Vacuum Window Research Articles

Characteristics of electron cyclotron resonance plasma sources

Electron cyclotron resonance plasmas were excited at a frequency of 2.45 GHz in chambers with diameters smaller and larger than cutoff ones for fundamental TE and TM modes. Microwaves were introduced in the plasma chambers from the cw generator via a vacuum window and the microwave transmission line. It was found that microwave breakdowns at pressures 10−5–10−1 Torr (argon, nitrogen, oxygen) occurred at sites where the static magnetic field was 875 G (or close). In chambers with diameter smaller than cutoff, the breakdown occurred only near the vacuum window. A high absorption level of microwave power (>95%) was achieved in all explored chambers at microwave power P=300–1000 W. It occurred at areas where the static magnetic field was ∼875 G and the plasma density was Ncr=7.4×1010 cm−3. A high microwave power absorption level resulted in high plasma density up to Ne=1012 cm−3 at P=1000 W. No saturation in Ne=f(P) was observed. The shape and location of the high plasma density area was determined by microwave matching tuning, magnetic field and its configuration, and source chamber size.

Beamline design for homogeneous irradiation of large exposure fields using synchrotron radiation

X-ray lithography tools using synchrotron radiation only allow high throughput if the wafer exposure takes place in normal atmosphere. In this case, two problems arise from the fact that synchrotron radiation has the shape of a horizontal stripe and is generated under vacuum conditions. The first problem is to find an effective way to homogeneously irradiate a large and square mask field. The second is to transfer the X-ray beam through the vacuum window in such a way, that the beam remains undisturbed. The problem of wide field exposure has been solved successfully using a scanning X-ray mirror. The reflectivity is, however, highly dependent on both the angle of incidence and the wavelength. In order to achieve a homogeneous illumination, the velocity profile of the mirror movement has to be adjusted to the change in the reflectivity as well as to the degradation of the mirror's surface due to cracked hydrocarbon products of the residual gas /1/, /2/. We have developed a high performance solution which enables compensation for all disturbing influences. The mirror drive is a computer-controlled piezo-translator. The vacuum window, which is made out of a thin foil (12.5 μm Kapton or 20 μm Beryllium), has to be at least as large as the mask field. Assuming a 32×32 mm 2 field area the window must be supported against atmospheric pressure. Usually a grid of thin bars is suitable. but this is printed on the wafer. We have developed a rotating vacuum window with a specially designed grid shape, which avoids the printing of the grid.

Alternative interpretation of the double atomic-field bremsstrahlung experiment.

A recent experiment reported a two-order-of-magnitude enhancement of the cross section for double atomic-field bremsstrahlung when compared to various theoretical formulations. In this paper we propose to explain the observed coincidence rate in terms of the coherent emission of two thick-target bremsstrahlung photons from the Mylar vacuum window by electrons elastically scattered in the target. The photon energy dependence of the coincidence rate predicted by this thick-target double bremsstrahlung (TTDB) model is estimated and compared to the previous experimental results for a gold target. Fairly good agreement is obtained with both the order of magnitude of the coincidence rate and its photon energy dependence, even though a crude approximation is used to estimate the TTDB distribution from the better known thick-target single bremsstrahlung spectrum.

Three‐dimensional electromagnetic analysis of a vacuum window involving waveguides

AbstractLocal heating of the pillbox‐type vacuum window employed in microwave heating of nuclear fusion apparatus TOKAMAK has been a problem when used under a high power, particularly under the ghost mode. This factor is used to determine the maximum power of the heating system. However, in the measurement in actual operation, only the electric field can be measured when the amplitude and reflection coefficient become small due to the loss. An analysis taking into consideration the dielectric loss is needed to determine the characteristics of the electromagnetic field distribution in the ghost mode. We have applied the spatial network method to the pillbox vacuum window excited by a rectangular waveguide. The method has a number of unique features as a time domain analysis method for three‐dimensional electromagnetic fields. After the basic treatment is described, the electromagnetic field of the ghost mode is simulated by the analysis taking into consideration the dielectric loss. The effectiveness of the three‐dimensional electromagnetic analysis for the entire system is presented.

