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Glow discharge techniques for conditioning high vacuum systems

Description: A review is given of glow discharge techniques which are useful for conditioning vacuum vessels for high vacuum applications. Substantial development of glow discharge techniques has been done for the purpose of in-situ conditioning of the large ultrahigh vacuum systems for particle accelerators and magnetic fusion devices. In these applications the glow discharge treatments remove impurities from vessel surfaces in order to minimize particle-induced desorption coefficients. Cleaning mechanisms involve a mixture of sputtering and ion- (or neutral) induced desorption effects depending on the gas mixture (ArO/sub 2/ vs. H/sub 2/) and excitation method (DC, RF, and ECR). The author will review the methodology of glow discharge conditioning, diagnostic measurements provided by residual gas and surface composition analysis, and applications to vessel conditioning and materials processing. 76 refs., 16 figs.
Date: March 1, 1988
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Partial pressure analysis of plasmas

Description: The application of partial pressure analysis for plasma diagnostic measurements is reviewed. A comparison is made between the techniques of plasma flux analysis and partial pressure analysis for mass spectrometry of plasmas. Emphasis is given to the application of quadrupole mass spectrometers (QMS). The interface problems associated with the coupling of a QMS to a plasma device are discussed including: differential-pumping requirements, electromagnetic interferences from the plasma environment, the detection of surface-active species, ion source interactions, and calibration procedures. Example measurements are presented from process monitoring of glow discharge plasmas which are useful for cleaning and conditioning vacuum vessels.
Date: November 1, 1984
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Pressure measurements in magnetic-fusion devices

Description: Accurate pressure measurements are important in magnetic fusion devices for: (1) plasma diagnostic measurements of particle balance and ion temperature; (2) discharge cleaning optimization; (3) vacuum system performance; and (4) tritium accountability. This paper reviews the application, required accuracy, and suitable instrumentation for these measurements. Demonstrated uses of ionization-type and capacitance-diaphragm gauges for various pressure and gas-flow measurements in tokamaks are presented, with specific reference to the effects of magnetic fields on gauge performance and the problems associated with gauge calibration.
Date: November 1, 1981
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Review of the wall problem and conditioning techniques for tokamaks

Description: A variety of conditioning techniques have been applied to tokamaks to control low Z impurities originating from vacuum vessel surface contaminants and to modify the working gas recycling properties. After chemical or physical treatment of the vacuum vessel hardware, in-situ vacuum vessel treatments are usually applied, including: (1) vacuum baking; (2) discharge cleaning; and (3) gettering. As the magnetic fusion program moves to the next generation of large test reactors, the implementation of any of the above techniques becomes a costly procedure. It is therefore prudent to understand the mechanisms and physical-chemical effects of a particular conditioning procedure and maximize the efficiency. A review is given of the present understanding of the physical mechanisms, the resulting surface effects, and benefits to plasma performance of the various conditioning procedures. Particular attention is given to the application of hydrogen glow discharge cleaning and the use of titanium gettering as conditioning procedures.
Date: May 1, 1980
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

A comparison of hydrogen vs. helium glow discharge effects on fusion device first-wall conditioning

Description: Hydrogen- and deuterium-fueled glow discharges are used for the initial conditioning of magnetic fusion device vacuum vessels following evacuation from atmospheric pressure. Hydrogenic glow discharge conditioning (GDC) significantly reduces the near-surface concentration of simple adsorbates, such as H/sub 2/O, CO, and CH/sub 4/, and lowers ion-induced desorption coefficients by typically three orders of magnitude. The time evolution of the residual gas production observed during hydrogen-glow discharge conditioning of the carbon first-wall structure of the TFTR device is similar to the time evolution observed during hydrogen GDC of the initial first-wall configuration in TFTR, which was primarily stainless steel. Recently, helium GDC has been investigated for several wall-conditioning tasks on a number of tokamaks including TFTR. Helium GDC shows negligible impurity removal with stainless steel walls. For impurity conditioning with carbon walls, helium GDC shows significant desorption of H/sub 2/O, CO, and CO/sub 2/; however, the total desorption yield is limited to the monolayer range. In addition, helium GDC can be used to displace hydrogen isotopes from the near-surface region of carbon first-walls in order to lower hydrogenic retention and recycling. 38 refs., 6 figs.
Date: September 1, 1989
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Turbomolecular pump vacuum system for the Princeton Large Torus

