High-power electrostatic free-electron maser as a future source for fusion plasma heating: Experiments in the short-pulse regime

Abstract

A high-power, frequency-tunable electrostatic free-electron maser, being developed at the FOM Institute for Plasma Physics "Rijnhuizen," shows lasing at various frequencies. An output power of 730 kW at 206 GHz is generated by a 7.2-A, 1.77-MeV electron beam, and 380 kW at 165 GHz is generated by a 7.4-A, 1.65-MeV electron beam. In the present experimental setup, without recovery of the spent electron beam power, the pulse length is limited to 12 micros. Nevertheless, the main issues, such as the possibility of high-power, single-mode operation and frequency tuning, have been confirmed. The experimental results and the dynamics of the laser process are well in accordance with simulations.

Mode dynamics in a free electron maser with broadband frequency-dispersive feedback

“…7 This type of feedback is used in the Fusion-FEM experiment. 8 In this work, we study problems of mode interaction and of frequency-tuning within the framework of a more general approach, which can describe the dynamics of a FEM-oscillator with a feedback system having any frequency-dispersion characteristic.…”

Design and beam transport simulations of a multistage collector for the Israeli EA-FEM

“…Variations in the order of 5% will be allowed on the potential of the electrode #1 to #3 (Table 2) while operating the FEM. Unlike other collectors employed in EA-FEL's [1,4] the presented one offers the flexibility of variable electrode positioning and voltage optimization for a final improvement on the collection efficiency accounting for real FEL operation conditions. This feature enables the design to be less sensitive for possible erroneous assumptions made on the energy spread and the beam phase-space at collector entrance.…”

“…It has been only recently that FEL devices based on superconducting energy recovery RF-linacs have successfully generated (quasi) cw, kilowatt laser beams [2]. There are currently experiments underway to demonstrate the ability of EA-Free Electron Masers (FEM) in providing high power mm-waves from kilowatts [3] to megawatts at pulse lengths up to 100 ms [4]. In Electrostatic Accelerators, sustaining ampere level electron pulses over tens of milliseconds with sufficiently stabilized terminal accelerating voltage requires highly efficient transport and collection (recirculation) of the beam charge with recovery rates R over 99%.…”

Long-Pulse Operation at Constant Output Power and Single-Frequency Mode of a High-Power Electrostatic Free-Electron Maser with Depressed Collector

“…Present generation gyrotrons, generating mm-wave output at the MW level at pulse lengths of seconds at frequencies up to 140 GHz, largely meet the first requirement, while high-power step-tunable gyrotrons (steps of several GHz) are in development [2,3]. A promising alternative is provided by the electrostatic Free-Electron Maser (FEM), featuring fast and continuous tunability as well as the possibility to reach higher frequencies (hundreds of GHz) as its main advantages [4]. At FOM such a device, the FOM Fusion FEM (FFF), has been developed with the aim to demonstrate single-mode operation and tunability at high-power, long pulse operation.…”

“…In our previous experiments [4] the electron beam recovery system was not yet installed. Consequently, the electron beam energy dropped sharply during the pulse, which had strong effects on the interaction mechanism; the amplification band shifted across the cavity resonance band and the mm-wave output power varied strongly [11].…”