SLAC, Menlo Park, California, USA
ANL, Lemont, Illinois, USA
Euclid TechLabs, LLC, Solon, Ohio, USA
MIT/PSFC, Cambridge, Massachusetts, USA
AAI/ANL, Lemont, Illinois, USA
UNIST, Ulsan, Republic of Korea
We present experimental studies of metamaterial (MTM) structures for wakefield acceleration. The MTM structure is an all-metal periodic structure with its period much smaller than the wavelength at X-band. The fundamental TM mode has a negative group velocity, so an electron beam traveling through the structure radiates by reversed Cherenkov radiation. Two experiments have been completed at the Argonne Wakefield Accelerator (AWA), namely the Stage-I and Stage-II experiments. Differences between the two experiments include: (1) Structure length (Stage-I 8 cm, Stage-II 20 cm); (2) Bunch number used to excite the structure (Stage-I up to 2 bunches, Stage-II up to 8 bunches). In the Stage-I experiment, two bunches with a total charge of 85 nC generated 80 MW of RF power in a 2 ns long pulse. In the Stage-II experiment, the highest peak power reached 380 MW in a 10 ns long pulse from a train of 8 bunches with a total charge of 224 nC. Acceleration of a witness bunch has not been demonstrated yet, but the extracted power can be transferred to a separate accelerator for two-beam acceleration or directly applied to a trailing witness bunch in the same structure for collinear acceleration.
MIT/PSFC, Cambridge, Massachusetts, USA
This work presents the development of the STARRE Lab, a facility at MIT for testing breakdown in high gradient accelerator structures at 110 GHz. The system utilizes a Laser-Driven Semiconductor Switch (LDSS) to modulate the output of a megawatt gyrotron, which generates 3 μs pulses at up to 6 Hz. The LDSS employs silicon (Si) and gallium arsenide (GaAs) wafers to produce nanosecond-scale pulses at the megawatt level from the gyrotron output. Photoconductivity is induced in the wafers using a 532 nm Nd:YAG laser, which produces 6 ns, 230 mJ pulses. A single Si wafer produces 110 GHz RF pulses with 9 ns width, while under the same conditions, a single GaAs wafer produces 24 ns 110 GHz RF pulses. In dual-wafer operation, which uses two active wafers, pulses of variable length down to 3 ns duration can be created at power levels greater than 300 kW. The switch has been successfully tested at incident 110 GHz RF power levels up to 720 kW.* The facility has been used to successfully test an advanced 110 GHz accelerator structure built by SLAC to gradients in excess of 220 MV/m.
*J.F. Picard et al., Appl. Phys. Lett. 114, 164102 (2019); doi: https://doi.org/10.1063/1.5093639
MIT/PSFC, Cambridge, Massachusetts, USA
Theoretical predictions of single-surface one-point multipactor modes have been confirmed in experiments with a 17 GHz standing wave single cell disk-loaded waveguide accelerator structure operated in gradient range of 45-90 MV/m. A dc-biased probe placed outside of a slit in the side wall of the structure was used to measure the internal dark current electron energy distribution. The results indicated that the electrons had kinetic energy up to about 50 eV, in agreement with our CST particle-in-cell (PIC) simulations. Further theoretical calculations were performed to calculate the frequency detuning introduced by the multipactor electron cloud on the cell side wall for different electron cloud thicknesses and densities. We found that the detuning (Δf/f) due to the electron cloud was small, about two orders of magnitude smaller than the reciprocal of the cavity loaded quality factor. This detuning is sufficiently small that it does not cause significant power reflection. Similar calculations were carried out for high gradient operation of accelerator structures at frequencies of 2.856 GHz and 110.0 GHz, showing similar small detuning by multipactor discharges.
THYBB4
SLAC, Menlo Park, California, USA
MIT/PSFC, Cambridge, Massachusetts, USA
MIT, Cambridge, Massachusetts, USA
INFN/LNF, Frascati, Italy
There is an ongoing interest in linear accelerators operating at 100s of GHz and THz frequencies due to their small size and potentially high efficiency. Vacuum RF breakdown is one of the fundamental factors limiting performance of these linacs. Accordingly, study of RF breakdown physics in mm-wave high gradient accelerating structures is needed, which includes understanding of dependencies of the breakdown rate on electromagnetic, geometric, and material properties. In our previous work, we have tested beam-driven 100 GHz and 200 GHz metallic accelerating structures. In this work we report results of high power tests of a 110 GHz single-cell standing wave accelerating cavity powered by a 1 MW gyrotron. The RF power is coupled into the accelerating structure using a "Gaussian to TM01" mode converter. In order to characterize high gradient behavior of the cavity, including the RF breakdown probability, we have measured RF signals and field-emitted currents. The cavity is driven by 10 ns, 100s of kilowatt pulses. These short pulses were cut from microsecond-long gyrotron pulses using a fast optical switch, with accelerating gradients up to 150 MV/m.