Description: A turbomolecular pump vacuum system has been designed and installed on the Princeton Large Torus (PLT). Four vertical shaft, oil-bearing, 1500 l/s turbomolecular pumps have been interfaced to the 6400 liter PLT Vacuum vessel to provide a net pumping speed of 3000 l/s for H/sub 2/. The particular requirements and problems of tokamak vacuum systems are enumerated. A vacuum control system is described which protects the vacuum vessel from contamination, and protects the turbomolecular pumps from damage under a variety of possible failure modes. The performance of the vacuum system is presented in terms of pumping speed measurements and residual gas behavior.
Date: October 1, 1977
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Deuterium pumping speed measurements on 77 K cryopanels and implications for D-T retention in neutral beam systems

Description: An upper limit for the pumping speed of deuterium on 77 K surfaces has been determined by in-situ pressure measurements in a TFTR neutral beam line pumped by 423 m/sup 2/ of LN/sub 2/-cooled cryopanels. The measurement has importance for estimating the tritium retention in the beam line following operation of the ion sources with tritium. No D/sub 2/ pumping was observed. An upper limit for D/sub 2/ pumping on 77 K surfaces of less than or equal to2.4 x 10/sup -7/ l/s cm/sup 2/ was determined, corresponding to a D/sub 2/ sticking coefficient of less than or equal to1.5 x 10/sup -8/. Based on the upper limit a D-T retention factor, equal to the ratio of retained D-T to D-T input, has been determined to be less than or equal to5 x 10/sup -3/. This upper limit for D-T retention bounds the tritium inventory within the beam line to a small fraction of the tritium throughput. Comparably small upper limits for hydrogenic sticking coefficients, of the order of 10/sup -6/ - 10/sup -10/, have been determined from a review of H/sub 2/O cryotrapping measurements at 77 K and from the physical adsorption studies of H/sub 2/ on H/sub 2/O at 4 K. 22 refs., 5 figs., 1 tab.
Date: November 1, 1988
Creator: Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Changes in surface conditions with first plasma in the Princeton Large Torus (PLT)

Description: In--situ measurements are reported of the changes in surface composition of 305 stainless steel exposed to the first plasma discharges in PLT. Auger electron spectroscopy of pre-exposure surfaces shows a multilayer coverage of carbon and a fractional monolayer coverage of oxygen. Surfaces subjected to increasing levels of plasma exposure show substantial removal of the carbon multilayer, increased fractional-monolayer coverages of oxygen and iron, and tenth-monolayer quantities of chlorine and sulfur. (auth)
Date: January 1, 1976
Creator: Dylla, H.F. & Cohen, S.A.
Partner: UNT Libraries Government Documents Department

Conditioning of ion sources for mass spectrometry of plasmas

Description: Mass spectrometry is a useful diagnostic technique for monitoring plasma species and plasma-surface interactions. In order to maximize the sensitivity of measurements of hydrogen-fueled fusion plasmas or hydrogen-based discharge cleaning and etching plasmas, the ion sources of mass spectrometers are operated at or near the high pressure limit of 10/sup -4/ Torr (10/sup -2/ Pa). Such high ambient pressures of hydrogen give rise to high background levels of residual gases such as H/sub 2/O, CO, and CH/sub 4/, due to surface reactions on the ion source electrodes. For a commonly used ion source configuration, the residual gas production is a linear function of the ambient H/sub 2/ pressure. Hydrogen conditioning can reduce the absolute residual gas levels. Steady-state residual gas production is observed in a conditioned ion source, which is related to a balance of diffusion and sorption on the electrode surfaces.
Date: February 1, 1983
Creator: Dylla, H.F. & Blanchard, W.R.
Partner: UNT Libraries Government Documents Department

Tritium retention in TFTR

Description: This report discusses the materials physics related to D-T operation in TFTR. Research activities are described pertaining to basic studies of hydrogenic retention in graphite, hydrogen recycling phenomena, first-wall and limiter conditioning, surface analysis of TFTR first-wall components, and estimates of the tritium inventory.
Date: April 1, 1988
Creator: Dylla, H.F. & Wilson, K.L. (eds.)
Partner: UNT Libraries Government Documents Department

Lower limits for parallel impurity fluxes in the PLT edge plasma

Description: Lower limits for time-resolved anti B-parallel impurity fluxes to solid collectors immersed in the PLT edge plasma have been determined as a function of minor radius. Using geometric shadowing of the probe surface, features attributed to finite ion gyroradii have been observed. Also, for r less than 48 cm, heat pulses of less than 2 ms duration and greater than 2.5 x 10/sup 3/ W/cm/sup 2/ intensity were found to occur at the termination of many discharges.
Date: May 1, 1978
Creator: Cohen, S.A. & Dylla, H.F.
Partner: UNT Libraries Government Documents Department

