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An instrumented limiter was used as a diagnostic to measure energy deposition and power fluxes in the scrapeoff layer. The limiter consisted of an array of 12 tiles, each equipped with a thermocouple. This arrangement represented a calorimeter array that yielded the total limiter energy and decay length of the power flux in the scrapeoff layer. Energy deposition and decay lengths are discussed for various plasma parameters.

Time-resolved measurements of impurity deposition were made in the plasma edge region of ISX-A. A cylindrical sample located 20 mm behind the limiter was exposed to the plasma in synchronization with the discharges to give a time resolution of ~8 ms. The cylinder was withdrawn to an analysis chamber where the impurities deposited as a function of time were determined by Auger electron spectroscopy. Later analysis by Rutherford ion backscattering was used to quantify the results. The primary metallic impurities were identified as titanium, iron and chromium. The time evolution of these impurities is correlated with conditions in the plasma during the discharge. All the impurities observed were found to accumulate on the sample at a minimum rate during the stable central portion of the discharge, and at a maximum rate at times when the plasma was unstable.

An experiment to test beryllium as a limiter material has been performed in the ISX-B tokamak. The effect of the plasma on the limiter and the effect of the limiter on the plasma were studied in detail. Heat and particle fluxes to the limiter were measured, and limiter damage by melting was documented as a function of power flux. Strong melting and evaporation of the limiter caused beryllium gettering of the vacuum vessel. Postmortem analysis of the limiter was performed to document the amount of retained hydrogen and the erosion and impurity deposition on the limiter. The effect of the limiter on the plasma performance was studied in terms of parameter space, impurity content, and confinement for the ungettered and gettered cases. Operational experience with beryllium in a fusion experiment is discussed.

The first pump limiter experiments were performed on ISX-B. Two pump limiter modules were installed in the top and bottom of one toroidal sector of the tokamak. The modules consist of inertia cooled, TiC-coated graphite heads and ZrAl getter pumps each with a pumping speed of 1000–2000 l/s. The objective of the initial experiments was the demonstration of plasma particle control with pump limiters. The first set of experiments were performed in ohmic discharges (OH) in which the effect of the pump limiters on the plasma density was clearly demonstrated. In discharges characterized by Ip = 110 kA, B T = 15 kG , n e = 1−5 × 10 13 cm −3 and t = 0.3 s, the pressure rise in the pump limiters was typically 2 mTorr with the pumps off and 0.7 mTorr after activating the pumps. When the pumps were activated, the line-average plasma density decreased by up to a factor 2 at identical gas flow rates. The second set of measurements were performed in neutral beam heated discharges (NBI) with injected powers between 0.6 MW and 1.0 MW. Due to a cooling problem on one of the ZrAl pumps, the NBI experiments were carried out with one limiter only. The maximum pressure observed in NBI-discharges was 5 mTorr without activating the pumps, i.e., approximately twice as high as in OH-discharges. The exhaust efficiency, which is defined as the removed particle flux divided by the total particle flux in the scrape-off layer, is estimated to be 5%.

The effects of low power discharge cleaning (Te=9–18 eV, ne=1010–1011 cm−3) on wall conditions in the ISX‐A tokamak have been studied in detail by means of Auger electron spectroscopy and residual gas analysis. It was found that discharge cleaning caused very little transport of metals and reduced the surface oxygen content on stainless steel samples located between the limiter and the wall to substoichiometric levels. This oxygen was probably removed from the samples as H2O molecules. On the other hand carbon, in the form of a metallic carbide, increased during discharge cleaning. The cleanliness of tokamak plasmas (Zeff=1.6–2) obtained after about three weeks of discharge cleaning was clearly correlated with improvements in the wall conditions and with the reduced level of residual gas species.

A surface analysis station was installed on ISX-A before start-up which provided data on samples of wall material and a variety of other materials exposed at positions between the first wall and the limiter inner radius. Samples were introduced, from air, into an ultra-high vacuum transport system without disturbing the vacuum in ISX-A or the analysis station. An Auger electron spectrometer in the analysis station along with other diagnostics provided information on the initial “clean up” as well as the subsequent routine operation of ISX-A. During the start-up period substantial sulfur contamination was evident but this was greatly reduced as hydrogen discharge cleaning proceeded. Samples of oxidized stainless steel were not cleaned to an oxygen-free status but the surface was reduced to a thin substoichiometric oxide. Carbon was significantly removed but not eliminated entirely. Routine tokamak operations were monitored by a means of a special composite sample which made possible estimates of both the state of cleanliness of the system and its rate of clean-up.

