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QED-1 device and measurements of gettering efficiency for a simulated divertor plasma

Description: The QED-1 device at PPL has provided gettering efficiency data for neutralized hydrogen plasma on titanium. The hollow-anode arcjet produces a plasma column 1 cm in diameter with 10/sup 12/ < n/sub e/ < 10/sup 15/ cm/sup -3/ and T/sub i/ approx.< T/sub e/ = 3-10 eV, confined by an axial magnetic field of 1-6 kG. The gettering measurements are based on monitoring neutral gas density with respect to time in the divertor simulation chamber of QED-1. The present results indicate that the plasma particles lose their charge and most of their energy when they strike the neutralizer plate.
Date: March 1, 1980
Creator: Owens, D.K. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Cross-field-current driven lower-hybrid instability and stochastic ion heating

Description: A suprathermal electron beam is injected parallel to vector B in a low-..beta.. plasma. The sharp space-potential drop across the surface of the beam generates a strong cross-field current which in turn drives a modified two-stream instability at the lower hybrid frequency. Intense stochastic ion heating is observed with the onset of the instability, and the heating rate is found to be proportional to the wave energy.
Date: March 1, 1977
Creator: Yamada, M. & Owens, D. K.
Partner: UNT Libraries Government Documents Department

Plasma-surface interactions in TFTR D-T experiments

Description: TFTR has begun its campaign to study deuterium-tritium fusion under reactor-like conditions. Variable amounts of deuterium and tritium neutral beam power have been used to maximize fusion power, study alpha heating, investigate alpha particle confinement, and search for alpha driven plasma instabilities. Additional areas of study include energy and particle transport and confinement, ICRF heating schemes for DT plasmas, tritium retention, and fusion in high {beta}{sub p} plasmas. The majority of this work is done in the TFTR supershot confinement regime. To obtain supershots, extensive limiter conditioning using helium fueled ohmic discharges and lithium pellet injection into ohmic and neutral beam heated plasmas is performed, resulting in a low recycling limiter. The relationship between recycling and core plasma confinement has been studied by using helium, deuterium and high-Z gas puffs to simulate high recycling limiter conditions. These studies show that confinement in TFTR supershots is very sensitive to the influx of neutral particles at the plasma edge.
Date: March 1, 1995
Creator: Owens, D.K.; Adler, H. & Alling, P.
Partner: UNT Libraries Government Documents Department

Parametric lower-hybrid instability driven by modulated electron beam injection

Description: A modulated electron beam is injected into a low ..beta.. plasma parallel to the confining field to investigate the energy-transfer-rate from the electron beam to the plasma. Parametric excitation of electrostatic lower-hybrid waves and ion cyclotron quasimodes is experimentally identified. The temperature of both ions and electrons is observed to increase significantly concomitant with the growth of the instability.
Date: August 1, 1978
Creator: Allen, G.R.; Owens, D.K.; Seiler, S.W.; Yamada, M.; Ikezi, H. & Porkolab, M.
Partner: UNT Libraries Government Documents Department

Transport simulations of ohmic pellet experiments on the TFTR, ASDEX, and ALCATOR-C tokamaks

Description: Transport simulations of ohmic gas-fuelled and pellet-fuelled experiments have been carried out to test a microinstability-based, profile-consistent model of anomalous transport in tokamaks. Predictions for experiments on the TFTR, ASDEX, and ALCATOR-C tokamaks were found consistent with the observed confinement and temperature measurements. 26 refs., 11 figs., 10 tabs.
Date: July 1, 1988
Creator: Redi, M.H.; Tang, W.M.; Owens, D.K.; Greenwald, M.; Gruber, O. & Kaufmann, M.
Partner: UNT Libraries Government Documents Department

