Vacuum Feedthrough Research Articles

Vessel design and interfaces development for the 1 MV ITER Neutral Beam Injector and Test Facility

In the framework of the design activities for the ITER Neutral Beam Injector (NBI) and full power neutral beam injector prototype, the vacuum vessel has been designed concurrently with the whole other components, and in particular with the Beam Source (BS) and the large Cryopumps, that strongly conditioned the design. The definition of the interfaces has been focused on the design for the 1 MV neutral beam injector prototype, anyway keeping to the absolute minimum the differences with respect to the ITER NBI Vessel. The Vacuum Vessel is composed of two separate parts which shall be welded on site: the Beam Line Vessel (BLV) and the Beam Source Vessel (BSV). Three main bolted lids are foreseen for horizontal and vertical access to the internal components. The vessel is composed of double wall and ribs in critical areas to minimize deformations and stresses under the atmospheric pressure load. New concepts for the Beam Source Support, Positioning and Tilting Systems have been developed and an engineering design has been carried out, able to satisfy precise requirements on stiffness, accuracy of regulation, vacuum compatibility, electric insulation and Remote Handling operation. These components and the BS have been fully integrated inside the BSV by means of support structures and vacuum feedthroughs for mechanical links allowing the transmission of motion and forces. The interfaces between the BLV and the Beam Line Components (BLCs) have been revised to be compatible with the new vessel design and the BLCs support frames. Further interfaces with the high voltage bushing, the vacuum pumping and the diagnostic systems have been considered. The number and the position of the diagnostic viewports were identified taking into account both diagnostics and structural requirements. Static, buckling and seismic analyses, based on EN 13445, have been performed considering operative and exceptional load cases. Requirements, criteria and design details are presented in the paper together with the main analyses results.

Development of imaging bolometers for magnetic fusion reactors (invited)

Imaging bolometers utilize an infrared (IR) video camera to measure the change in temperature of a thin foil exposed to the plasma radiation, thereby avoiding the risks of conventional resistive bolometers related to electric cabling and vacuum feedthroughs in a reactor environment. A prototype of the IR imaging video bolometer (IRVB) has been installed and operated on the JT-60U tokamak demonstrating its applicability to a reactor environment and its ability to provide two-dimensional measurements of the radiation emissivity in a poloidal cross section. In this paper we review this development and present the first results of an upgraded version of this IRVB on JT-60U. This upgrade utilizes a state-of-the-art IR camera (FLIR/Indigo Phoenix-InSb) (3-5 microm, 256 x 360 pixels, 345 Hz, 11 mK) mounted in a neutron/gamma/magnetic shield behind a 3.6 m IR periscope consisting of CaF(2) optics and an aluminum mirror. The IRVB foil is 7 cm x 9 cm x 5 microm tantalum. A noise equivalent power density of 300 microW/cm(2) is achieved with 40 x 24 channels and a time response of 10 ms or 23 microW/cm(2) for 16 x 12 channels and a time response of 33 ms, which is 30 times better than the previous version of the IRVB on JT-60U.

Swagelok Ultra-Torr based feed-through design for coupling optical fibre bundles into vacuum systems

We report the design of an inexpensive and flexible fibre-optic bundle high vacuum feed-through that can be constructed from easily available Swagelok Ultra-Torr™ stock parts. The fibre bundle extends through the vacuum envelope and thus does not suffer the transmission losses encountered when using face-to-face couplings. The flexibility of construction allows the fitting to be customised to suit all flange designs such as CF, KF or custom mountings. The use of Viton™ o-rings allows modest baking (max 200 °C) for high vacuum applications and fibre bundles can be easily repositioned, removed or replaced.