In-situ impurity measurements in PDX Edge plasma

Description: The surface analysis station of PDX combines several surface analysis techniques (AES, XPS, SIMS) for in-situ measurement of impurity fluxes in the edge-plasma. The major impurities deposited on a sample surface during nondiverted PDX discharges are oxygen, titanium (limiter material) and chlorine. The impurity fluxes measured at different radial positions decreased by a factor of ten from the plasma edge to the wall. The sample surface collecting the impurity ions is located behind a circular aperture. The observed broadening of the deposition profile of Ti relative to the aperture diameter enables an estimate to be made of the ratio of charge state/energy of Ti ions in the edge plasma. Time-resolved analyses of the deposited impurities are presented which indicate that the time behavior for various impurities may be quite different for different impurity species. This aspect is discussed in relation to probable impurity release mechanisms.
Date: July 1, 1980
Creator: Staib, P.; Dylla, H.F. & Rossnagel, S.M.
Partner: UNT Libraries Government Documents Department

X-ray photoelectron spectroscopy (XPS) studies of oxygen and carbon bonding to tokamak walls

Description: The binding energies of surface atoms of stainless steel samples exposed to hydrogen plasma discharges in PDX are measured using X-ray Photoelectron Spectroscopy. After glow discharge conditioning the O/sub 1s/ line is completely shifted to a higher binding energy which is an indication of the formation of hydroxides. The Fe/sub 2p/ lines show a partial reduction of iron. After high power discharges, the oxygen concentration increases and the binding energy of the deposited O atoms corresponds to a metallic oxide. The deposited limiter material Ti is also completely oxidized. For both kinds of discharges the binding energies of Cr/sub 2p/ and C/sub 1s/ remain essentially unmodified.
Date: November 1, 1979
Creator: Staib, P.; Dylla, H.F. & Rossnagel, S.M.
Partner: UNT Libraries Government Documents Department

Compatibility of the Zr-Al alloy with a tokamak plasma environment

Description: We have investigated the compatibility of the Zr-Al alloy bulk getter with a tokamak plasma environment, where the hydrogenic fluxes are sufficient to cause embrittlement in relatively short times. Under normal operating conditions with the getters activated, it is necessary to regenerate the absorbed hydrogenic species before the embrittlement limit is reached. We present a method for determining the loading under tokamak conditions where the Zr-Al surface characteristics can change. During glow discharge cleaning and pulse discharge cleaning, it is not convenient to regenerate. We find, however, that during the cleaning operations the getter self-inerts, thus limiting the loading. We present data and a model which accounts for this behavior in terms of impurity adsorption on the room temperature getter surface during the cleaning operations.
Date: December 1, 1981
Creator: Knize, R.J.; Cecchi, J.L. & Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Measurement of H/sub 2/, D/sub 2/ solubilities in Zr-Al

Description: We have measured solubility constants for hydrogen and deuterium in the Zr-Al alloy to be: K/sub H/ = exp (11.1(5) - 16900(600)/T9 (Torr/(Torr-1/g)/sup 2/) and K/sub D/ = exp (12.2(8) - 16800(600)/T) (Torr/Torr-1/g)/sup 2/) respectively, where K is defined implicitly by P = Kq/sup 2/ with P the equilibrium pressre (in Torr) and q the bulk concentration (in Torr-l/g). These values, in conjunction with a model for the solubility, predict that the constant for tritium, K/sub T/ approx. 4 K/sub H/. Consequently, the regeneration of tritium will be faster by a factor of four over that for hydrogen at the same temperature, or the tritium regeneration temperature can be reduced approx. 50/sup 0/K compared to hydrogen for the same regeneration time.
Date: December 1, 1981
Creator: Knize, R.J.; Cecchi, J.L. & Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Measurements of the hydrogenic recombination coefficient for the TFTR vacuum vessel