Initial conditioning of the TFTR vacuum vessel

Description: We report on the initial conditioning of the Tokamak Fusion Test Reactor (TFTR) vacuum vessel prior to the initiation of first plasma discharges, and during subsequent operation with high power ohmically-heated plasmas. Following evacuation of the 86 m/sup 3/ vessel with the 10/sup 4/ 1/s high vacuum pumping system, the vessel was conditioned by a 15 A dc glow discharge in H/sub 2/ at a pressure of 5 mTorr. Rapid-pulse discharge cleaning was used subsequently to preferentially condition the graphite plasma limiters. The effectiveness of the discharge cleaning was monitored by measuring the exhaust rates of the primary discharge products (CO/C/sub 2/H/sub 4/, CH/sub 4/, and H/sub 2/O). After 175 hours of glow discharge treatment, the equivalent of 50 monolayers of C and O was removed from the vessel, and the partial pressures of impurity gases were reduced to the range of 10/sup -9/-10/sup -10/ Torr.
Date: January 1, 1984
Creator: Dylla, H. F.; Blanchard, W. R.; Krawchuk, R. B.; Hawryluk, R. J. & Owens, D. K.
Partner: UNT Libraries Government Documents Department

Measurement of limiter heating due to fusion product losses during high fusion power deuterium-tritium operation of TFTR

Description: Preliminary analysis has been completed on measurements of limiter heating during high fusion power deuterium-tritium (D-T) operation of TFTR, in an attempt to identify heating from alpha particle losses. Recent operation of TFTR with a 50-50 mix of D-T has resulted in fusion power output ({approx} 6.2 MW) orders of magnitude above what was previously achieved on TFTR. A significantly larger absolute number of particles and energy from fusion products compared to D-D operation is expected to be lost to the limiters. Measurements were made in the vicinity of the midplane ({plus_minus} 30{degree}) with thermocouples mounted on the tiles of an outboard limiter. Comparisons were made -between discharges which were similar except for the mix of deuterium and tritium beam sources. Power and energy estimates of predicted alpha losses were as high as 0.13 MW and 64 kJ. Depending on what portion of the limiters absorbed this energy, temperature rises of up to 42 {degrees}C could be expected, corresponding to a heat load of 0.69 MJ/m{sup 2} over a 0.5 sec period, or a power load of 1.4 MW/m{sup 2}. There was a measurable increase in the limiter tile temperature as the fusion power yield increased with a more reactive mixture of D and T at constant beam power during high power D-T operation. Analysis of the data is being conducted to see if the alpha heating component can be extracted. Measured temperature increases were no greater than 1 {degree}C, indicating that there was probably neither an unexpectedly large fraction of lost particles nor unexpected localization of the losses. Limits on the stochastic ripple loss contribution from alphas can be deduced.
Date: March 1995
Creator: Janos, A.; Owens, D. K.; Darrow, D.; Redi, M.; Zarnstorff, M. & Zweben, S.
Partner: UNT Libraries Government Documents Department

Particle balance in a TFTR supershot

Description: Particle balance has been studied self-consistently for a TFTR supershot during a relatively steady-state phase of the neutral beam injection. The TRANSP analysis code was used to model plasma parameters within the last closed flux surface, deriving time-dependent plasma profiles from measurements. The poloidal flux surfaces were obtained from TRANSP and an equivalent-filament analysis code. The edge plasma and recycling were modeled using the combined B2/DEGAS code with boundary conditions from the TRANSP modeling. The edge and scrape-off modeling results are compared with measurements of the D[alpha] emission and temperature increases in the inner limiter.
Date: November 1, 1992
Creator: Budny, R.V.; Coster, D.; Stotler, D.; Bell, M.G.; Janos, A.C. & Owens, D.K.
Partner: UNT Libraries Government Documents Department

Particle balance in a TFTR supershot

Description: Particle balance in a TFTR supershot is studied self-consistently. The TRANSP analysis code is used to model plasma parameters within the last closed flux surface, deriving time-dependent plasma profiles from measurements. The poloidal flux surfaces are derived using TRANSP and an equivalent-filament analysis code which distributes axisymmetric currents to match measurements of the poloidal field and flux and the total plasma current. The plasma in the edge and scrape off regions are modeled during a relatively steady state phase of the neutral beam injection using the B2 code which calculates plasma profiles in 2 dimensions. The recycled hydrogenic neutrals from the limiter are modeled with the DEGAS neutrals code. The recycling rates within the last closed flux surface are input into TRANSP. The edge and scrape off modeling results are compared with those from TRANSP in the main plasma and with measurements of the D{sub {alpha}} emission and thermocouple measurements of temperature increases in the inner limiter. The recycling coefficients at the last closed flux surface and at the limiter are discussed.
Date: January 1, 1992
Creator: Budny, R.V.; Coster, D.; Stotler, D.; Bell, M.G.; Janos, A.C. & Owens, D.K.
Partner: UNT Libraries Government Documents Department

