High-power free-electron maser operated in a two-mode frequency-multiplying regime

The frequency multiplication effects in high-power free-electron masers (FEM) with Bragg cavities were studied to provide the advance of the oscillators into short-wavelength bands. Theoretical analysis of frequency-multiplying FEMs was carried out within the framework of the averaged coupled-wave approach. Proof-of-principle experiments were performed based on a moderately relativistic induction linac LIU-3000 (JINR). As a result, an FEM multiplier operated with a megawatt power level in the 6-mm and 4-mm wavelength bands at the second and third harmonics, respectively, was realized. The possibility of using two-mode bichromatic FEMs for powering a double-frequency accelerating structure was discussed.

Figure 1

Scheme of the FEM multiplier with a helical wiggler and a two-mirror Bragg resonator.

Figure 2

Dispersion diagram illustrating the rf wave/e-beam interaction in the FEM multiplier. The solid lines correspond to the operating ${\mathrm{TE}}_{1,1}$ and ${\mathrm{TE}}_{2,1}$ modes and the dashed lines show the electron beam dispersion curves.

Figure 3

The results of simulations of the FEM multiplier based on LIU-3000 within the framework of the averaged approach. Dependences on the axial coordinate $z$ of (a) the LF power ($P_{LF}$) and HF powers $P_{HF2,3}$ at the second ($P_{HF2}$) and third ($P_{HF3}$) harmonics, (b) amplitudes of the rf current harmonics ($G_i$) and (c) amplitudes of the forward- and backward-propagating LF waves with respect to the position of the Bragg resonator.

Figure 4

The results of experimental studies of the FEM multiplier based on the LIU-3000 accelerator. Typical oscilloscope traces of the beam current $I_{\mathrm{beam}}$ and the rf pulses in the two-wave generation regime of the fundamental harmonic at 24 GHz ($P_{LF}$) with (a) the second harmonic at 48 GHz ($P_{HF2}$) and (b) the third harmonic at 72 GHz ($P_{HF3}$).

Figure 5

Scheme of the planned experiment on testing a double-frequency accelerating structure by means of the FEM multiplier based on LIU-3000.