I.M. Yakover

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.

Resonator design and characterization for the Israeli tandem electrostatic FEL project

Abstract The design and measurements of a resonator operating near 100 GHz and intended for use in a tandem FEL are presented. The designed resonator employs two parallel curved plates as a waveguide. In FEL operation the TE 01 mode is excited. The resonator employs two wave splitters as reflectors. The wave splitters are segments of an overmoded rectangular waveguide which is connected at one end to the waveguide as described above, and is shorted at the other end by a metal plate with an aperture in the center for e-beam passage. Gain calculations were made in the low gain regime. At the operating frequency the curvature of the plates and the gap size were chosen so as to maximize the gain. A multimode analysis of the wave splitter was made. Calculations show, that the optimal splitter width and length allow achievement of very low diffraction losses at the aperture (∼ 2%). This means that the aperture can be made sufficiently large to allow efficient beam entrance into the resonator without degrading its Q -factor. A resonator prototype was constructed and its performance was evaluated experimentally.

Efficient electrostatic-accelerator free-electron masers for atmospheric power beaming

The electrostatic-accelerator free-electron laser (EA-FEL) operating at mm wavelength is considered as a source for energy transfer through the atmosphere to a high altitude platform. The high average power and high efficiency attainable from appropriately designed EA-FEL make it a suitable candidate as an efficient source of mm-waves for power beaming from a ground station. Various aspects of the FEL as a high power oscillator (operating voltage, e-beam current, gain and efficiency) are reviewed; design tradeoffs are described. The study includes consideration of typical requirements of power beaming to a high altitude platform such as atmospheric absorption versus frequency and transmitting and receiving antenna requirements. A conceptual design of a compact, moderate voltage (0.5-3 MeV), high current (1-10 Amp) EA-FEM operating in the mm-wavelength band is presented as an efficient power source for space beaming. The FEM design parameters are presented based on analytical and numerical models. Expected performance parameters of an FEL (gain, energy conversion efficiency, average power) are discussed as related to the proposed application.

Study of waveguide resonators for FEL operating at submillimeter wavelengths

Abstract Theoretical investigations of waveguides for FEL operating at submillimeter wavelengths were made. Several types of overmoded waveguides, i.e. rectangular, circular, parallel curved plates waveguide and metal-dielectric waveguide, were studied in order to achieve the best gain/loss ratio in FEL operation. Gain calculations were made in the low gain regime. Waveguide ohmic losses were calculated taking into account the anomalous skin effect and the influence of surface roughness. Numerical calculations made for the parameters of the Israeli tandem electrostatic FEL show that a parallel curved plates waveguide and a metal-dielectric waveguide provide good gain/loss ratios up to frequencies of the order of 1000 GHz (while conventional rectangular and circular waveguides do not).

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.

Spontaneous emission in waveguide free-electron masers near waveguide cutoff

In this work spontaneous emission is investigated in a waveguide free-electron maser, taking into account previously untreated interaction effects in the vicinity of the waveguide cutoff frequency. Our study is based on the exact waveguide excitation equations, formulated in the frequency domain for a single electron moving in a planar magnetostatic wiggler. An analytical solution of the amplitude of the excited waveguide mode in the frequency domain was obtained using the Green function technique and allows us to calculate the spectral density of the radiated power and the time-dependent radiated field with good accuracy using a numerical inverse Fourier transform. The obtained solution shows that for TE-modes the spectral density of the radiated energy tends to infinity at the cutoff frequency of a lossless waveguide. The character of this singularity is, however, such that the total radiated energy is finite. The radiated electromagnetic field in the time domain has the form of very long (of the order of tens of characteristic times on the scale of Lwc, where Lw is the wiggler length and c is the speed of light) pulse, lagging behind the electron, at the carrier of cutoff frequency, in addition to two finite wave packets, corresponding to the two synchronism frequencies. The results of a numerical calculation of the radiated energy spectral density and of the radiated electromagnetic field in the time domain are presented.

Visualization and simulation of electron beam transport along a FEL planar wiggler

Abstract We have developed and employed a special apparatus for viewing and investigating the motion and behavior of an e-beam inside a planar wiggler. The total current I(z), the current density J(x, y, z), the beam position and cross-section shape were investigated along an evacuated waveguide cavity located on the axis of the wiggler of the TAU 70 KeV FEM. The beam cross-section image could be viewed on the screen through a window at the end of the evacuated waveguide using a CCD camera equipped with a zoom lens. An exact 3D simulation code (PTIMF) developed in our group for simulation of electron trajectories inside and along the wiggler, was used to calculate trajectories of electrons in the wiggler region under conditions similar to those prevailing in the experiment. The correlation of experimental results with those predicted from simulations was good. It is presented and discussed in this paper.

Enhanced super-radiant emission of FEM near waveguide-cutoff and near zero-slippage conditions

We report on super-radiance obtained from the TAU FEM just above waveguide cutoff and near grazing intersection. Grazing intersection (or ‘‘Zero Slippage’’) is defined as the point at which the two synchronous frequencies merge to one frequency. In this case, the radiated power frequency can be tuned over a very wide band by change of the premodulation frequency. Near the lower synchronous frequency, the super-radiance power is much greater and the spectral width is much narrower than those at the higher synchronous frequency. The super-radiance emission near cutoff (lower synchronous frequency) and near to the upper synchronous frequency was measured and compared to those predicted by an analytical model for a wide range of frequencies. r 2002 Published by Elsevier Science B.V.

Theoretical investigation of a free-electron maser operating with a TEM transmission line

The possibility of using the transverse electric magnetic (TEM) transmission line in free-electron masers (FEM) is discussed. It is shown that at the centimeter and long-millimeter wavelengths such transmission lines allow one to combine the advantages of an open cavity and a waveguide-based resonator. A particular case of an FEM-based on the use of a shielded two-wire transmission line is investigated theoretically. A mathematical approach that allows one to calculate transmission-line parameters important to the FEM application is developed. It is based on the use of the integral equation technique and on a new representation of the Green function of the internal region of a circle, which was obtained in this paper. Numerical analysis of effective mode area, wave impedance, and attenuation constant was made for the odd TEM mode, which is excited in FEM operation. The FEM under research at Tel Aviv University was considered as an example. The frequency dependence of gain for an FEM operating in the linear regime was calculated. That the obtained gain value is much higher than the ohmic losses in the transmission line shows the possibility of using the TEM transmission line in this FEM.

Space Charge Effects in a Prebunched Beam Free Electron Maser (FEM)

Publisher Summary This chapter describes space charge effects in a prebunched beam free-electron maser (FEM). An electron beam prebunched at a frequency near to the synchronous free-electron laser frequency and passing through a magnetic undulator emits coherent synchrotron undulator radiation at the bunching frequency. In the collective regime, the spectral properties of the emitted super radiant power depend on the electron beam current density. For a low-density electron beam, the monochromatic super-radiant emission power is maximal at a bunching frequency equal to the electron-beam synchronism frequency, independent of the beam current. However, for a dense electron beam, the space charge effects become dominant and synchronism takes place with either the slow or the fast space-charge (Langmuire) wave, which propagate on the electron beam with different phase velocities. Consequently, radiated power maxima can occur at two well-separated frequencies.