Description: Characteristic values of the recombination rate coefficient for hydrogen and deuterium in stainless steel have been measured for the inner wall of the TFTR vacuum vessel for vessel temperatures of 25 to 100 C. In situ measurements of k/sub r/ are important for predicting the hydrogen isotope retention in the wall as a function of time, temperature, and discharge exposure, particularly because existing laboratory measurements of k/sub r/ for stainless steel span a range of four orders of magnitude. The measurement technique involved the observation of the decrease in hydrogen pressure during a glow discharge in the TFTR vacuum vessel with an initial static gas fill. The resulting values of k/sub r/ at 25 C are in the range of (0.4 to 4) x 10/sup -27/cm/sup 4/-s/sup -1/ assuming a value of the hydrogenic diffusivity of 2 x 10/sup -12/cm/sup 2/-s/sup -1/ at room temperature. No significant isotopic dependence was observed and the temperature dependence of k/sub r/ is consistent with the literature value (0.5 eV) of the activation energy. The implications of this range of values of k/sub r/, for the estimation of the in-vessel tritium inventory following D-T operation in TFTR are discussed.
Date: December 1, 1983
Creator: Dylla, H.F.; Cecchi, J.L. & Knize, R.J.
Partner: UNT Libraries Government Documents Department

Effect of hydrogen glow discharge conditioning on Zr/Al getter pumps

Description: Zr/Al bulk getter pumps are presently being considered for use in the Tokamak Fusion Test Reactor (TFTR) to reduce impurities and limit the recycling of hydrogenic species. It is necessary that these pumps not be adversely affected by the hydrogen glow discharge cleaning (GDC) which is planned as part of the routine TFTR vessel wall conditioning. The GDC procedure involves the use of a dc glow discharge with a 400 V bias voltage. The total fluence of hydrogenic ions given to the affected surfaces during a typical conditioning period is 10/sup 18/ cm/sup -2/. We have investigated the effects of typical GDC runs on a getter-pump module containing 25 g of Zr/Al mounted in a 100 liter test stand. Pumping speed, capacity, and regeneration characteristics have been studied after various exposures to GDC.
Date: January 1, 1981
Creator: Dylla, H.F.; Cecchi, J.L. & Ulrickson, M.
Partner: UNT Libraries Government Documents Department

Hydrogen isotope analysis by quadrupole mass spectrometry

Description: The analysis of isotopes of hydrogen (H, D, T) and helium (/sup 3/He, /sup 4/He) and selected impurities using a quadrupole mass spectrometer (QMS) has been investigated as a method of measuring the purity of tritium gas for injection into the Tokamak Fusion Test Reactor (TFTR). A QMS was used at low resolution, m/..delta..m < 150, for quantifying impurities from m/q = 2 to 44, and at medium resolution, m/..delta..m approx. 600, for determining concentrations of HD in /sup 3/He, and /sup 4/He in HT/D/sub 2/.
Date: March 1, 1981
Creator: Ellefson, R.E.; Moddeman, W.E. & Dylla, H.F.
Partner: UNT Libraries Government Documents Department

AES study of the adsorption of O/sub 2/, CO, CO/sub 2/, and H/sub 2/O on indium

Description: The adsorption of various oxygen-containing gases on clean polycrystalline In has been studied. For the In-O/sub 2/ system, the relationship between N(E) areas and dN(E)/dE peak heights for the In MNN Auger transition was found to be linear. At saturation, an oxygen/indium ratio of 3/2 was measured, consistent with the formation of the stable oxide In/sub 2/O/sub 3/. Low coverage sticking coefficients were found to be approximately .014 for O/sub 2/, approximately .004 for H/sub 2/O, approximately 5 x 10/sup -5/ for CO, and approximately 2 x 10/sup -5/ for CO/sub 2/. In each case, chemical shifts and peak distortions in the N(E) and dN(E)/dE spectra have been measured for the gas-saturated In, the largest being approximately 3 eV for the MNN transition of O/sub 2/-saturated surface.
Date: October 1, 1978
Creator: Rossnagel, S.M.; Dylla, H.F. & Cohen, S.A.
Partner: UNT Libraries Government Documents Department

A high-average-power FEL for industrial applications

Description: CEBAF has developed a comprehensive conceptual design of an industrial user facility based on a kilowatt UV (150-1000 nm) and IR (2-25 micron) FEL driven by a recirculating, energy-recovering 200 MeV superconducting radio-frequency (SRF) accelerator. FEL users{endash}CEBAF`s partners in the Laser Processing Consortium, including AT&T, DuPont, IBM, Northrop-Grumman, 3M, and Xerox{endash}plan to develop applications such as polymer surface processing, metals and ceramics micromachining, and metal surface processing, with the overall effort leading to later scale-up to industrial systems at 50-100 kW. Representative applications are described. The proposed high-average-power FEL overcomes limitations of conventional laser sources in available power, cost-effectiveness, tunability and pulse structure. 4 refs., 3 figs., 2 tabs.
Date: December 31, 1995
Creator: Dylla, H.F.; Benson, S. & Bisognano, J.
Partner: UNT Libraries Government Documents Department

