied by us experimentally. Special modifications of the existing TAU FEM facility were made in order to investigate FEM radiation near cutoff. Some rectangular waveguide components, used for the radiation coupling from the resonator to the detection system, were replaced by components based on a double-ridged waveguide having a lower cutoff frequency and a wider frequency band. FEM operation in the vicinity of the waveguide cutoff frequency was studied in free-running oscillator configuration using a unique experimental setup enabling time-dependent spectral measurements within a single radiation pulse. The observed FEM radiation was of good spectral purity without any significant mode competition.

Performance improvement of FEMs by prebunching of the electron beam

Abstract FEM performance enhancement achieved by use of r.f. prebunching of the e-beam in the FEM developed at TAU was investigated theoretically and experimentally. For FEM operation as an oscillator, use of e-beam prebunching enables stable, coherent, high output power throughout the r.f. output pulse at any selected oscillator eigenfrequency for which the net gain is above unity. Prebunching enables faster r.f. output power buildup. An eigenfrequency of maximum efficiency and power output can be selected by e-beam prebunching at or near that eigenfrequency. FEM efficiency is thus, considerably improved. By contrast, FEM operation without prebunching leads to saturation in the highest gain mode, giving a lower efficiency. Frequency “hopping” of the FEM r.f. output between various eigenfrequencies is also attainable via prebunching. FEM operation as a high gain amplifier between the premodulator input and the FEM output is reported for our FEM operating with an e-beam current of only 0.6 A. High FEM gain, broad bandwidth, high power operation possibilities at millimeter waves are also described.

Development of a tandem electrostatic accelerator quasi-cw FEL

Abstract The EN tandem electrostatic accelerator at the Weizmann Institute of Science has been converted into an electron accelerator with beam power recovery. We report on the design and performance of the accelerator as well as on a new approach to stable, long-pulse operation of this class of machines. The long-pulse model of operation offers interesting possibilities for the operation of free electron lasers, in particular the study of high-coherence, single-mode operation.

A novel free-electron maser as a high power microwave source of sophisticated signals

A novel Free Electron Maser (FEM) developed at Tel-Aviv University and operating in C-band is described. The FEM, utilizing prebunching of its e-beam at the operating frequency, can operate as an oscillator or as an amplifier. In first experiments the FEM was operated as an oscillator without and with e-beam prebunching. Without prebunching the signal builds up out of noise in resonator modes for which the gain is highest. Mode competition and single mode evolution were observed. With prebunching the FEM oscillator could be locked to the prebunching frequency which could be at or near every eigenfrequency of the resonator for which the round trip net gain is larger than the loss. Use of the prebuncher as a signal modulator to obtain AM, FM and PM modulation is described. Frequency-agile operation can be achieved by switching of the prebuncher locking frequency. An FEM utilizing a broadband prebuncher enables smart signal modulation and frequency agility, together with a wideband interaction circuit should allow development of high power, wide band sophisticated signal FEMs.

Laser‐induced electron source in a vacuum diode

Experiments were conducted in which a high‐power CO2 TEA laser interacted with metallic cathode in a high‐vacuum (10−8 Torr) diode. For power densities lower than 5×107 W/cm2, no current was detected. For power densities in the range of 5×107–5×108 W/cm2, the Cu cathode emitted a maximum current of 40 mA. At a higher power density level, a circuit‐limited current of 8 A was detected. The jump of a few orders of magnitude in the current is attributed to breakdown of the diode gap. The experimental results are similar to those of a triggered vacuum gap, and a thorough comparison is presented in this paper. The influence of the pressure in the vacuum chamber on the current magnitude shows the active role that adsorbed gas molecules have in the initial breakdown. When the cathode material was changed from metal to metal oxide, much lower laser power densities were required to reach the breakdown current region.