# Gyrotrons as High-Frequency Drivers for Undulators and High-Gradient Accelerators
## 4 Gyrotron-Driven Ultra-Compact Particle Accelerators
The probability of RF breakdown in a single-cell standing wave accelerator cavity powered by a 110 GHz gyrotron has been carried out in [57]. It has been reported that the pursued high-frequency accelerator concept is characterized by a high degree of tunability in both power and pulse width. Pulse widths from <3 ns to 3 us have been generated at continuously-variable power levels up to 650 kW. The accelerator structure tested at STARRE Lab has already demonstrated accelerating gradients of greater than 220 MV/m. The above-mentioned advanced accelerator concept at sub-THz frequencies utilizes a laser-driven semiconductor switch (LDSS) to modulate the output of a megawatt-class gyrotron (which generates 3 us pulses with a repetition frequency of up to 6 Hz) and to produce megawatt-level, nanosecond-duration, sub-THz pulses [58, 59, 60, 61]. This concept is illustrated in Figure 5. Such a setup reflects the gyrotron radiation by inducing temporary reflectance in a semiconductor wafer generating there an electron-hole plasma. The LDSS are employing silicon (Si) or gallium arsenide (GaAs) wafers. For both semiconductor materials, a higher value of reflectance was observed with increasing 110-GHz beam intensity [58]. Furthermore, using two active wafers, pulses of variable length down to 3 ns duration have been produced. Recently, following the same approach and using a quasi-optical setup, reliable coupling of an unprecedented RF power (up to 575 kW) to the mm-wave accelerating structure has been demonstrated [59; 60; 61]. Figure 6 shows the standing wave accelerating structure which consists of a single-cell copper cavity and a Gaussian to TE\({}_{11}\) to TM\({}_{01}\) mode converter. The accelerator structure is powered by 110 GHz, 10-ns long RF pulses. The latter are chopped from 3 \(\upmu\)s pulses from a gyrotron oscillator using an LDSS (as already described in [58]). In this study, an unprecedented high gradient up to 230 MV/m that corresponds to a peak surface electric field of more than 520 MV/m has been achieved. The LDSS is a Si wafer illuminated by an intense Nd:YAG laser with a wavelength of 523 nm that produces 6 ns pulse with an energy of 230 mJ. The RF breakdown rates (BDRs) in this millimeter-wave accelerating cavity powered by a megawatt gyrotron have been measured in [60]. The observed BDR in the last 10\({}^{3}\) shots (after being previously conditioned by 10\({}^{5}\) shots) at a gradient of 230 MV/m is about 10\({}^{-3}\) [1/pulse]. Figure 5: Schematic of the LDSS and basic parameters used in [60]. Reprinted with permission from Ref. [60]. Copyright 2019 American Physical Society. Input laser pulse: 215 mJ, 6 ns, peak intensity at wafer 3.5 MW/cm\({}^{2}\), energy density 15.3 mJ/cm\({}^{2}\); incident gyrotron pulse 525 kW, 3 \(\upmu\)s; output pulse 410 kW, 9 ns. Figure 6: High-gradient mm-wave accelerating cavity test setup. Reprinted with permission from Ref. [59]. Copyright 2020 American Institute of Physics. The relativistic gyrotrons developed at IAP-RAS [62, 63] have a great potential for feeding the new generation of linear electron accelerators. These radiation sources operate with electron beams with energies of 250-300 keV at a wavelength of 10 mm with an efficiency of 50% [62] and at a wavelength of 3 mm with an efficiency of around 20% [63]. The possibility of reaching an output power of 200-250 MW by a relativistic gyrotron at a wavelength of 10 mm has been demonstrated by numerical simulations [64]. Such a powerful radiation source could be used for driving RF undulators and accelerating structures. Another advanced concept is to use a dielectric-loaded room-temperature accelerating structure of standing-wave type. Extremely high-quality factor and high shunt impedance can be realized utilizing ultralow-loss (tan\(\delta\) = 10\({}^{-5}\)-10\({}^{-6}\)) dielectrics (diamond, TiO\({}_{2}\). doped Al\({}_{2}\)O\({}_{3}\) ceramics) [65]. The structure is composed of stacking dielectric discs with a diameter of \(\sim\)4 mm.