High-power Windows Research Articles

Evaluation of low-loss alumina material for high-power RF windows

Conventional RF vacuum windows are made of metalized ceramics, hermetically brazed to a pillbox cavity. High-power windows, operating in ultra-high frequency (UHF) band, require the fabrication of ceramic disks with diameters on the order of 9”. Furthermore, a Titanium Nitride (TiN) multipactor suppression coating must be applied to the ceramic surfaces. The large size and complex internal geometry of these windows create challenges in validating the coating in the fully fabricated assembly. This study evaluates a novel low-loss alumina AO479U, provided by Kyocera, and a reactive sputtering process suitable to deposit a 10-20 nm thick TiN coating on a large diameter window. The paper will report the changes in the TiN coating through chemical cleaning and vacuum braze processes using stylus profilometry, optical microscopy, Scanning Electron Microscopy (SEM), and Rutherford Backscattering Spectroscopy (RBS).

Silicon carbide for high-power applications at MM and THz ranges

The dielectric loss and refractive index in the monocrystal silicon carbide of the polytype 6H–SiC were studied in the 6–380GHz frequency range and the 300–850 K temperature interval using high-quality resonator techniques. At low frequencies (f<10GHz), the loss tangent varies as 1/f. At high frequencies (f>50GHz), the loss increases with frequency. The loss nature in wide frequency and temperature ranges is considered. The possibility to apply silicon carbide for production of high-power windows is investigated.

Development of high power window prototypes for ECH&CD launchers

Torus window prototypes for electron cyclotron (EC) heating and current drive (H&CD) systems are developed for different launcher concepts in accordance with the specific transmission requirements. For ITER, single-disk windows are designed for 2 MW power transmission at fixed frequency (170 GHz) in two variants. For the RS launcher, the remote steering unit is placed close to the torus window in the launcher back-end which calls for a large window aperture (95 mm) to avoid beam vignetting at the extreme steering angles of ±12°. For the FS launcher with the steering mechanism placed in the front shield of the launcher, the aperture and the disk thickness are reduced as the torus window is integrated into a cylindrical waveguide configuration with an inner diameter of 63.5 mm. The reduced absorbed power and cooling paths allow to consider indirect cooling as an alternative to edge cooling which eliminates the risk of tritium contact to the cooling water in case of crack formation in the diamond disk. For the multifrequency torus window at ASDEX Upgrade (aperture: 80 mm), a double-disk configuration is realised with the disk separation of 5 mm, fine tuneable over ±1 mm. Prototypes of the high power windows were manufactured. First results of the current thermo-hydraulic and of the mm-wave performance tests support the feasibility of the concepts. For the ITER torus windows, the tool development is described which provides on-site replacement by automated cutting and welding tools.

CVD diamond windows studied with low- and high-power millimeter waves

As part of long-pulse high-power gyrotron development low- and high-power millimeter-wave characterization has been performed on bare and brazed chemical vapor deposition (CVD) diamond disks. The dielectric property measurements performed with low-power open resonator studies demonstrate the availability of large area CVD diamond disks fulfilling the requirements for high-power windows. In brazed components, additional surface losses are put to evidence. Their contribution to enhanced dielectric absorption differ characteristically between existing brazing techniques. A particular acid and hydroxide treatment for reducing the observed surface terms was studied for brazed windows at different stages of their integration into gyrotron tubes, such as premounting, onsite, and post-dismantling. Video inspections of the output and transmission windows in high-power tests gave evidence of stationary light emission phenomena from irregularly distributed spot-like centres in the output window. The absence of a similarly stationary phenomenon in all disks studied in transmission rules out typical defect structures such like nondiamond like phase inclusions or specifically terminated surface bonds as a critical source. Local temperature peaks and the presence of only singular light emission events in the transmission windows hint at contamination effects by particles which are only stationary in vacuum conditions.

Thermal and electrical analysis of alumina and beryllia coax high-power windows under irradiation

Characteristics of dielectric insulators in vacuum windows of coaxial 10 to 100 MHz transmission lines in high-power steady-state use under irradiation are simulated with respect to electric, nuclear, mechanical, and thermal properties. Neutron fluence /spl sim/5/spl times/10/sup 18/ n/cm/sup 2/ at the window is obtained to be sufficiently small to allow beryllia, but not alumina, to be used as dielectric. In beryllia (10/sup -3/ displacements per atom (dpa) due to irradiation) or in un-irradiated alumina (97.5% purity), the temperature is found to rise by not more than 125/spl deg/C with maximum stress <140 MPa for 50 kV peak voltage at 60 MHz, provided niobium, titanium or materials with similar thermal expansion coefficients are used in water cooled conductors. The tangential electric field is kept well below the surface discharge limit 2 MV/m by using potential rings together with a sufficiently large inclination angle of the conical ceramic with respect to the radial coaxial direction, but high normal fields exceeding the vacuum breakdown limit are obtained near the potential rings. Abandoning the potential rings and deforming the equipotential lines by shaping the ceramic-conductor joint can reduce both tangential and normal field components below the breakdown limit, which appears to be in agreement with recent voltage test experiments.

