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Under some conditions, a very large influx of carbon into TFTR occurs during neutral 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 threshold, at about 1650°C, is consistent with radiation enhanced sublimation (RES). The other, at about 2300°C, appears to be thermal sublimation of carbon from the limiter. The carbon influx can be quantitatively accounted for by taking laboratory values for RES rates, making reasonable assumptions about the extent of the blooming area and assuming unity carbon recycling at the limiter. Such high carbon recycling is expected, and it is shown that, in target plasmas at least, it is observed on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces that are 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 1989–1990. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.

In the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 26, 11 (1984)], the highest neutron source strength Sn and D–D fusion power gain QDD are realized in the neutral‐beam‐fueled and heated ‘‘supershot’’ regime that occurs after extensive wall conditioning to minimize recycling. For the best supershots, Sn increases approximately as P1.8b. The highest‐Q shots are characterized by high Te (up to 12 keV), Ti (up to 34 keV), and stored energy (up to 4.7 MJ), highly peaked density profiles, broad Te profiles, and lower Zeff. Replacement of critical areas of the graphite limiter tiles with carbon‐fiber composite tiles and improved alignment with the plasma have mitigated the ‘‘carbon bloom.’’ Wall conditioning by lithium pellet injection prior to the beam pulse reduces carbon influx and particle recycling. Empirically, QDD increases with decreasing pre‐injection carbon radiation, and increases strongly with density peakedness [ne(0)/〈ne〉] during the beam pulse. To date, the best fusion resu...

Measurements by moveable Langmuir probes and edge spectroscopy diagnostics have documented the conditioning effect of low density helium-initiated discharge sequences on the Tokamak Fusion Test Reactor (TFTR) edge plasma. Langmuir probe measurements show in general that the edge electron density n{sub e} decreases by less than a factor of 2 while the edge electron temperature T{sub e} doubles. Radial profiles to the plasma boundary show that the density scrape-off length increases somewhat while the temperature scrape-off length decreases substantially. The particle flux density is unaffected. The spectral emission of C 2 decreases by a factor of 2, a much smaller change than that exhibited by the D{sub {alpha}} signal. These results complement previous accounts of the conditioning technique. Comparisons of these He conditioning measurements are made to edge measurements during a deuterium density scan experiment, showing many similarities, and to an existing edge model of the conditioning process, showing qualitative agreement. 20 refs., 5 figs.

Magnetohydrodynamic (MHD) activity within three zones (core, half‐radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half‐radius the m/n=3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM’s) are found to have a precursor mode with a frequency between 50–200 kHz and a mode number m/n=1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, ...

Helium pumping on argon frost has been investigated on Tokamak Fusion Test Reactor (TFTR) neutral beam injectors and shown to be viable for limited helium beam operation. Maximum pumping speeds are ∼25% less than those measured for pumping of deuterium. Helium pumping efficiency is low, >20 argon atoms are required to pump each helium atom. Adsorption isotherms are exponential and exhibit a twofold increase in adsorption capacity as the cryopanel temperature is reduced from 4.3 K to 3.7 K. Pumping speed was found to be independent of cryopanel temperature over the temperature range studied. After pumping a total of 2000 Torr l of helium, the beamline base pressure rose to 2×10−5 Torr from an initial value of 10−8 Torr. Accompanying this three order of magnitude increase in pressure was a modest 40% decrease in pumping speed. The introduction of 168 Torr l of deuterium prior to helium injection reduced the pumping speed by a factor of two with no decrease in adsorption capacity.