Details of a water target used in a high-current application for a neutrino-physics experiment and the direct measurement of the incident proton beam by the induced change in the water conductivity

A water target was constructed for use in a low-energy neutrino physics experiment at the Los Alamos Meson Physics Facility. For 800 MeV incident protons, a water target may be close to optimal in the production of positive pions that subsequently decay in flight, yielding muon neutrinos, especially if used in conjunction with a pion-focusing device. We discuss the special features of the beam vacuum window, the target and water cooling system that were designed to withstand damage from radiation and mechanical stress caused by the large proton currents used in the experiment. Our target also included internal monitors to measure changes in water conductivity induced by the passage of the proton beam. This novel monitoring technique permitted a direct measure of the intensity of protons inside the target and gave a measure of the beam alignment.

Cryogenic vacuum window

A procedure is described for assembly of an inexpensive, easy to assemble waveguide window that has both low microwave losses and small reflectivity. This window is vacuum, superfluid-helium-tight, and withstands repeated thermal cycling to helium temperatures.

Elliptical spectrograph/gated microchannel-plate detector for time-resolved spectral measurements in the x-ray region

Time-resolved measurements from an elliptical crystal spectrograph are used to diagnose x-ray laser experiments on a gas puff Z pinch. The elliptical spectrograph (1.2-m working distance, eccentricity 0.9586) observes the 1-keV region, covering a range of λ/2d = 0.5 to 0.9. A thin filter (0.2 μm Al on 2 μm Kimfol) stretched across the spectrograph exit slit acts as a low-energy x-ray cutoff and as a vacuum window, allowing the detector to be at high vacuum regardless of the pressure in the experimental chamber. The detector consists of a seven-frame microchannel-plate intensifier system. A pulser is used to gate each of seven striplines on the microchannel plate, providing nanosecond resolution. With this instrument we are able to measure the pump radiation from the imploding plasma (Ne or Ar) and converter layer (Al), and to study the lasant ionization state (Ne-like Ni or Cu).

X-ray pulse perturbations by line-of-sight closure effects (abstract)

In experiments with intense x-ray pulses, perturbations of the signal in traversing a diagnostic line-of-sight channel connecting source and detector can be large, requiring detailed computations to infer true characteristics of the signal. These perturbations fall in the most difficult class, being highly nonlinear (especially with respect to frequency) and time dependent. They cause the transmission of collimators and vacuum windows to change continuously as these apertures heat, ionize, and expand under intense irradiation; similarly the fluorescence efficiency of a scattering foil, the transmission of a filter, the reflectivity of a multilayer mirror, etc., can change significantly over the x-ray pulse duration. We discuss the problem of modeling x-ray transport through the ablation plasmas created by the intense signal, focusing on the questions: What are the sources of systematic error in our calculations that limit our ability to correct for aperture closure effects and how can the size of perturbations be minimized by careful design of the line-of-sight channel? This work was performed under the auspices of the U. S. DOE by the LLNL under Contract No. W-7405-ENG-48.

Toroidal and poloidal soft x-ray imaging system on the DIII-D tokamak

A toroidal soft x-ray imaging system is being added to the currently installed poloidal soft x-ray system on the DIII-D tokamak. The poloidal array is used to determine the poloidal mode structure and location of internal helical MHD perturbations in the plasma. The new array will add toroidal mode identification capability. The four detector arrays are toroidally spaced in a manner that allows identification of toroidal mode numbers up to 24. Beryllium vacuum windows separate the detectors from the tokamak vacuum and also serve as low-energy filters. The separate detector vacuum chambers can be filled with a gas that changes the low-energy cutoff of the system. By proper selection of the gas and pressure the low-energy cutoff can be chosen over the entire range of the detector sensitivity (500–1200 eV). This capability can be used to produce crude x-ray spectra for the entire imaging system or for gain control.