Recent advanced technology in electron cyclotron heating systems

Abstract Electron cyclotron resonance heating and electron cyclotron current drive are of growing importance among the heating and current drive methods for fusion plasmas. To obtain full advantage of the two main features of these techniques, namely the absence of antenna structures near the plasma boundary and the possibility of localized power deposition, the launching of a well-collimated beam with a controlled direction and polarization is necessary. This requires the generation and transmission of high power millimetre waves with high mode purity. Step-tunable single-mode gyrotron oscillators capable of high average power (1 MW unit−1) continuous operation in the millimetre wave range (100 GHz–180 GHz) are currently under development. 140 GHz gyrotrons with 0.58 MW output power in the gaussian free space TEM00 mode with pulse length τ up to 2.0 s and efficiency η = 34% are commercially available. The corresponding parameters of a 159 GHz gyrotron are Pout = 0.5 MW, τ = 0.7s and η = 30%. High order rotating TE modes (e.g. TE22,6 at 140 GHz) are used as working modes in the cavities of the tubes. Improved internal quasi-optical mode transducers generate the TEM00 output mode with efficiencies of 90%–95% and separate the electron beam from the r.f. beam, thus allowing large continuous wave relevant collectors compatible with designs providing potential depression. Face-cooled double-disc sapphire windows, cryogenically edge-cooled single-disc sapphire windows (liquid N2, liquid Ne or liquid He cooling), distributed windows (metal-ceramics) and single-disc diamond windows are under investigation in order to solve the window problem. Long-distance high power millimetre wave transmission from the source to the plasma device with very low ohmic losses and high mode purity can be accomplished by 1. (1) open, quasi-optical TEM00 transmission through a gaussian beam waveguide using focusing reflectors as phase correcting elements, and 2. (2) closed, highly overmoded, circumferentially corrugated or dielectrically lined, tubular HE11 mode waveguide. In this paper we report the state of the art in the development of high power millimetre wave sources and windows and discuss the components and the characteristics of TEM00 and HE11 transmission systems and antennas.

Precise Millimeter-Wave Measurements of Complex Refractive Index, Complex Dielectric Permittivity and Loss Tangent of GaAs, Si, SiO/sub 2/, A1/sub 2/O/sub 3/, BeO, Macor, and Glass

Highly accurate continuous spectra of the complex refractive index and complex dielectric permittivity are given in the millimeter range for a variety of potentially useful materials. The absorption coefficient is found to increase monotonically with increasing frequencies. Small amounts of glassy inclusions or water were found to increase losses at all frequencies, but impurities and radiation damage (except in semiconductors) have not yet proved to be detrimental to performance. Materials have been found for which the millimeter-wave losses can be tolerated when used as dielectric waveguide, high-power windows, and other applications. Nominal consideration must be given, however, to the conditions of preparation and the nature of contaminants, The measurements were made in a modular, polarizing, dispersive Fourier-transform spectrometer.

Resonant Modes in Waveguide Windows

Analysis and experimental verification of a class of resonant fields, called ghost-modes, occurring in waveguide dielectric windows are presented. Numerical solutions for a simple geometry are given through universal curves. Knowledge about ghost-modes has importance to designers of high-power windows. It also leads to a measuring technique for dielectric constants through a frequency measurement.

Problems related to very high power windows at microwave frequencies

A discussion is given of the types of windows used on the 20 MW klystrons which serve as sources of radio-frequency power for the high-energy linear electron accelerator at Stanford University. Both ceramic disk windows and ceramic cone windows have been used for this purpose, and several methods of mounting the window in the output waveguide of the tube have been tried. The choice of both physical and electrical characteristics of the windows are dictated by considerations of size, power level, irradiation by electrons and X-rays, vacuum requirements and the like. The single most important requirement of these windows is that they should have long life, since window life is now the major factor in life of the tubes. Results are presented on average life of the windows over the period of the last few years. The causes of failure are not completely understood at the present time, but a discussion is given of the typical kinds of failure and the factors which influence window life. The goal of window research at Stanford is to increase the reliable operation life of windows from the present value of approximately 1900 h to perhaps 5000 h.

Discussion on “High-power windows at microwave frequencies”

Discussion on “High-power windows at microwave frequencies”

High-power windows at microwave frequencies

During the past ten years the Microwave Laboratory at Stanford University has been engaged in development work on high-power klystrons. One of these is used in the linear accelerator and is operated at 20 MW (peak) and a few kilowatts average. The others, now produced commercially, operate between 1 and 3 MW (peak) in the X-, S- and L-bands. The characteristics of the output windows of these tubes are briefly reviewed, and the information obtained on life and the mechanism of failure of the windows used for the linear-accelerator klystron is considered in detail. From the operation of approximately 120 windows used with the accelerator tubes, an average window life of nearly 2000 hours has been observed. Most of the windows used with the accelerator consist of a ceramic disc in a circular waveguide. Most failures appear to start as a series of small holes in the ceramic, one of which eventually punctures the disc. The general direction of the puncture is normal to the direction of the E-field in the guide. The reasons for these failures are not yet clearly understood. Experimental and theoretical investigations are continuing on the effects of the impurities of the material, particle bombardment, X-ray bombardment, and geometry of the window. We expect, when a better understanding of the mechanism of failure has been achieved, that the average window life can be increased to a minimum of 5 000 hours.