Simple versatile vacuum feedthrough

Abstract

Long pulse test of the KSTAR ICRF antenna with water-cooling

The KSTAR ICRF (ion cyclotron range of frequency) antenna is being developed and tested for a high-power and long pulse operation. In the previous test campaign, the standoff capability was increased to 31.2 kV p (kV peak) from 24.3 kV p for a 300 s pulse duration by applying water-cooling to the ICRF antenna, but it was limited by an overheating of the vacuum feedthrough (VF) and the transmission line of the unmatched section which did not have any cooling channel. Prior to the RF test campaign in 2005, water-cooling system for the VF and the transmission line of the unmatched section was developed to remove the dissipated RF heat load. For a cooling of the central conductors of the VF and transmission line, the joining part of the two transmission lines was carefully designed and fabricated to ensure no water leakage and to have a tight electrical contact. During the RF testing, a cooling water was fed by a coaxial tube inside the central conductors and it flowed out through the space between the center conductor and the coaxial tube. Outer conductors near the current maximum were also water-cooled by using Al cooling blocks which had a cooling channel inside them. The high voltage tests with long pulse durations were performed at a frequency of 30 MHz. A bottom half of the current strap-1 was connected to the RF source and the other three straps were shorted at the input ports. The achieved standoff voltage was 41.3 kV p for a 300 s operation, which was much higher than that of the previous campaign. The maximum standoff voltage for a 20 s pulse was 46.0 kV p. A voltage of 41.3 kV p is equivalent to a heating power of 7.4 MW in the case of a plasma loading of 6 Ω/m. To simulate a steady state operation, much longer pulse tests were also performed. As a result, we achieved standoff voltages of 35.0 kV p for 600 s and 27.9 kV p for 1000 s.

Progress of the KSTAR ICRF components development for long pulse operation

Abstract The ICRF heating and current drive system for the KSTAR tokamak is being developed to support long pulse, high β, advanced tokamak physics experiments. Key ICRF components such as a water-cooled, high power density (∼10 MW/m2) antenna, a vacuum feedthrough for 1 MW of RF power transmission, and the tuning and matching components for slow/fast matching, are developed. High voltage characteristics and long pulse operation capability are investigated in KAERI test stand. Test results show that they have high voltage stand-off characteristics with the peak voltage over 30 kV and they can be used for long pulse (300 s) high RF power transmission.

Supersonic gas injector for fueling and diagnostic applications on the National Spherical Torus Experiment

A prototype pulsed supersonic gas injector (SGI) has been developed for the National Spherical Torus Experiment (NSTX). Experiments in NSTX will explore the compatibility of the supersonic gas jet fueling with the H-mode plasma edge, edge localized mode control, edge magnetohydrodynamic stability, radio frequency heating scenarios, and start-up scenarios with a fast plasma density ramp up. The diagnostic applications include localized impurity gas injections for transport and turbulence experiments and edge helium spectroscopy for edge Te and ne profile measurements. Nozzle and gas injector design considerations are presented and four types of supersonic nozzles are discussed. The prototype SGI operates at room temperature. It is comprised of a small graphite Laval nozzle coupled to a modified commercial piezoelectric valve and mounted on a movable vacuum feedthrough. The critical properties of the SGI jet—low divergence, high density, and sharp boundary gradient, achievable only at M>1, have been demonstrated in a laboratory setup simulating NSTX edge conditions. The Mach numbers of about 4, the injection rate up to 1022 particles/s, and the jet divergence half angle of 6° have been inferred from pulsed pressure measurements.

Development of a KSTAR ICRF antenna for long pulse operation

A prototype ion cyclotron range of frequency (ICRF) antenna with RF power of 6 MW has been developed for the long pulse (300 s), high power operation in the Korea superconducting tokamak advanced research (KSTAR) tokamak. Cooling paths in the antenna were carefully designed to remove the dissipated RF power loss. An RF power test has been performed to estimate the standoff capability of the antenna. A high power RF test at a frequency of 30 MHz gives a standoff voltage of 30.5 kVp for 60 s and 23.2 kVp for 300 s (without cooling). During the RF pulse, the peak voltage, forward/reflected powers, temperature of the antenna, and gas pressure are measured. A vacuum feedthrough of 1 MW RF power has been developed, which has two alumina ceramic cylinders and an O-ring seal. For cooling of the ceramic parts, dry air is injected into the ceramic surface through two outer nozzles. Independent cooling water channels are installed to cool the inner conductor of the feedthrough. RF high voltage tests show that stable operation is possible, with a peak voltage of 28.9 kVp for 300 s, without any severe damage.

Calorimeter probe for the DIII-D divertor

Heat flux measurements of the DIII-D divertor plate have been obtained with 6 mm spatial resolution using a calorimeter probe. These measurements complement the infrared camera system normally used for heat flux measurements on DIII-D but at higher-spatial resolution. The calorimeter probe is inserted into the tokamak from below to a position which is flush with the lower divertor plate tiles using the divertor materials experimental station (DiMES). The DiMES mechanism allows for retraction of the probe behind a gate valve and removal from the tokamak for modification or calibration. A 6 mm diameter insulated graphite cylinder for collecting energy is mounted within a standard DiMES sample. A 0.8 mm diameter thermocouple, installed 4 mm below the surface, provides a measurement of the temperature during and after the plasma exposure. The 80 ms time constant for the measurement is fast enough to determine heat flux changes during the 5 s plasma discharge and heat flux profiles have been obtained using both fixed strike points and slow strike point sweeps across the calorimeter. Special electronics and isolation is necessary as the sample is in direct electrical contact with the plasma. The calorimeter observes approximately 100 °C temperature rise over one tokamak discharge. The thermocouple signals are typically less than 1 mV and must be amplified near the vacuum feedthrough, passed through a low-pass filter to eliminate magnetic pickup, isolated, and sent to the data acquisition system approximately 8 m away. Initial measurements are included.