Parallel detection for high-resolution electron energy loss studies in the scanning transmission electron microscope

A parallel detection system has been added to the Wien Filter electron spectrometer on the dedicated scanning transmission electron microscope at IBM. The system uses an intensified diode array that is optically coupled to a single-crystal YAG screen by a vacuum window and an f/1.4–22 camera lens. The YAG screen shares the spectrometer image plane with energy selecting slits used for bright field STEM images and for single-channel energy loss analysis. Energy calibration to ±0.05 eV for scans over arbitrary energy ranges is accomplished by combining many parallel spectra having different energy centers. This process can remove channel gain variations, background variations, and nonlinear energy scale effects.

X-ray lithography using broadband sources

A variety of source technologies are currently being proposed for x‐ray lithography. Some are broadband sources, producing a continuum spectrum or a broad distribution of wavelengths. The performance of a system using a broadband source is strongly affected by the vacuum window, mask substrate, and resist composition because of the wavelength‐selective transmission and absorption of these layers. A desired ‘‘optimum’’ spectrum of energy deposited in the resist can be obtained by proper choice of source parameters, vacuum window, and mask substrate. The calculated performances of different source technologies are compared, using source parameters appropriate to each technology. The effect of vacuum window and mask substrate on performance is shown for a representative broadband source spectrum. The feasibility of x‐ray lithography sources for manufacturing use should be viewed in terms of theoretical 0.25‐μm patterning capability, which is found to depend principally on three factors: power, geometry (isotropic or collimated), and whether a vacuum window is required.

Three-dimensional analysis of a vacuum window connected to waveguide

For an effective additional heating system in the experimental tokamak-type system the pillbox-type vacuum window is proposed to isolate each part. L.J.B. Bergeron's formulation (1941) of the pillbox-type vacuum window connected to cylindrical waveguides is described, and the fundamental characteristics of the electromagnetic field within this system are shown. Local heating at the surface of the dielectric disk may be decreased by reducing circumferential dependence by using cylindrical waveguides. A unified analysis involving the pillbox window and both the input-side waveguide and the output-side one has been performed using the presented method. The results obtained present the complex variations of the distribution of the field in the overall analyzed region as a function of the length of the pillbox window. These results illustrate the characteristics of the three-dimensional analysis. The analyzed model is described; in particular, the boundary conditions of the conductor are explained. The numerical results of the field distributions are shown by the longitudinal magnetic field variations and the electric field variations in the various cross sections in the pillbox window.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Collimated electron beams and their associated penumbra widths.

The Fermi-Eyges multiple-scattering theory for electrons is applied to calculate profiles of collimated electron beams. The dose profile below the collimator is a convolution of the intensity distribution of the electrons at the level of the collimator and the distribution arising from the propagation of a Gaussian point source from the collimator to the level of the calculation. The electrons at the level of the collimator possess an angular distribution characteristic of the configuration of the electron beam at the vacuum window. Hence, the dose profile and its associated penumbra width can be expressed in terms of the angular moments of the distribution of the electrons at the collimator. The dependence of the penumbra width on the configuration-dependent angular spread of the electrons at the collimator accounts for differences in the size of the penumbra between two broad-beam configurations. These differences are also seen experimentally. We have also studied the dependence of the angular moments of the electrons upon scattering foils present above the collimator and the position of the beam-broadening device in the accelerator head.

Large composite infrared high-vacuum windows made of NaCl

We describe the fabrication of large NaCl vacuum windows (384 × 70 × 30 mm) made by cementing together two halves of 192 mm length. These windows have been used successfully for more than 2 years on the TCA tokamak to provide diagnostic access.