Evolution of TFTR scrape-off plasmas with neutral beam injection

Description: Langmuir probe radial scans were achieved for several types of neutral-beam-heated 0.8 MA discharges in TFTR. The I(V) characteristics were analyzed to derive N/sub e/ and T/sub e/ profiles and extrapolated to infer the global particle confinement time tau/sub p/(t). A one-dimensional edge model was used to infer transport coefficients before and during neutral beam injection. The model indicates that ionization in the scrape-off increases the ion flux to the limiter by a factor approx.2 and alters the profiles.
Date: September 1, 1986
Creator: Budny, R.; Heifetz, D.; Kilpatrick, S.; Manos, D.; Owens, D.K.; Papes, R. et al.
Partner: UNT Libraries Government Documents Department

Gas-fueling studies in the PDX tokamak

Description: The characteristics of gas-fueling of high power discharges in the PDX tokamak have been investigated using gas-flow, neutral pressure, plasma density, and H..cap alpha.. emission measurements. The efficiency of gas-fueling was measured for various plasma configurations by comparison of the measured gas-influx rates to the particle exhaust rates inferred from particle decay time measurements. We observe that the fueling efficiency decreases significantly with increasing plasma density as the ionization length for thermal neutrals becomes shorter than the width of the boundary plasma. Gas fueling rates required to maintain a given plasma density are considerably higher (by factors of 5 to 10) for diverted discharges compared to limiter discharges. This result is attributed to a lower effective recycling coefficient for diverted plasmas. We discuss the dependence of the particle balance on the following experimentally measured parameters: the particle containment time, system-pumping speed, and neutral pressure in the vicinity of the active pumps.
Date: August 1, 1982
Creator: Dylla, H.F.; Blanchard, W.R.; Budny, R.; Fonck, R.J.; Owens, D.K. & Schmidt, G.L.
Partner: UNT Libraries Government Documents Department

Reduction of recycling by pumping at the PDX limiter

Description: We have installed two arrays of Zr-Al getters adjacent to the PDX limiter to affect the pumping of neutrals formed in this region. The projected area of the getters is approximately 0.4% of the plasma area, and the measured H/sub 2/ pumping speed is 16,000 l/sec. During ohmic discharges, the getters reduced the electron density decay time from 340 to 180 msec. This result, combined with H/sub ..cap alpha..'/ data indicates that the recycling coefficient decreases by 10% or more.
Date: February 1, 1983
Creator: Cecchi, J.L.; Knize, R.J.; Dylla, H.F.; Fonck, R.J.; Owens, D.K. & Sredniawski, J.J.
Partner: UNT Libraries Government Documents Department

Particle balance in a TFTR supershot

Description: Particle balance has been studied self-consistently for a TFTR supershot during a relatively steady-state phase of the neutral beam injection. The TRANSP analysis code was used to model plasma parameters within the last closed flux surface, deriving time-dependent plasma profiles from measurements. The poloidal flux surfaces were obtained from TRANSP and an equivalent-filament analysis code. The edge plasma and recycling were modeled using the combined B2/DEGAS code with boundary conditions from the TRANSP modeling. The edge and scrape-off modeling results are compared with measurements of the D{alpha} emission and temperature increases in the inner limiter.
Date: November 1, 1992
Creator: Budny, R. V.; Coster, D.; Stotler, D.; Bell, M. G.; Janos, A. C. & Owens, D. K.
Partner: UNT Libraries Government Documents Department

Particle balance in a TFTR supershot

Description: Particle balance in a TFTR supershot is studied self-consistently. The TRANSP analysis code is used to model plasma parameters within the last closed flux surface, deriving time-dependent plasma profiles from measurements. The poloidal flux surfaces are derived using TRANSP and an equivalent-filament analysis code which distributes axisymmetric currents to match measurements of the poloidal field and flux and the total plasma current. The plasma in the edge and scrape off regions are modeled during a relatively steady state phase of the neutral beam injection using the B2 code which calculates plasma profiles in 2 dimensions. The recycled hydrogenic neutrals from the limiter are modeled with the DEGAS neutrals code. The recycling rates within the last closed flux surface are input into TRANSP. The edge and scrape off modeling results are compared with those from TRANSP in the main plasma and with measurements of the D{sub {alpha}} emission and thermocouple measurements of temperature increases in the inner limiter. The recycling coefficients at the last closed flux surface and at the limiter are discussed.
Date: May 1, 1992
Creator: Budny, R. V.; Coster, D.; Stotler, D.; Bell, M. G.; Janos, A. C. & Owens, D. K.
Partner: UNT Libraries Government Documents Department