Over the past year, extensive operating experience has been gained with coated graphite limiters in the ISX-B and Doublet III tokamaks. A total of approximately 20,000 tokamak discharges have been performed with TiC coated limiters, and the plasma performance and capability with these limiters has been improved over that obtained with metallic limiters. While none of the limiters have failed while in service, the TiC coating and in some cases the graphite substrate have experienced damage. This damage consists of grooves formed in the coating and an area of coating melt and graphite cracking. The groove damage is believed to have been caused by disruptions and the melt region by the high heat loads during neutral beam heated discharges.

Hydrogen recycling from walls and limiters of tokamaks constitutes the major source of plasma particles after the initial few milliseconds of a plasma pulse. The physical processes involved in recycling are thus very important to the control of plasma density and composition. Isotope changeover experiments in the laboratory show that thermally activated processes such as recombination and diffusion play significant roles in recycling. These processes are important not only during the plasma pulse but also continue after the plasma pulse ends, thus the length of time between pulses, the wall temperature, and the vacuum pumping rate all influence changeover dynamics. This paper shows that for type 304 stainless steel walls changeover can range from ≳95% to

The first year of operation of Doublet III, a large noncircular tokamak (R=1.43 m, a=0.45 m, BT ≤2.6 T), was highlighted by the achievement of a plasma current in excess of 2 MA. This is partially attributed to the low Zeff discharges made possible by wall conditioning using low temperature, low power discharge cleaning, accompanied by preconditioning of the wall and by baking. Approximately 500 h of hydrogen discharge cleaning and 25 h of oxygen discharge cleaning combined with tokamak plasma operations were required to obtain these low Zeff tokamak discharges. During the initial cleanup of the wall, copious quantities (>100 monolayers) of carbon were removed, ultimately with greatest efficiency by oxygen discharge cleaning. During the conditioning process, monitoring the impurity removal rate with the residual gas analyzer (RGA) provided the most understandable measure of the rate of the cleanup, while an in situ Auger analysis surface diagnostic supplied and operated by ORNL was most useful in identify...

For the past eighteen months a variety of low-Z coatings have been tested for service as limiters in both ohmically-heated and beam-heated ISX-B plasmas. To date TiC, TiB 2 , and B coatings on a graphite substrate have been examined with TiC providing the best results of the three. The history of these materials in ISX-B is reviewed and compared with previously used stainless steel limiters with particular emphasis on machine performance.

An experiment has been performed on the Impurity Study Experiment (ISX‐B) tokamak to test beryllium as a limiter material. Beryllium is an attractive candidate for a limiter and has been proposed for use in the Joint European Torus (JET) experiment. A temperature‐controlled, segmented, beryllium top‐rail limiter was located inside the plasma radius described by the existing titanium limiters. An extended set of diagnostics was added for measurement of scrapeoff and limiter parameters. These included visible and infrared monitoring systems, probes, and surface analysis experiments. Tokamak experiments included parameter surveys of both Ohmically heated and neutral‐beam‐heated plasmas and an extended fluence test of the limiter. The most significant effect of operation with a beryllium limiter was the reduction in low‐Z impurities caused by gettering action of beryllium deposited on the liner walls. The experiment required the design and implementation of contamination control apparatus and work procedures ...

It is shown that for type 304L stainless steel at room temperature and ion energies of 100 eV a large part of the recycling occurs with hydrogen residence times in the wall of about 100 ms. The use of wall temperatures from 80 to 500 K permits differentiation between thermally activated processes and ion bombardment-induced phenomena. For low energy ions (100 eV) onto 300 K stainless steel walls recycling increases to more than 90% in a few tenths of a second at; 3 × 10 16 ions cm −2 s −1 . For ions up to 300eV at this flux there is little energy dependence. Both thermally- and ion-induced processes can be important at room temperature, but under the conditions of this study thermal processes greatly enhance the recycle rates at 300 and 500 K. Preliminary results indicate that recombination of atomic hydrogen to molecular hydrogen can be rate controlling in some practical regimes.

Results of experimental measurements of arcing in ISX-B are described. Samples of stainless steel were exposed at the plasma edge and arcing currents to the sample were measured. Currents from several amperes up to 100 A were observed. It was possible to trigger arcs during well-behaved discharges by applying a high voltage to the sample. Elemental analysis of surface features of samples has shown that sulfur segregates to surfaces of arc craters in type 304 stainless steel. Detection of sulfur on surface samples may be related to arcing during cleanup processes in tokamaks.

Impurities coming into the plasma from the walls of present‐day toroidal plasma confinement devices modify plasma behavior substantially. Small fractions of high‐Z ions in the plasma greatly decrease plasma temperatures and increase plasma energy losses. We are attempting to identify and control the sources of impurities from the ’’first‐wall’’ in ORMAK. Auger electron spectroscopy, soft x‐ray appearance‐potential spectroscopy, and other surface‐sensitive techniques have been used to characterize the surface composition of the first wall and to develop methods to remove carbon and oxygen. Oxygen glow‐discharge cleaning has been shown, in the laboratory, to be an effective way of removing carbon from gold films (simulated ORMAK liner material) and the use of oxygen‐discharge cleaning in ORMAK has resulted in a decrease in plasma contamination, a 50% increase in plasma current, and an accompanying increase in plasma temperature. In spite of these improvements the walls of ORMAK are far from clean. Substanti...