We describe a > 100 Watt broadband THz source that takes advantage of the relativistic enhancement of the radiation from accelerating electrons according to the formula assigned the name of Sir Joseph Larmor[1, 2]. This is in contrast to the typical 1 milliwatt sources available in a laboratory. Specifically, for relativistic electrons the emission is enhanced by the fourth power of the increase in mass. Thus for 100 MeV electrons, for which the mass increases by a factor of ∼ 200, the enhancement is > 109. The experiments use a new generation of light source called an energy recovery linac (ERL) [3], in which bunches of electrons circulate once, but in which their energy is recovered. In such a machine the electron bunches can be very much shorter than those, say, in storage rings or synchrotrons.The Jefferson Lab facility operates in new limits of emission from relativistic particles involving both multiparticle coherence and near-field emission in which the velocity (Coulomb) term in the classical electrodynamical theory becomes as important as the acceleration term (synchrotron radiation).The sub-picosecond pulses of light offer unique capabilities in 2 specific areas, namely time-resolved dynamics, and imaging. High resolution THz spectroscopy has recently revealed sharp vibrational modes for many materials including malignant tissue, proteins, DNA, pharmaceuticals and explosive materials. Energetically the THz range embraces superconducting bandgaps, and regions of intense interest in the understanding of systems in which correlated motions of electrons are important, such as colossal magneto-resistive and high-Tc materials. The very high power levels of the new source will allow non-linear effects to be observed as well as the creation of novel states of materials, including electric-field driven localization[4]. We will give examples of existing work in these areas and present opportunities afforded by the new source.

The kW‐class infrared (IR) Free Electron Laser (FEL) at Jefferson Lab had the capability of producing intracavity Thomson scattering of the IR off the electron beam thus producing high average flux, sub‐picosecond x‐rays. We have measured these x‐rays and demonstrated the energy tuneability range from 3.5 keV to 18 keV. The corresponding flux and brightness has been estimated and will be discussed. This year, 2002, the FEL was disassembled and has been reconfigured to produce 10 kW average power IR. We present the estimated x‐ray capabilities for the new FEL and discuss potential applications.

Field emission is one of the principal phenomena limiting the operating voltage of practical electron guns. There is interest in developing photoemission cathode based DC electron guns employing cathode field strengths and cathode-anode voltages well above the present state-of-the-art. Such electron sources could provide high brightness, high average current beams for energy recovered superconducting linear accelerators, for applications in next generation light sources, electron cooling, and electron-ion colliders. We have studied the effect of plasma-source ion implantation on the field emission behavior of large area stainless steel electrodes. Our apparatus allows the operation of disc-shaped electrode pairs with 100 cm/sup 2/ uniform field area to 125 kV. The cathode electrode is biased at high voltage, and the anode is electrically isolated, allowing measurement of the field emission current. Electrodes were either mechanically polished, or implanted with nitrogen ions. Two separate ion implanted electrodes have shown negligible field emission at 20 MV/m, and emission between 0.5 and 1.8 pA/cm/sup 2/ during multi-hour runs at 30 MV/m. These electrodes show very little conditioning effect.

RF plasma‐assisted CVD and sputter deposition of amorphous boron‐carbon layers similar to those being used in the TFTR tokamak at the Princeton Plasma Physics Laboratory have been performed. The initial stoichiometry has been determined using Rutherford backscattering spectrometry and elastic recoil detection. Films have also been implanted with deuterium in order to determine H‐isotope pumping capacity. These studies, in addition to characterizations made of layers collected on probes in TFTR, have been used to optimize the boronization parameters and to better understand the effects of boronization on TFTR.

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 H2O, CO, and CH4, 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 H2O, CO, and CO2; 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.

A particle scoop limiter with a graphite face backed by a 50 liter volume for collecting particles was used in PDX. Experiments were performed to test its particle control and power handling capabilities with up to 5 MW of D° power injected into D+ plasmas. Line average plasma densities of up to 8 × 1013 cm−3 and currents up to 450 kA were obtained. Plasma densities in the scoop channels greater than 2 × 1013 cm−3 and neutral densities in the scoop volume greater than 5 × 1014 cm−3 were observed. There is evidence that recycling may have occurred in the scoop channels for several discharges with large line-averaged plasma density. At beam powers up to 2.5 MW, energy confinement times above 40 ms were deduced from magnetics measurements and from transport analysis. Pressures in the vacuum vessel were in the 10 −5 Torr range, and recycling source neutral densities in the central plasma were low.