A tritium vessel cleanup experiment in TFTR

Description: A simple tritium cleanup experiment was carried out in TFTR following the initial high power deuterium-tritium discharges in December 1993. A series of 34 ohmic and deuterium neutral beam fueled shots was used to study the removal of tritium implanted into the wall and limiters. A very large plasma was created in each discharge to ``scrub`` an area as large as possible. Beam-fueled shots at 2.5 to 7.5 MW of injected power were used to monitor tritium concentration levels in the plasma by detection of DT-neutrons. The neutron signal decreased by a factor of 4 during the experiment, remaining well above the expected T-burnup level. The amount of tritium recovered at the end of the cleanup was about 8% of the amount previously injected with high power DT discharges. The experience gained suggests that measurements of tritium inventory in the torus are very difficult to execute and require dedicated systems with overall accuracy of 1%.
Date: March 1, 1995
Creator: Caorlin, M.; Kamperschroer, J.; Owens, D.K.; Voorhees, D.; Mueller, D.; Ramsey, A.T. et al.
Partner: UNT Libraries Government Documents Department

Gas fueling studies in the PDX tokamak: II

Description: Measurements of the gas fueling characteristics of the PDX tokamak have been extended in parameter range. An earlier study presented the gas fueling efficiency for H/sub 2/ and D/sub 2/ for the standard PDX divertor configurations with a large conductance between the divertor and main plasma chambers. This study presents the observed variations in H/sub 2/, D/sub 2/, and He neutral pressure and ionization light emission for divertor configurations with a restricted conductance between the divertor and main plasma chambers. The restricted conductance improved the divertor/main-chamber compression ratio by more than an order of magnitude. For the same plasma density, gas fueling from the divertor chamber was twice as efficient as fueling from the main chamber. At the highest plasma densities that were investigated, anti n/sub e/ approx. = 4 x 10/sup 13/ cm/sup -3/, a decrease in the plasma temperature in the divertor was indicated by a decrease in the ionization light at the divertor throat for D/sub 2/ and He fueled discharges. These observations are consistent with a Monte-Carlo model of neutral gas transport in the divertor.
Date: January 1, 1984
Creator: Dylla, H.F.; Bell, M.G.; Fonck, R.J.; Jaehnig, K.; Kaye, S.M.; Owens, D.K. et al.
Partner: UNT Libraries Government Documents Department

First-wall and limiter conditioning in TFTR

Description: A progress report on the experimental studies of vacuum vessel conditioning during the first year of TFTR operation is presented. A previous paper described the efforts expended to condition the TFTR vessel prior to and during the initial plasma start-up experiments. During the start-up phase, discharge cleaning was performed with the vessel at room temperature. For the second phase of TFTR operations, which was directed towards the optimization of ohmically heated plasmas, the vacuum vessel could be heated to 150/sup 0/C. The internal configuration of the TFTR vessel was more complex during the second phase with the addition of a TiC/C moveable limiter array, Inconel bellows cover plates, and ZrAl getter pumps. A quantitative comparison is given on the effectiveness of vessel bakeout, glow discharge cleaning, and pulse discharge cleaning in terms of the total quantity of removed carbon and oxygen, residual gas base pressures and the resulting plasma impurity levels as measured by visible, uv, and soft x-ray spectroscopy. The initial experience with hydrogen isotope changeover in TFTR is presented including the results of the attempt to hasten the changeover time by using a glow discharge to precondition the vessel with the new isotope.
Date: October 1, 1984
Creator: Dylla, H.F.; Blanchard, W.R.; Hawryluk, R.J.; Hill, K.W.; Krawchuk, R.B.; Mueller, D. et al.
Partner: UNT Libraries Government Documents Department