Low-Z coatings on graphite substrates have been developed for testing as limiters in the Impurity Study Experiment (ISX-B) tokamak. Laboratory and tokamak testings have been accomplished. The laboratory tests included thermal shock experiments by means of pulsed e-beam irradiation, arcing experiments, and hydrogen and xenon ion erosion experiments. The tokamak testing consisted of ohmically heated plasma exposures with energy depositions up to 10 kJ/discharge on the limiters. The coatings, applied by chemical vapor deposition, consisted of TiB 2 and TiC deposited on POCO graphite substrates. The limiter samples were interchanged through the use of a transfer chamber without atmospheric exposure of the ISX-B tokamak. Limiter samples were baked out in the transfer chamber before use in the tokamak. Provisions for both heating and cooling the limiter during tokamak discharges were made. Initial testing of the limiter samples consisted of exposure to only ohmically heated plasmas; subsequent testing will be performed in neutral-beam-heated plasmas having up to 3 MW of injected power. Bulk and surface temperatures of the samples were measured to allow the determination of energy deposition. Extensive plasma and edge diagnostics were used to evaluate the effect of the limiter on the plasma (e.g. vacuum ultraviolet spectrometry to determine plasma impurity concentrations, Thomson scattering to determine Z effective, IR camera to measure limiter surface temperature, and laser fluorescence spectrometry to determine neutral impurity concentration and velocity distribution in the limiter region).

Optical constants of Na and Li in the form of evaporated thick solid films have been determined between 0.6 and 3.8 eV by ellipsometry. Measurements were made at room temperature through the quartz substrate at the quartz-metal interface. The optical conductivity of Na agrees well with the earlier data of Smith, while that of Li exhibits considerable differences from the results of Mathewson and Myers. The parameters of the nearly-free-electron model have been derived from the real part of the dielectric function, and the results compared with previous data.

The recycle of hydrogen from walls and limiters plays a fundamental role in the operation of tokamaks for several reasons: (1) the average proton confinement time in the plasma is short with respect to plasma pulse length, therefore each proton is, on the average, recycled several times to and from the walls; (2) the walls may elastically or inelastically reflect the hydrogen but, according to our measurements, half or more of the hydrogen incident on the walls may be buried in the wall, thermalized, and most will return to the plasma volume on a time scale which is on the order of 10−1 s. We have studied details of the hydrogen recycle phenomena from 304L stainless steel using a plasma-wall interaction simulator. Low energy (30–200 eV) atomic and molecular hydrogen ions were used. The evolution rate from surfaces bombarded with a flux of 6 × 1015ions/cm2 · s has been measured for several conditions and appears to be limited by complex diffusion phenomena and perhaps, in some cases, by surface recombination rates. The evolution rate reveals effects due to diffusion from the highly supercharged near surface region. Several mechanisms were discussed but it is not possible to identify which are responsible for the prompt desorption processes.

The angular and spectral distributions of photons emitted by Ag foils 660 A and 1 980 A in thickness when bombarded by 40 keV electrons have been determined experimentally in the wavelength region from 2 500 A to 5 600 A. The spectral distribution of light polarized in the plane of incidence showed a sharp peak at 3 300 A ± 12 A, this value being an average over photon directions from 10° to 50° from the foil normal. In addition to the peak, the spectrum showed a continuum which decreased slowly with increasing wavelength and a deep minimum at 3 200 A with a rise in the shorter wavelengths. The angular distribution of photons emitted at the peak wavelength showed a maximum at 30° from the foil normal with zero intensity at 0° and near 90°, whereas the photons emitted at other wavelengths in the continuum, e.g., 2 700 A and 4 500 A, were most intense at 50° from the foil normal. The absolute photon yield was determined by calibrating the spectrometer, analyzer, and photomultiplier with a tungsten filament lamp obtained from the U. S. National Bureau of Standards. For photon directions from 10° to 40° the photon yield was found to agree with the theoretical predictions in all respects at all wavelengths except at that of the sharp peak where the experimental values were about 30 % lower.

The Impurity Study Experiment (ISX‐A) was a tokamak designed to study impurity sources, plasma wall interactions, impurity behavior within the plasma, and related phenomena. An Auger electron spectrometer, attached to the tokamak by means of a UHV sample transfer system, has been the primary tool used for surface studies of the first wall. Stainless steel samples positioned at the wall edge were used to follow the progressive ’’cleanup’’ of the torus wall. Cleaning consisted of low energy hydrogen discharge plasmas, and later titanium gettering. The principal contaminants initially were carbon, oxygen, and sulfur. As cleaning proceeded, the surface oxide layers were reduced to substoichiometric levels, and the carbon deposits appeared to be converted to metallic carbides. The sulfur was largely removed. A decrease in the effective nuclear charge of the plasma could be correlated with these surface changes. Discharge cleaning reduced Zeff from ∠4 to 1.6 over a period of several months while titanium getter...

