Design of a High-Q Diamond-Loaded Cavity for a Third-Harmonic Subterahertz Gyrotron Driven by a Low-Power Electron Beam

A continuous-wave (CW) high-harmonic gyrotron driven by a low-power electron beam is a compact radiation source demanded by terahertz applications. Its physical feasibility, however, is hampered by ohmic losses and mode competition in the gyrotron cavity. An ultralow-loss diamond loading of the cavity can give a clue to this problem. This article is concerned with theoretical aspects of mode selection and design for a gyrotron cavity loaded with coaxial rod made of chemical vapor deposition (CVD) diamond. As an example, the design of a high-Q diamond-loaded cavity for a third-harmonic 658-GHz gyrotron powered by a 0.1-A, 15-kV electron beam is presented. It is shown that the designed cavity enables the gyrotron to produce up to 116-W output power in a single oscillating mode

Investigations on RF Behavior of a V-Band Second Harmonic Gyrotron for 100/200 kW Operation

This article presents the investigations on RF-behavioral aspects for the possible operation of a V -band, continuous wave (CW) second harmonic gyrotron for plasma diagnostic application. Keeping in view the design goals and constraints, initial design studies for the mode selection and the computation of starting currents are carried out. From these studies, two possible modes, namely, TE 7,3 and TE 8,3 are considered for the second harmonic operation. Later, the cold cavity design and self-consistent calculations are carried out for the selected operating modes. All the computations are performed using the latest version of our in-house code Gyrotron Design Studio Second Harmonic Version 2020 (GDS2H-2020) with Glidcop as the cavity material. The RF behavior studies confirm the feasible operation of such a second harmonic gyrotron with power levels in excess of 115.52/217.64 kW with the chosen modes of operation

Realistic Design Studies on a 300-GHz, 1-MW, DEMO-Class Conventional-Cavity Gyrotron

This article presents the realistic initial design studies of a 300-GHz, 1-MW, conventional-cavity gyrotron for its probable application in the next-generation thermonuclear fusion reactors. Keeping the design goals, parameters, and constraints in view, the very high-order TE 49,18 mode is chosen as the operating mode after a careful mode-selection calculation considering realistic ohmic cavity losses. After mode selection and mode competition studies, the cold-cavity design and initial design of a triode-type magnetron injection gun (T-MIG) and a gyrotron magnet are carried out and an electron beam radius of 8.11 mm is obtained with 2.4% velocity spread. Furthermore, investigation on RF behavior of the cavity is performed with the T-MIG beam parameters. By varying the nominal beam parameters, single-mode self-consistent calculations are conducted and achieved the desired output power. Then, multimode time-dependent self-consistent calculations are carried out before and after space-charge neutralization (SCN) with realistic velocity spread (up to 6%) and different beam radii for the assessment of the start-up scenario. Before SCN without velocity spread, the beam voltage is depressed to 70.08 kV and 0.72-MW output power is obtained, whereas with velocity spread (6%), 0.69-MW output power is obtained with 8.11 mm of beam radius. After 60% of SCN in the start-up scenario with velocity spread (6%), the beam voltage increases to 74.83 kV, and thereby, an output power of 0.91 MW is obtained

Structural and functional characterization of the two phosphoinositide binding sites of PROPPINs.

β-propellers that bind polyphosphoinositides (PROPPINs), a eukaryotic WD-40 motif-containing protein family, bind via their predicted β-propeller fold the polyphosphoinositides PtdIns3P and PtdIns(3,5)P2 using a conserved FRRG motif. PROPPINs play a key role in macroautophagy in addition to other functions. We present the 3.0-Å crystal structure of Kluyveromyces lactis Hsv2, which shares significant sequence homologies with its three Saccharomyces cerevisiae homologs Atg18, Atg21, and Hsv2. It adopts a seven-bladed β-propeller fold with a rare nonvelcro propeller closure. Remarkably, in the crystal structure, the two arginines of the FRRG motif are part of two distinct basic pockets formed by a set of highly conserved residues. In comprehensive in vivo and in vitro studies of ScAtg18 and ScHsv2, we define within the two pockets a set of conserved residues essential for normal membrane association, phosphoinositide binding, and biological activities. Our experiments show that PROPPINs contain two individual phosphoinositide binding sites. Based on docking studies, we propose a model for phosphoinositide binding of PROPPINs

Design verification of the gyrotron diamond output window for the upgrade of the ECRH system at W7-X

The 10 MW electron cyclotron resonance heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) currently relies on the successful operation of continuous wave (CW) 1 MW, 140 GHz gyrotrons which have chemical vapor deposition (CVD) diamond output windows cooled by the industrial silicon oil Dow Corning 200(R) 5 cSt. The window features a 1.8 mm thick diamond disk brazed to two copper cuffs with an aperture of 88 mm, which are then integrated in a steel housing. In the context of the upgrade of the ECRH system towards higher microwave power, this gyrotron design has been significantly advanced to fulfill the requirement of 1.5 MW CW operation, still at 140 GHz. A prototype of this new gyrotron is under development at Thales, France. This paper reports the computational fluid dynamics (CFD) conjugated heat transfer and structural analyses of the diamond window performed using the commercial code ANSYS V19.2 to investigate its performance at 1.5 MW operation. Furthermore, sensitivity studies were also carried out with respect to the absorbed power in the disk and the mm-wave beam radius at the window location. These analyses showed that the window design of the existing 1 MW gyrotrons still works quite well at higher power operation, thus verifying the performance of the window. Even in the worst case scenario of 1.5 kW absorbed power, the maximum temperature of 215 °C at the disk center can be safely accepted, being below the conservative limit of 250 °C for CVD diamond. In addition, the non-axial symmetric thermal gradients due to the geometry of the cooling channels lead to thermal stresses in the disk and the cuffs. However, they are much lower than the limits. The copper cuffs experience plasticity deformation in the region of the interface with the diamond disk up to a value of about 1.5 mm