A Vacuum Feedthrough Insulator at a Voltage of ∼1 MV

The design and results of tests of a vacuum feedthrough insulator for a pulsed electron accelerator with a voltage of ∼1 MV, a current of ∼20 kA, and a half-height voltage-pulse duration of ≥ 400 ns are presented. A method for distributing the voltage over the sections of the insulator due to an electron emission from specially created surfaces is proposed.

Development of High Power RF Transmission Components for Long Pulse Operation

The high power RF transmission components are developed for transmitting MW level of RF power continuously in ICRF heating and current drive system. A water-cooled co-axial transmission line with the special connector for water sealing while maintaining electrical contact, a vacuum feedthrough of 1 MW RF power which. has two alumina (Al2O3, 97%) ceramic cylinders and O-ring seal instead of a brazed seal for good mechanical and thermal strength, and the phase shifter using liquid instead of gas for insulating dielectric medium were developed. RF tests show that stable operation is possible with the peak voltage over 30 kV for 300 seconds.

Fabrication of prototype ICRF antenna for KSTAR and first RF test

A prototype ICRF antenna has been fabricated and is being tested for the development of long pulse (300 s), high power ICRF system of KSTAR. The antenna has many cooling channels inside the current strap, Faraday shield, cavity wall and vacuum transmission line to remove the dissipated RF loss power and incoming plasma heat loads. The antenna is installed in the RF test chamber evacuated by 2000 l/s turbomolecular pump and consistency with the design requirement for long pulse, high power operation is being investigated through the RF test. With the half of a current strap connected to the matching circuit via a vacuum feedthrough and a directional coupler, and the other seven ports shorted at the input ports, intermediate RF power test has been performed at f=32 MHz. During the RF pulse, the maximum peak voltage, forward/reflected powers, temperature on the antenna and gas pressure are measured. Even the initial test results promise that the long pulse, high power operation of the ICRF system will be obtainable for the advanced tokamak operation of KSTAR tokamak.

High vacuum feedthrough for angular, linear, and rotary motion

An angular motion feedthrough capable of supporting a large diameter linear motion feedthrough has been designed and built for the large experiment on instabilities and anisotropies at West Virginia University. With a combination of linear, angular, and rotary motion, a probe can be positioned in the vacuum chamber within a cone-shaped volume.

The motional stark effect diagnostic at TEXTOR -94

Knowledge of the current distribution in a tokamak is indispensable for an understanding of the energy transport in the plasma and controlling instabilities. A diagnostic exploiting the Motional Stark Effect (MSE) has the potential to obtain this quantity. This is based on a measurement of the polarisation of the Balmer-α light emitted by neutral particles injected into tokamaks. On the tokamak TEXTOR-94 a new MSE system is under development which exploits the full spectral information, along with its polarisation. This system consists of 30 radial channels, measuring the spectrum at two orthogonal angles. This is accomplished by inserting a Glan-Laser prism in the optical path. All the optics is housed in five camera modules and located inside the vessel to improve the radial resolution and minimise changes in polarisation due to optical elements. Fibres transfer the light by a vacuum feedthrough towards a spectrometer. The simple construction of the fibre feedthrough and its reliable operation opens the possibility to observe plasma light at all conceivable angles and positions, irrespective of suitable locations of windows.

A novel angular motion vacuum feedthrough

A novel angular motion feedthrough to vacuum has been designed and implemented at the Large Plasma Device at UCLA. The mechanism is easy and inexpensive to build. If linear motion capability is added, then one can arbitrarily and precisely position a probe within a specific volume in a vacuum chamber.