TiC-coated graphite limiters are currently in wide use as tokamak limiters. These limiters usually suffer a groove type of damage caused by plasma disruptions. A surface texturing treatment is described that reduces the occurrence of this type of damage.

Samples of polished and oxidized 304 stainless steel, aluminum, and gold have been subjected to bombardment by hydrogen ions in order to simulate effects expected in tokamak fusion devices. This paper reports on the sputtering and chemical effects of bombardment with 100 and 200 eV ions to fluence levels of 2 x 10/sup 21/ ions/cm/sup 2/ and 4 x 10/sup 21/ ions/cm/sup 2/, respectively. Under these conditions gold is badly blistered and eroded, aluminum is slightly blistered at 100 eV and substantially eroded at 200 eV but few changes are seen in the polished stainless steel. The oxide film (produced by a /sup 1///sub 2/ hr exposure to air at 500/sup 0/C) on the polished sample was, however, removed and only a very thin layer of substoichiometric oxide remained on the surface after exposure to either 100 or 200 eV hydrogen plasmas. Sample temperatures during bombardment were about 50/sup 0/C for the 100 eV ions and 65/sup 0/C for the 200 eV ions.

The ORMAK vacuum liner is constructed of stainless steel, overcoated with a thin platinum diffusion barrier and a final layer of gold. Gold was selected as the vacuum surface because it is chemically inert to the adsorption of common gases. However, gold surfaces do adsorb hydrocarbons, and carbon (along with oxygen) was the principal plasma contaminant during the first two years of ORMAK operation. Upon switching discharge cleaning gases from hydrogen to oxygen, carbon levels dropped until carbon is no longer a significant contaminant. Residual hydrocarbons can now be controlled by either hydrogen or oxygen discharge cleaning. The principal measured plasma contaminant in ORMAK is now oxygen. Samples taken from the ORMAK liner and analyzed by Auger electron spectroscopy reveal the presence of iron and oxygen. There is evidence from a SXAPS (Soft X-ray Appearance Potential Spectroscopy) probe of iron and chromium diffusion from the stainless steel through the gold surface in spite of the platinum diffusion barrier. The Fe and Cr provide surface oxidation sites, and SXAPS analysis shows that these metals exist as oxides. In order to investigate tokamak impurity problems further, the ISX (Impurity Study Experiment) tokamak is presently under construction. It will provide a cleaner and more flexible vacuum system in which to conduct studies of surfaces and plasma impurities. The operating characteristics will be much the same as those of ORMAK (with ohmic heating) in terms of size, plasma current, and plasma temperature.

Thesis submitted to Univ. of Tennessee by L. C. Emerson. An experimental investigation of the visible and ultraviolet light emitted when a charged particle moves across a boundary between two media with different dielectric properties was carried out. The spectral distributions of the light from evaporated foils of copper, germanium, silver, tin, and antimony bombarded by a 1.5-microamp beam of electrons were measured as a function of electron energy between 25 and 100 kev. The analysis was carried out with a Seya-Namioka vacuum ultraviolet spectrometer, Glan-Foucault prism polarizer, and a quartz- window photomultiplier. Calibration of the optical system with a NBS tungsten filament lamp enabled the intensity measurements to be carried out on an absolute basis. The experimental results for light polarized parallel to the plane containing the photon and the electron were compared with the calculated intensity of transftion radiation, and in general the agreement was found to be excellent. The component of the photon intensity polarized perpendicular to this plane was compared with the calculated intensity of the optical portion of the bremsstrahlung spectrum. The predicted dependence on electron energy, photon wavelength, and foil thickness was observed although the measured yield was higher than that predicted by theory.more » The possibility of contributions from Cherenkov radiation and plasma radiation is also considered, but it is shown that it would be unlikely for these radiations to be observed under the experimental conditions of this study. (auth)« less

Exact theoretical expressions for scattering of light incident at oblique angles on a cylinder are derived and compared with measurements on aluminized quartz fibers (diameters 0.9–2.2 μm) using a He-Ne laser. The free-electron gas model was used for the dielectric response function for Al. Fitting the angular positions of observed maxima and minima in the scattered intensity to the calculated values gave a sensitive method for determining fiber radii, reproducible within ± 3%–5%. These radii agreed with values from Fraunhofer diffraction at normal incidence within ± 10%–15%. The role of surface plasmon excitation in the scattering of light from cylinders is discussed briefly.