The local density increment from an ablated deuterium pellet in the TFTR tokamak

Description: Details of the process by which ionized pellet ablatant increases the density in a tokamak have been observed for the first time. For mild ablation rates, this process is not simply diffusive. Instead, the ionized ablatant takes the form of a temporally symmetric pulse which propagates over large distances along magnetic field lines. This pulse has been observed both interferometrically and with an x-ray camera. These observations are not in qualitative agreement with models which predict a step-like densification of the plasma by the pellet. 11 refs., 4 figs.
Date: May 1, 1991
Creator: Mansfield, D.K.; Janos, A.; Owens, D.K.; Schmidt, G.L.; Bell, M.G.; Cavallo, A. et al.
Partner: UNT Libraries Government Documents Department

Enhanced carbon influx into TFTR supershots

Description: Under some conditions, a very large influx of carbon into TFTR occurs during beam injection into low recycling plasmas (the Supershot regime). These carbon blooms'' result in serious degradation of plasma parameters. The sources of this carbon have been identified as hot spots on the TFTR bumper limiter at or near the last closed flux surface. Two separate temperature thresholds have been identified. One, at about 1650{degree}C, is consistent with radiation enhanced sublimation. The other, at about 2300{degree}C, appears to be thermal sublimation of carbon from the limiter. To account for the increased density caused by the blooms, near unity recycling of the carbon at the limiter by physical sputtering is required; this effect is expected from laboratory measurements, and we believe we are seeing it on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces nearly perpendicular to the magnetic field lines. Such surfaces may have local power depositions two orders of magnitude higher than usual. The TFTR team modified the limiter during the opening of Winter 1989--90. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.
Date: December 1, 1990
Creator: Ramsey, A.T.; Bush, C.E.; Dylla, H.F.; Owens, D.K.; Pitcher, C.S. & Ulrickson, M.
Partner: UNT Libraries Government Documents Department

Pellet ablation and temperature profile measurements in TFTR

Description: Single and multiple deuterium pellets have been injected into a variety of TFTR plasmas, including ohmically heated plasmas with wide range of electron temperatures, neutral beam heated plasmas at several NBI powers and high T/sub e/, post NBI plasmas. Pellet penetration into these plasmas was determined by measuring the pellet speed and duration of the H/sub ..cap alpha..//D/sub ..cap alpha../ light emission during pellet ablation in the plasma. These penetration measurements are compared to the predicted penetration computed using the ablation model developed by Oak Ridge National Laboratory. The plasma density profiles before and after pellet injection are used to estimate the number of particles deposited in the plasma. The plasma particle increase compared to the estimated number of atoms in the pellet yields a measure of the fueling efficiency of pellets in TFTR. The ablation cloud parameters are discussed based on polychromater measurements of the H/sub ..cap alpha..//D/sub ..cap alpha../ line emission from the neutral cloud surrounding the pellet. The electron temperature profile evolution after pellet injection is examined for the case of multiple pellet injection into an ohmically heated plasma. The ORNL pellet ablation code was used to compare measured pellet penetration depths with a theoretical model. The measured input parameters to the model are the electron density and temperature profiles, the neutral beam heating profile, the neutral density profile, the pellet size, pellet speed and pellet composition. The free parameter in the model is the thickness of the neutral cloud surrounding the pellet. This parameter is adjusted to arrive at a reasonable agreement between measured and calculated pellet penetration depths. The output of the model which is directly comparable to experiment is the calculated ablation rate. It is assumed that the broad-band H/sub ..cap alpha..//D/sub ..cap alpha../ emission is proportional to the ablation rate.
Date: January 1, 1988
Creator: Owens, D.K.; Schmidt, G.L.; Cavallo, A.; Grek, B.; Hulse, R.; Johnson, D. et al.
Partner: UNT Libraries Government Documents Department

First-wall conditioning for enhanced confinement discharges and the DT experiments in TFTR

