Nanosecond Pulses in a THz Gyrotron Oscillator Operating in a Mode-Locked Self-Consistent $Q$-Switch Regime

An experimental study of a nanosecond pulsed regime in a THz gyrotron oscillator operating in a self-consistent $Q$-switch regime has been carried out. The gyrotron is operated in the ${\mathrm{TE}}_{7,2}$ transverse mode radiating at a frequency of 260.5 GHz. The 5 W nanosecond pulses are obtained in a self-consistent $Q$-switch regime in which the cavity diffraction quality factor dynamically varies by nearly 2 orders of magnitude on a subnanosecond time scale via the nonlinear interaction of different mode-locked frequency-equidistant sidebands. The experimental results are in good agreement with numerical simulations performed with the TWANG code based on a slow time scale formulation of the self-consistent time-dependent nonlinear wave particle interaction equations.

Figure 1

Uncoupled dispersion relation in the $\omega -k_{\parallel }$ space for the operating point exhibiting the self-consistent $Q$-switch regime. The beam line represents the Doppler shifted electron beam dispersion relation $\omega ={\Omega }_c+k_{\parallel }{v}_{\parallel }$, with ${\Omega }_c$, $k_{\parallel }$, $v_{\parallel }$ being the relativistic cyclotron angular frequency, the parallel wave vector, and parallel velocity, respectively. The beam energy and pitch angle are 15.5 keV and 1.9, respectively. The cavity magnetic field is $B_0=9.59\mathrm{T}$ and the beam current is $I_b=75\mathrm{mA}$. The filled dots on the rf-wave dispersion relation (${\mathrm{TE}}_{7,2}$ transverse mode) represent the discrete frequencies of longitudinal modes considering a finite length cavity, $L=25\mathrm{mm}$, with a parallel wave number $k_{\parallel }=q\pi /L$, with $q$ being the longitudinal index. The large red dot is the SQS operating point.

Figure 2

 Schematic diagram of the experimental setup used for analyzing the THz radiation spectrum. The fast Schottky diode signal is acquired with a fast oscilloscope ($10\mathrm{Gs}/\mathrm{s}$, 8 bit A/D converter) ensuring a time resolution of 100 ps. A low phase noise synthesized local oscillator (LO) drives the harmonic mixer ($\mathrm{n}=24$, $f_{\mathrm{LO}}=10.8\mathrm{GHz}$) of the heterodyne receiver. The IF signal of the heterodyne receiver is acquired at the same acquisition rate. This allows us to perform fast spectroscopy (FFT) with a time resolution of typically 100 ns. Using the beam splitter setup, an absolute measurement of the average gyrotron rf power is made with a Scientech calorimeter (model 360 401).

Figure 3

Experimental time traces of the SQS regime. (a) In red, fast Schottky diode (dc 20 GHz bandwidth) and, in blue, the IF of the heterodyne receiver. (b) Spectrogram with a time resolution of 100 ns calculated from the IF signal shown in (a). The vertical line indicates the time, $t=65\mu \mathrm{s}$, at which the 30 ns time window shown in (a) has been taken. (c) Frequency spectrum (FFT on a 100 ns time window) of the IF signal at $t=65\mu \mathrm{s}$. DNP 369 identifies the shot number in the acquisition data base.

Figure 4

Simulation results of the SQS regime calculated with TWANG. The system parameters are the same as for the experiment, and a cavity wall conductivity of ${\sigma }_{\mathrm{dc}}=2.9\times {10}^7[\mathrm{S}/\mathrm{m}]$ has been considered. From top to bottom are the time traces of (a) cavity stored rf energy, (b) electronic efficiency, (c) radiated rf power measured at the cavity exit, (d) self-consistent diffraction quality factor. (e) rf-field profile at the times $t_1$ and $t_2$ for which the maximum and minimum radiated rf power is predicted. (f) Radiation frequency power spectrum relative to a reference frequency of 260.53 GHz. This spectrum is computed at the exit of the interaction space.

Figure 5

For the same electron beam parameters as considered before, but only varying the cavity magnetic field the experiment-theory comparison of the sideband frequency separation is shown. The experimental points are indicated in blue. The theoretical points have been calculated using the TWANG code.