Thermal desorption measurements of hydrogen-isotope retention in Alcator-A

Description: Results are presented for deuterium retention measurements in type 304 stainless steel exposed in the edge plasma of the Alcator-A tokamak. Stainless steel samples, at 300K, with surface conditions characterized by Auger electron spectroscopy, were introduced into the limiter shadow region for exposure to deuterium plasmas, and then withdrawn into an appended UHV chamber for thermal desorption measurements. For unoxidized samples exposed at the vacuum vessel wall radius, 1 x 10/sup 15/Dcm/sup -2/ was retained after exposure to high density (anti n/sub e/ = 3 x 10/sup 14/cm/sup -3/) discharges. Exposure to consecutive series of N discharges produced retention proportional to N/sup 1/2/.
Date: December 1, 1979
Creator: Razdow, A.M.; Marmar, E.S.; Cohen, S.A. & Dylla, H.F.
Partner: UNT Libraries Government Documents Department

Glow discharge conditioning of the PDX vacuum vessel

Description: A glow discharge technique has been developed and applied to the conditioning of the large (38 m/sup 3/) Poloidal Divertor Experiment (PDX) vacuum vessel. The discharge parameters and working gas (H/sub 2/) were chosen to maximize C and O removal and minimize metal sputtering. The glow discharge was produced by biasing one or two internal anodes at 400 V to sustain a discharge current of 2 to 4 A per anode. Purified H/sub 2/ at a pressure of 3 x 10/sup -2/ torr was flowed through PDX at approx. 10 t-l/s. The effectiveness of the glow discharge conditioning was monitored by measuring impurity gas (CH/sub 4/, C/sub 2/H/sub 4/, and CO) exhaust rates by mass spectrometry and C and O surface removal rates by in-situ AES and XPS.
Date: March 1, 1980
Creator: Dylla, H.F.; Cohen, S.A.; Rossnagel, S.M.; McCracken, G.M. & Staib, P.
Partner: UNT Libraries Government Documents Department

Long-term changes in the sensitivity of quadrupole mass spectrometers

Description: We routinely use quadrupole mass spectrometers (QMS) to monitor vacuum conditions, gas purity, and plasma-wall interactions in the Tokamak Fusion Test Reactor (TFTR) at Princeton. Two QMS systems have been operating on TFTR continuously for a two-year period. Both QMS systems are absolutely calibrated at weekly intervals using a six-part standard gas mixture. The calibration procedure is based on the use of transfer standards (ion gauge and capacitance manometer) that are calibrated against a primary standard (spinning rotor gauge) on an external vacuum system. We have identified variations in the efficiency of the QMS ionizer and drifts in the sensitivity of the electron multiplier ion detector to be the major reasons for the observed changes in overall OMS sensitivity. Weekly variations in sensitivity greater than 100% have been observed following system bakeout at 150/sup 0/C and with the use of rhenium filaments which were initially in the QMS ionizer. Operation of the QMS systems with tungsten filaments and at constant temperature has yielded more stable operation with weekly sensitivity changes generally being less than 10%. 7 refs., 7 figs.
Date: February 1, 1986
Creator: Blanchard, W.R.; McCarthy, P.J.; Dylla, H.F.; LaMarche, P.H. & Simpkins, J.E.
Partner: UNT Libraries Government Documents Department

Hydrogen isotope exchange and conditioning in graphite limiters used in TFTR

Description: Isotopic exchange experiments performed in TFTR are used to examine the outgassing and diffusive properties of graphite used as the plasma limiter. Changeover from hydrogen to deuterium for different periods ranges from approx.600 to 60 plasma discharges, which appears to be correlated to the limiter temperature. We present a simple analytical model that predicts a fast transient (approx.10 plasma discharges) changeover where the deuterium fueling dilutes the adsorbed and near-surface hydrogen, and a slowly changing term where bulk hydrogen diffuses to the surface. Using this model we can extract an activation energy for diffusion of 0.15 +- 0.02 eV. We hypothesize that interpore diffusion for this porous (approx.15%) material is consistent with our observations. 19 refs.
Date: February 1, 1986
Creator: LaMarche, P. H.; Dylla, H. F.; McCarthy, P. J. & Ulrickson, M.
Partner: UNT Libraries Government Documents Department