Vacuum chambers in composite material

Vacuum chambers and windows required for colliding beam experiments in elementary particle physics research should be as transparent as possible for the secondary particles produced in the collisions. Until now, very thin wall chambers were made of stainless steel, aluminum alloys, titanium, or beryllium. Beryllium was the best for transparency but by far the most expensive. To provide an alternative method with good transparency at low cost, CERN has developed a new generation of chambers, made of composite materials and intended primarily for the proton–antiproton mode of operation of the Super Proton Synchrotron. These chambers consist of very thin metallic pipes for vacuum, reinforced by a carbon fiber structure for mechanical stability and clad with a second metallic layer to give electromagnetic screening. Chamber elements have been made by industry and tested extensively at CERN. These elements are up to 1.8 m in length with circular cross-section diameters of 50 to 150 mm and have 0.5–1.5 mm thick walls of carbon fiber/epoxy composite. The combined metallic layers have only 70–120 μm total thickness which is sufficient to meet the requirements regarding gas desorption in ultrahigh vacuum and electromagnetic shielding of the collider detector arrays from the circulating bunched beam. The chambers are bakable up to 130 °C, have a global leak rate over a typical 10-m section of less than 1×10−10 mbar l/s and an outgassing rate of about 10−12 mbar l/s cm2. They are almost as transparent as Be chambers but cost only about 15% as much. A set of carbon fiber vacuum chambers was glued together to a total length of 13 m and successfully tested.

Applications for gyrotrons in fusion research†

High power microwave sources in the cm to mm wavelength range are in demand for heating plasmas in fusion research. In this paper a brief description of the physics of the absorption of the power and some special applications to the present state of microwave heating of tokamak plasmas is given. The need in the near future is for sources capable of providing power of the order of 1 MW per unit for many seconds. Such devices may not require the development of a vacuum window in the first instance.

Maintainability Features of the Compact Ignition Tokamak

The Compact Ignition Tokamak (CIT) is a deuterium-tritium (D-T) device envisaged to be the next experimental reactor in the US Fusion Program. The reactor will initially operate in a nonactivated hydrogen phase for approximately two years. This will permit verification of the integrity of the total system and allow hands-on repair to equipment which has experienced shakedown and early operation failures. Once D-T operations commence, reactor maintenance will require remote handling techniques. An evaluation has been completed to determine what maintenance operations must be performed on the CIT. A maintenance philosophy has been developed which is based upon the use of manipulator systems and robotics in the test cell. Replacement of life-limited equipment will be accomplished using a modular design approach for components, with simple remotely operable interfaces. Examples of operations to be done remotely include: (1) replacing of rf antennae and Faraday shields, (2) uncoupling diagnostic and fueling penetrations, (3) removing of all port covers, and (4) replacing first wall armor tiles, optical mirrors, and vacuum windows.

Grating spectrometer installation for electron cyclotron emission measurements on the DIII-D tokamak using circular waveguide and synchronous detection

The grating spectrometer installation on the DIII-D tokamak uses fundamental circular waveguide propagating the TE11 lowest-order mode followed by oversized circular guide carrying the low-loss TE01 mode. The short section of fundamental guide permits use of an electronic chopper operating at 100 kHz for both calibration and plasma operation. By using ac-coupled amplifiers tuned to the chopping frequency, the background signal generated in the indium antimonide detectors by neutrons and x rays is automatically subtracted and the system noise bandwidth is reduced. Compared with a quasi-optical system, the much smaller fundamental horn and front-end waveguide allow the waveguide system to be located outside a gate valve. With this configuration the entire waveguide run, including the actual horn and vacuum window used during plasma operations, can be included in the calibration setup.

An Analysis of a Vacuum Window for Lower Hybrid Heating

One of the key devices in lower hybrid heating systems, the vacuum window, is analyzed in this paper. We solved the pill-box vacuum window regarded as a typical boundary value problem for microwave circuits, and evaluated its equivalent circuit parameters and field distributions inside the window. Numerical results are also verified by experiment.

Far-infrared laser scattering in the ACT-I toroidal device

A far-infrared laser scattering diagnostic has been built for the ACT-I toroidal device. The optical system uses a passively stabilized 447-μm CH3I laser. A polyethylene etalon is the beam splitter. The vacuum windows are plastic (TPX), which we found has the vacuum property Q 6.5×10−9 Torr l/s/cm2. Using paraboloidal and ellipsoidal mirrors for detection optics improves the signal strength and allows a better rf enclosure design for the detector. The diagnostic was tested by scattering from an ion Bernstein wave, a technique which can be used for ion temperature diagnostics.