Free electron laser pumped by a powerful traveling electromagneticwave

A three-wave free-electron laser (FEL) was operated with a powerful 8.4-GHz electromagnetic pump wave replacing the usual magnetostatic wiggler. The presence of a uniform axial magnetic field B₀ produced cyclotron-harmonic idler waves. Peaks in the emission spectrum corresponding to cyclotron harmonics were observed covering a frequency range from 16.5 to 130 GHz. The frequency spectrum of this novel FEL mechanism was tuned continuously by the variation of B₀     

Starting energy and current for a large diameter finite lengthbackward wave oscillator operated at the fundamental mode

We study the starting conditions for a large diameter (diameter/wavelength=4.8) finite length backward wave oscillator designed for 24-GHz operation at the fundamental TM₀₁ mode. This geometry is very promising for high power handling capability. We analyze two separate threshold conditions. First, finite length effects give rise to a threshold in electron beam energy below which oscillations cannot be sustained at any beam current. The second is the more familiar current threshold known as a start current. It is also found that the growth rate for the fundamental mode can be much larger than those of other higher order modes thus leading to coherent operation of large diameter sources free from mode competition     

Electromagnetic properties of open and closed overmoded slow-waveresonators for interaction with relativistic electron beams

Specific slow wave structures are needed in order to produce coherent Cherenkov radiation in overmoded relativistic generators. The electromagnetic characteristics of such slow wave, resonant, finite length structures commonly used in relativistic backward wave oscillators have been studied both experimentally and theoretically. In experiments, perturbation techniques were used to study both the fundamental and higher order symmetric transverse magnetic (TM) modes. Finite length effects lead to end reflections and quantization of the wave number. The effects of end reflections in open slow wave structures were found from the spectral broadening of the discrete resonances of the different axial modes. The measured axial and radial field distributions are in excellent agreement with the results of a 2-D code developed for the calculation of the fields in these structures     

A high-power millimeter-wave sheet beam free-electron laseramplifier

The results of experiments with a short period (9.6 mm) wiggler sheet electron beam (1.0 mm×2.0 cm) millimeter-wave free electron laser (FEL) amplifier are presented. This FEL amplifier utilized a strong wiggler field for sheet beam confinement in the narrow beam dimension and an offset-pole side-focusing technique for the wide dimension beam confinement. The beam analysis herein includes finite emittance and space-charge effects. High-current beam propagation was achieved as a result of extensive analytical studies and experimental optimization. A design optimization resulted in a low sensitivity to structure errors and beam velocity spread, as well as a low required beam energy. A maximum gain of 24 dB was achieved with a 1-kW injected signal power at 86 GHz, a 450-kV beam voltage, 17-A beam current, 3.8-kG wiggler magnetic field, and a 74-period wiggler length. The maximum gain with a one-watt injected millimeter-wave power was observed to be over 30 dB. The lower gain at higher injection power level indicates that the device has approached saturation. The device was studied over a broad range or experimental parameters. The experimental results have a good agreement with expectations from a one-dimensional simulation code. The successful operation of this device has proven the feasibility of the original concept and demonstrated the advantages of the sheet beam FEL amplifier. The results of the studies will provide guidelines for the future development of sheet beam FELs and/or other kinds of sheet beam devices     

Design of high-average-power near-millimeter free electron laseroscillators using short period wigglers and sheet electron beams

The design and feasibility of a 1-MW continuous-wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration includes a short-period (I_w~ 1 cm) planar wiggler, a sheet electron beam, a 0.5-1.0-MV thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial DC power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low (⩽10-100 W/cm²) at 1/MW output power, while thermal heat transfer studies conservatively indicate that wall cooling up to 1500 W/cm² should be possible. Experiments have convincingly verified theory and simulations which predict that negligible body currents will be achievable with low-emittance low-space-charge sheet beams. High-voltage sheet beam gun design studies indicate that the required beam quality can be achieved with CW compatible devices. The spent beam energy distribution is consistent with highly efficient spent beam energy recovery, and the proposed resonator cavity should provide mode discrimination and beam/RF separation capability     

Design of a high-efficiency low-voltage axially modulatedcusp-injected second-harmonic X-band gyrotron amplifier

We present the theoretical design of a second-harmonic small-orbit gyrotron amplifier which utilizes the interactions between a 35-kV 4-A beam and a TE₀₁₁ cavity to produce over 70 kW of amplified power at 9.9 GHz in a 1.83-kG magnetic field. One of the novel features of this device is that the electron gun produces an axially streaming annular beam which is velocity modulated by a short TM_0n0 input cavity. Perpendicular energy is imparted to the beam via a nonadiabatic magnetic transition at the end of a 13-cm drift region. An electronic efficiency of 53% is predicted with a large signal gain near 20 dB by a single particle code which takes into account nonideal effects associated with finite beam thickness and finite magnetic field transition widths