Nanosecond rf-Power Switch for Gyrotron-Driven Millimeter-Wave Accelerators

The development of alternative mm-wave high-gradient, >200 MV/m, accelerating structures offers a promising path to reduce the cost and footprint of future TeV-scale linear colliders, as well as linacs for industrial, medical, and security applications. The major factor limiting accelerating gradient is vacuum rf breakdown. The probability of such breakdowns increases with pulse length. For reliable operation, millimeter-wave structures require nanoseconds-long pulses at the megawatt level. This power is available from gyrotrons, which have a minimum pulse length on the order of microseconds. To create shorter pulses and to reliably detect rf breakdowns, we developed the following devices: a laser-based rf switch capable of selecting 10 ns long pulses out of the microseconds long gyrotron pulses, thus enabling the use of the gyrotrons as power sources for mm-wave high-gradient linacs, and a shot-to-shot sub-THz spectrometer with high-frequency resolution, capable of detecting pulse shortening due to rf breakdowns. We will describe the principle of operation of these devices and their achieved parameters. We also report on the experimental demonstration of these devices with the high-power gyrotron at the Massachusetts Institute of Technology. In the experiments, we demonstrate nanosecond rf power modulation, shot-to-shot measurements of the pulse spectra, and detection of rf breakdowns.

Figure 1

Simplified geometry (left) and Brewster angle incidence (right) configurations of gated window. COMSOL multiphysics simulation results show the transmission (top right) of 100 GHz through the intrinsic semiconductor and reflection (bottom right) from the interface in the photoconductive state.

Figure 2

Power reflection coefficient for GaAs (left) and Si (right) as a function of frequency for various carrier concentrations from intrinsic state (10¹² m⁻³ in GaAs and 10¹⁶ m⁻³ in Si) to a highly photoconductive state 10²⁵ m⁻³.

Figure 3

Power reflection coefficient evaluated at 110 GHz for GaAs (left) and Si (right) as functions of incidence angle for various carrier concentrations from intrinsic state (10¹² m⁻³ in GaAs and 10¹⁶ m⁻³ in Si) to a highly photoconductive state 10²⁵ m⁻³.

Figure 4

Power reflection coefficient evaluated at 110 GHz for GaAs and Si as functions of carrier concentration (left) and laser illumination intensity (right) in the Brewster’s angle configuration. Note the apparent switch in responsivity from left to right.

Figure 5

Schematics of the laser-driven rf switch. 1 – Input THz window, 2 – Polarizer, 3 – THz lens no. 1, 4 – Wafer, 5 – THz lens no. 2, 6 – THz lens no. 3, 7 – Mirror, 8 – THz lens no. 4, 9 – Output THz window, 10 – THz load, 11 – Optical lens, 12 – Laser dump, 13 – Beam expander, 14 – Alignment mirror no. 1, 15 – Alignment mirror no. 2, 16 – Alignment mirror no. 3, 17 – 532 nm laser. Laser light is shown with green lines, THz signal with red lines.

Figure 6

Calculated spectra of the single-frequency pulses with different pulse length.

Figure 7

Detector part of the spectrometer. 1 – 30-dB horn antenna, 2 – Band-pass filter, 3 – Directional coupler, 4 – IQ-mixer, 5 – Local oscillator, 6 – Schottky amplitude detector.

Figure 8

The schematics (left) and the prototype (right) of the spectrometer data acquisition electronic board.

Figure 9

Test-synthesized 200-MHz signal digitization (left) and spectroscopy (right).

Figure 10

Oscillograms of the reflected THz signals after laser illumination of GaAs wafer (top) and a silicon wafer (bottom) measured with the Schottky amplitude detector. Note the different time scale on the plot demonstrating the significantly slower recombination time of Si relative to GaAs.

Figure 11

Transmitted signal measured by Schottky detector as a function of laser pulse energy using the maximum spot size of 4.7 cm (left) and as a function wafer spot diameter (right).

Figure 12

Laser beam spot profile measured at the wafer (left – as is, right – with a detector card).

Figure 13

The experimental set up of the high-power tests of the laser-gated rf switch at MIT PSFC lab.

Figure 14

Measured peak amplitude at the output of the switch as a function of gyrotron power. The vertical lines correspond to accelerating gradients of 100 and 200 MV/m in the SLAC mm-wave accelerating structure.

Figure 15

Scope traces of envelopes for nonbreakdown (top) and breakdown (bottom) shots. The plot represents the following traces: blue – pulse envelope, yellow – I signal, green – Q signal, pink – FFT of I signal. Time scale of the traces is 500 ns, and voltage scale is 50 mV for I and Q signals, and 10 mV for the envelope.

Figure 16

Measured THz signal parameters. Top: single pulse spectra during regular (blue) and rf breakdown (red) events. Bottom: Pulse length (blue) and frequency spectrum FWHM width (red) of 500 consecutive rf pulses. Drops in pulse lengths and spikes in spectra represent breakdown events.