Description: The conditioning techniques applied to the TFTR first-wall configuration that will be in place for the DT experiments in 1990--1991 are reviewed. Of primary interest is the helium conditioning procedure that was developed to control hydrogenic recycling from the graphite, inner-wall bumper limiter. Operation of TFTR over the plasma density range for gas-fueled ohmic plasmas, /bar n//sub e/ = (2 /minus/ 5) /times/ 10/sup 19/ m/sup /minus/3/, typically results in hydrogenic recycling coefficients near unity. The use of the helium conditioning procedure produced recycling coefficients as low as 0.5, and decreased the minimum ohmic plasma density to /bar n//sub e/ = 0.5 /times/ 10/sup 19/ m/sup /minus/3/ at I/sub p/ = 0.8 MA. Low density ohmic target plasmas with low recycling conditions are prerequisite conditions for the enhanced confinement (e.g., ''supershot''), neutral-beam-heated discharges observed in TFTR during 1986-1987, which is the primary mode being considered for study in the DT experiments. The recycling changes induced by the helium conditioning procedure are believed to be the result of a plasma pumping effect in the graphite induced by He and C ion desorption of hydrogenic species from the near-surface (< 20 nm) layer of the limiter. The capacity of the conditioned limiter to pump gas-fueled, and neutral-beam-fueled discharges is compared. The helium conditioning technique is also beneficial for isotopic exchange and for minimizing the in-vessel tritium inventory. 31 refs., 7 figs., 2 tabs.
Date: November 1, 1988
Creator: Dylla, H.F.; Ulrickson, M.; Bell, M.G.; Owens, D.K.; Buchenauer, D.; Budny, R.V. et al.
Partner: UNT Libraries Government Documents Department

Methods for the measuring surface tritium inside TFTR using beta decay

Description: Three potential methods for evaluating the surface tritium content of the TFTR vacuum vessel are described, each based on a different technique for measuring the in situ beta emission from tritium. These methods should be able to provide both a local and a global assessment of the tritium content within the top {approximately}1{mu}m of the inner wall surface.
Date: March 1, 1995
Creator: Zweben, S.J.; Johnson, D.W.; Hill, K.W.; Ku, L.P.; Lemunyan, G.; Loesser, D. et al.
Partner: UNT Libraries Government Documents Department

Collisional stochastic ripple diffusion of alpha particles and beam ions on TFTR

Description: Predictions for ripple loss of fast ions from TFTR are investigated with a guiding center code including both collisional and ripple effects. A synergistic enhancement of fast ion diffusion is found for toroidal field ripple with collisions. The total loss is calculated to be roughly twice the sum of ripple and collisional losses calculated separately. Discrepancies between measurements and calculations of plasma beta at low current and large major radius are resolved when both effects are included for neutral beam ions. A 20--30% reduction in alpha particle heating is predicted for q{sub a} = 6--14, R = 2.6 m DT plasmas on TFTR due to first orbit and collisional stochastic ripple diffusion.
Date: July 1, 1995
Creator: Redi, M.H.; Zarnstorff, M.C.; White, R.B.; Budny, R.V.; Janos, A.C.; Owens, D.K. et al.
Partner: UNT Libraries Government Documents Department

Material behavior and materials problems in TFTR (Tokamak Fusion Test Reactor)

Description: This paper reviews the experience with first-wall materials over a 20-month period of operation spanning 1985--1987. Experience with the axisymmetric inner wall limiter, constructed of graphite tiles, will be described including the necessary conditioning procedures needed for impurity and particle control of high power ({le}20 MW) neutral injection experiments. The thermal effects in disruptions have been quantified and no significant damage to the bumper limiter has occurred as a result of disruptions. Carbon and metal impurity redeposition effects have been quantified through surface analysis of wall samples. Estimates of the tritium retention in the graphite limiter tiles and redeposited carbon films have been made based on analysis of deuterium retention in removed graphite tiles and wall samples. New limiter structures have been designed using a 2D carbon/carbon (C/C) composite material for RF antenna protection. Laboratory tests of the important thermal, mechanical and vacuum properties of C/C materials will be described. Finally, the last series of experiments in TFTR with in-situ Zr/Al surface pumps will be described. Problems with Ar/Al embrittlement have led to the removal of the getter material from the in-torus environment. 53 refs., 8 figs., 3 tabs.
Date: May 1, 1988
Creator: Dylla, H.F.; Ulrickson, M.A.; Owens, D.K.; Heifetz, D.B.; Mills, B.E.; Pontau, A.E. et al.
Partner: UNT Libraries Government Documents Department