The flux of deuterium in the plasma edge several centimeters outside the limiter has been measured using collector probes during neutral beam heating experiments on the PDX tokamak and rf heating experiments on the PLT tokamak. The dependence of the flux on the distance from the plasma was determined, and the time dependence of the flux was measured with a time resolution of 90 ms. In PDX the deuterium flux decreases rapidly with increasing distance from the plasma. The deuterium flux increased strongly when the beams came on and decreased when they turned off. The depth distribution of the deuterium in the samples, measured using SIMS, shows that when the beams are on, about 30% of the deuterium incident on the probe is superthermal deuterium from the beams. In PLT the deuterium flux decreased only slightly with increasing distance from the plasma. The ICRH heating in PLT caused an increase of about 30% in the flux of deuterium to the samples and in the plasma density. In both machines the deuterium fluxes were fairly low (≲1016 D/cm2s) at the positions sampled.

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, ne = (2–5) × 1019m−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 ne = 0.5 × 1019m−3 at IP = 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, pellet-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.

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.

Neutral-beam heating of plasmas in the Tokamak Fusion Test Reactor at low preinjection densities (n/sub e/(0)approx. =10/sup 19/ m/sup -3/) were characterized by T/sub e/(0) = 6.5 keV, T/sub i/(0) = 20 keV, n/sub e/(0) = 7 x 10/sup 19/ m/sup -3/, tau/sub E/ = 170 msec, ..beta../sub t//sub h//sub e//sub t//sub a/ = 2, and a d(d,n)/sup 3/He neutron emission rate of 10/sup 16/ sec/sup -1/. The ion temperature and the deuterium-fusion neutron yields were significantly higher than for previous tokamak experiments. The low initial densities were achieved by operation of the Tokamak Fusion Test Reactor with low plasma currents (less than or equal to1 MA) and by extensive limiter conditioning.

During the 1987 run, TFTR reached record values of QDD, neutron source strength, and Ti(0). Good confinement together with intense auxiliary heating has resulted in a plasma pressure greater than 3*105 Pascals on axis, which is at the ballooning stability boundary. At the same time improved diagnostics, especially ion temperature profile measurements, have led to increased understanding of tokamak confinement physics. Ion temperature profiles are much more peaked than previously thought, implying that ion thermal diffusivity, even in high ion temperature supershot plasmas, is greater than electron thermal diffusivity. Based on studies of the effect of beam orientation on plasma performance, one of the four neutral beamlines has been re-oriented from injecting co-parallel to counter parallel, which will increase the available balanced neutral injection power from 14 MW to 27 MW. With this increase in balanced beam power, and the addition of 7 MW of ICRF power it is planned to increase the present equivalant QDT of 0.25 to close to break-even conditions in the coming run.

Hα emission is a useful indicator of hydrogen ionization since the energy required to excite hydrogen to the n=3 level is approximately the same as to ionize the hydrogen atom. Other aspects of recycling, such as plasma scrapeoff parameters, wall particle fluxes, and wall retention rates can be inferred from spatial distributions of Hα emission. There are two Hα diagnostics on the Tokamak Fusion Test Reactor (TFTR) observing the inner limiter: a relatively calibrated wide‐angle television camera, and the absolutely calibrated Hα interference filter array (HAIFA) diagnostic, both viewing the inner limiter. Hα observations by these instruments have been analyzed using the degas neutral transport code. The core recycling rate, and ion and neutral particle limiter fluxes, have been deduced from the calculations. The results are very sensitive to the ion flux distribution, and therefore also provide information on the plasma scrapeoff conditions. A survey was made of over 500 TFTR discharges. Hα emission appears to increase in proportion to n2e. The ratio of the core ionization to average Hα emission was calculated to be roughly constant over a large range of discharges.