Spectra polarimetry of the motional Stark effect at TEXTOR-94

The Balmer-α light emitted by neutral particles injected into tokamaks is polarized with respect to the Lorentz electric field experienced by these atoms: El=ν×B, known as the motional Stark effect (MSE). On TEXTOR-94 a new MSE system is under development which exploits the full spectral information, along with its polarization. The advantages of exploiting the full spectral information are obvious: beam velocity, observation volume, absolute value of B, the plasma radial electric field and the beam density can in principle all be extracted from the measurement, whereas the polarization can be detected as several lines simultaneously. The TEXTOR-94 MSE system consists of 30 radial channel, of which the full Balmer-α spectrum is measured at two orthogonal angles. This is accomplished by inserting a Glan–Laser prism in the optical path. All the optics is located inside the vessel to improve the radial resolution and minimize changes in polarization due to optical elements. Fibers transfer the light by a vacuum feedthrough towards a spectrometer. It is shown that the measured spectra can be adequately fitted. The accuracy for determining the safety factor q is estimated to be 10%–15% and for the radial electric field to be 30 kV/m.

Cryogenic substrate cooling or substrate heating without vacuum feedthroughs

Cryogenic substrate cooling or substrate heating without vacuum feedthroughs

Use of Goubau line to couple microwave signals generated by resonant laser-assisted field emission

Simulations suggest that ultrawide band signals from DC to over 100 THz could be generated by photomixing in resonant laser-assisted field emission. However, measurements have been limited by the coupling to the apex of the emitting tip where these signals are generated. Goubau line, which is a broad-band surface-wave transmission line, could be formed by coating the emitting tip with a dielectric. We have modeled the use of Goubau line including (1) determining the coupling efficiency of the transition at the apex of the tip, (2) tapering the transmission line from the shank to the apex, and (3) horn transitions to low-impedance coaxial line at the vacuum feedthrough for the tip electrode. Our simulations suggest that a single device could provide efficient coupling over a bandwidth of two decades in frequency, and that this method is suitable for operating frequencies from 100 MHz to 10 THz.

Steady-State Tests of High-Voltage Ceramic Feedthroughs and Coaxial Transmission Line for ICRF Heating System of the Large Helical Device

Steady-state ion cyclotron range of frequency (ICRF) heating technologies have been developed to heat plasma for >30 min in the Large Helical Device (LHD). Steady-state-operation tests of high voltages up to 40 kV0p for >30 min were carried out on radio-frequency (rf) vacuum feedthroughs and a coaxial transmission line in a test set. Four types of ceramic feedthroughs, each having a 240-mm diameter, were tested. Cone-type alumina ceramic and cylinder-type silicon nitride composite-ceramic feedthroughs produced good performances of 40 kV/30 min and 50 kV/10 s. The others had vacuum leaks when subjected to long-pulse duration. The temperature of the cone-type alumina ceramic feedthrough was measured during the ICRF operations. By using gas-cooling techniques, the temperature increase of the ceramic was substantially reduced and saturated within 20 min. Without any gas-cooling techniques, the temperature increased linearly and did not saturate. Therefore, this approach could not be used for steady-state operation. The rf dissipation on the ceramic was calculated using the ANSYS finite element computer code. It was found that damaged feedthroughs had local high heat spots, which could result in vacuum leaks. A 240-mm-diam water-cooled coaxial transmission line was designed and tested for steady-state operation. Specially designed connector components and Teflon insulator disks were tested. During the test operation, the insulation gases of nitrogen, sulfur hexafluoride, and carbon dioxide were used to compare their insulation capabilities for steady state. For the duration of a 10-s pulse, these gases performed well up to 60 kV0p. However, for steady-state operation, carbon dioxide gas could not withstand voltages >40 kV0p. The connector components of the transmission line performed without problems below 50 kV0p and 1 kA0p for 30-min steady-state operation. The performance of the feedthroughs and transmission line exceeded the specifications for steady-state heating in the LHD.

Axial diagnostic package for Z

We have developed and fielded an axial diagnostic package for the 20 MA, 100 ns, z-pinch driver Z. The package is used to diagnose dynamic hohlraum experiments which require an axial line of sight. The heart of the package is a reentrant cone originally used to diagnose ion-beam-driven hohlraums on PBFA-II. It has one diagnostic line of sight at 0°, two at 4°, four at 6°, and four at 9°. In addition it has a number of viewing, alignment, and vacuum feedthrough ports. The front of the package sits approximately 1.5 m from the pinch. This allows much closer proximity to the pinch, with inherently better resolution and signal, than is presently possible in viewing the pinch from the side. Debris that is preferentially directed along the axis is mitigated by two apertures for each line of sight, and by fast valves and imaging pinholes or cross slits for each diagnostic. In the initial run with this package we fielded a time-resolved pinhole camera, a five-channel pinhole-apertured x-ray diode array, a bolometer, a spatially resolved time-integrated crystal spectrometer, and a spatially and temporally resolved crystal spectrometer. We will present data obtained from these diagnostics in the dynamic hohlraum research conducted on Z.