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Superconducting Radiofrequency Cavities
Published in David A. Cardwell, David C. Larbalestier, Aleksander I. Braginski, Handbook of Superconductivity, 2022
A drawback of thin-film Nb cavities is the rapid decrease of Q0 with increasing Eacc, which makes the technology less attractive for high-gradient accelerators. This issue has been studied for many years but a definitive answer is yet to be found. Ongoing R&D aims at improving the quality of the Nb films mainly by developing techniques which can produce Nb ions attracted to the substrate by a bias and which can be done in ultra-high vacuum, without a working gas. The techniques being explored are high-power pulsed magnetron sputtering, cathodic arc and post-ionization of evaporated Nb by an electron cyclotron resonance process [20]. Measurements on samples show the possibility of producing Nb films on Cu with higher RRR (∼100) and larger grain size (1–5 μm) than were obtained by conventional sputtering [21].
Generation, Removal, and Passivation of Plasma Process Induced Defects
Published in Kazumi Wada, Stella W. Pang, Defects in Optoelectronic Materials, 2021
A schematic of the system which consists of an ECR source and an rf-coupled stage is shown in Figure 2. Microwave power at a frequency of 2.45 GHz is coupled into the ECR cavity through a rectangular waveguide and a copper probe [24, 25]. The cavity is a hollow cylindrical structure whose top height is adjustable and whose bottom consists of a quartz dome. The quartz dome is transparent to microwave power which allows the plasma to be ignited in the vacuum chamber. The cavity mode is tuned to minimize the reflected power and provide a stable or resonant electromagnetic mode by adjusting the top height of the cavity and the length of the copper probe in the cavity. The electromagnetic mode can be determined by measuring the microwave power radially and axially along the cavity. Electron cyclotron resonance occurs due to the matching of the microwave frequency to the magnetic field that is generated by the permanent magnets surrounding the quartz disk. At resonance, the electrons are rotating in phase with the electric field. This allows the electrons to gain sufficient energy to ionize the gases in the chamber and create the plasma efficiently.
The fabrication of recessed gate GaN MODFET's
Published in Jong-Chun Woo, Yoon Soo Park, Compound Semiconductors 1995, 2020
Jinwook Burm, William J. Schaff, Lester F. Eastman, Hiroshi Amano, Isamu Akasaki
Two sets of MODFET's were fabricated on GaN/Al0.27Ga0.73N heterostructures, one with, and one without a gate recess etch. Electron Cyclotron Resonance (ECR) etching was used for the gate recess. Ti/Au was used for the ohmic pad and Ti/Pd/Au was used for the gate. The gate I-V curve showed low leakage Schottky characteristics with and without recess etch. After the recess etch, the peak gm improved from 23mS/mm to 47mS/mm, as the gate bias for the peak gm shifted from -2.3 V to 1.5 V. At gate lengths of 25μm, ft of 11GHz and fmax of 21GHz were measured for non-recessed gates. For 0.4μm recessed gates, ft of 14GHz and fmax of 42.5GHz were achieved. The ECR recess process did not introduce damage resulting in measurable electrical degradation of the electron supply layer or 2DEG. Through the subsequent annealing after the proton bombardment, a severe degradation of the electrical isolation was observed, showing more than factor of 600 reduction of resistance with 450°C 15sec anneal.
Plasma Wall Interaction of New Type of Divertor Heat Removal Component in LHD Fabricated by Advanced Multi-Step Brazing (AMSB)
Published in Fusion Science and Technology, 2023
Masayuki Tokitani, Yukinori Hamaji, Yutaka Hiraoka, Yuki Hayashi, Suguru Masuzaki, Hitoshi Tamura, Hiroyuki Noto, Teruya Tanaka, Tatsuya Tsuneyoshi, Yoshiyuki Tsuji, Gen Motojima, Hiromi Hayashi, Takanori Murase, Takeo Muroga, Akio Sagara, Tomohiro Morisaki
Three flat bar–type components composed of the AMSB joint structure of W/GlidCop/SUS, the so-called new type of divertor heat removal component, were placed in a horizontal row, as shown in Fig. 1, and then inserted into the divertor strike position of the LHD through a retractable material probe system, schematically shown in Fig. 2. The probe system was designed so that the W surface and the magnetic field lines of the divertor leg crossed at 45 deg. The magnetic field line direction, as drawn in Fig. 2, is the clockwise (CW) direction of the LHD helical magnetic field. The three components were completely the same design, and were distinguished as components A, B, and C. The size of the W plates was 20 × 20 × 5 mm3, and the numbers of each W array were named as positions 1 to 7, as shown in Figs. 1 and 2. The components were exposed to 1180 shots (~8000 s in total) of NBI-heated plasma discharges in the 2020FY plasma campaign. Electron cyclotron resonance heating (ECH) was sometimes used as an auxiliary heating.
MEPhIST-0 Tokamak for Education and Research
Published in Fusion Science and Technology, 2023
S. Krat, A. Prishvitsyn, A. Alieva, N. Efimov, E. Vinitskiy, D. Ulasevich, A. Izarova, F. Podolyako, A. Belov, A. Meshcheryakov, J. Ongena, N. Kharchev, A. Chernenko, R. Khayrutdinov, V. Lukash, D. Sinelnikov, D. Bulgadaryan, I. Sorokin, K. Gubskiy, A. Kaziev, D. Kolodko, V. Tumarkin, A. Isakova, A. Grunin, L. Begrambekov, R. Voskoboinikov, A. Melnikov
Experimentally, the observed vertical magnetic field is significantly higher (Fig. 8). The data were obtained by placing a line of Hall magnetic probes into a vented vacuum chamber of the tokamak and running a typical current pulse through the toroidal coils. The difference between the calculated predictions and the observed results can be attributed to the presence of the electrically conductive vacuum vessel and inter-chamber elements, as well as the error margin of the magnetic sensors’ vertical alignment. One can see that there is a significant difference in the magnetic field distributions for the empty vacuum vessel and the vacuum vessel with an ion-cyclotron resonance (ICR) antenna in it. The ratio of the vertical magnetic field to the toroidal magnetic field is significantly higher than expected, on the order of magnitude of a single digit percent. However, several areas of the zero or near-zero magnetic field were also observed. This indicates that successful plasma breakdown should be possible.
Core-Pedestal Plasma Configurations in Advanced Tokamaks
Published in Fusion Science and Technology, 2023
Ehab Hassan, C. E. Kessel, J. M. Park, W. R. Elwasif, R. E. Whitfield, K. Kim, P. B. Snyder, D. B. Batchelor, D. E. Bernholdt, M. R. Cianciosa, D. L. Green, K. J. H. Law
Then an introduction to the core transport (TGLF) is provided in Sec. III.C. The codes that model the H/CDs at the core region of the plasma, such as TORIC, TORAY, GENRAY, and NUBEAM, are introduced with some scanning results for the ion-cyclotron resonance heating (ICRH), electron-cyclotron resonance heating (ECRH), helicon waves (HCs) heating, lower-hybrid (LH) heating, and neutral beam injection (NBI) heating in Secs. III.D, III.E, III.F (III.F.1 and III.F.2), and III.G, respectively. Then a comprehensive view for integrating all the prescribed components to achieve a set of noninductive scenarios for the core plasma are described in detail in Sec. IV. The new scenarios of broad current profiles provide a new set of operating points in regard to electron density, plasma current, and total injected power compared to the previously studied case in Ref. 20.