An upper limit for the pumping speed of deuterium (D) on 77 K surfaces has been determined by in situ pressure measurements in a (TFTR) neutral beam line pumped by 423 m2 of liquid‐nitrogen‐cooled cryopanels. The measurement has importance for estimating the tritium (T) retention in the beam line following operation of the ion sources with tritium. No D2 pumping was observed. An upper limit for D2 pumping on 77 K surfaces of ≤2.4×10−7 l/s cm2 was determined, corresponding to a D2 sticking coefficient of ≤1.5×10−8. 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 ≤5×10−3. 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 10−6 to 10−10, have been determined from a review of H2O cryotrapping measurements at 77 K and from the physical adsorption studies of H2 on H2O at 4 K.

TSTR (Tokamak Fusion Test Reactor) has operated since December 1982 with ohmically heated plasmas. Routine operation with feedback control of plasma current, position and density has been obtained for plasmas with Ip800 kA, a = 68 cm, R = 250 cm, and Bt=27 kG. A maximum plasma current of 1 MA was achieved with q2.5. Energy confinement times of ~150 msec were measured for hydrogen and deuterium plasmas with e = 2 x 1013 cm-3, Te(0) 21.5 keV, Ti(0) = 1.5 keV and Zeff1 3. The preliminary results suqgest a size-cubed scaling from PLT, and are consistent with Alcator C scaling where T ~ nR2a. Initial measurements of plasma disruption characteristics indicate current decay rates of ~ 800 kA in 8 ms which is within the TFTR design requirement of 3 MA in 3 ms.

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.

Isotopic exchange experiments performed in the TokamakFusion Test Reactor (TFTR) are used to examine the outgassing and diffusive properties of graphite used as the plasmalimiter. Changeover from hydrogen to deuterium for different periods ranges from ∼600 to 60 plasma discharges, which appears to be correlated in the limitertemperature. We present a simple analytical model that predicts a fast transient (∼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 (∼15%) material is consistent with our observations.

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.

Recent experiments on TFTR have extended the operating regime of TFTR in both ohmic- and neutral-beam -heated discharges. The TFTR tokamak has reached its original machine-design specifications ( I p = 2.5 MA and B T = 5.2 T). Initial neutral-beam -heating experiments used up to 6.3 MW of deuterium beams. With the recent installation of two additional beamlines, the power has been increased up to 11 MW. A deuterium pellet injector was used to increase the central density to 2.5 x 10 20 m -3 in high-current discharges. At the opposite extreme, by operating at low plasm a current ( I p ~ 0.8 MA) and low density ( n e~ 1 x 10 19 m -3 ), high ion temperatures (9 + 2 keV) and rotation speeds (7 x 10 5 m s -1 ) have been achieved during injection. In addition, plasma-compression experiments have demonstrated acceleration of beam ions from 82 to 150 keV, in accord with expectations. The wide operating range of TFTR, together with an extensive set of diagnostics and a flexible control system, has facilitated transport and scaling studies of both ohmic- and neutral-beam -heated discharges. The result of these confinement studies are presented.

During the q-mode in TFTR, the Dα emissions decrease and the electron density, Zineff, and confinement times increase, as seen during the H-mode in other tokamaks. The interaction of surface islands with the limiter is believed to increase the carbon and electron concentration. Analysis of the deuterium recycling indicates that the recycling coefficient at the last closed flux surface decreases about 15% during the q-mode.

Carbon samples were exposed to the plasma edge in the PLT tokamak at various distances from the plasma. The amount of deuterium retained was subsequently determined using nuclear reaction analysis. The deuterium retention was found to saturate with increasing exposure time. Values for the energies and fluxes of the deuterium incident on the samples were determined by comparing the observed retention behavior with a saturation model for the retention. The deuterium fluxes and energies were found to decrease from o~4 × 1017 D/cm2 s and kT~270 eV at the radius of the limiter to o~0.8 × 1017 D/cm2 s and kT~90 eV at the radius of the wall. At these energies the fraction of the incident deuterium directly reflected from carbon is predicted to be 30 to 40%. From the deuterium energies and fluxes, values for the power flux and erosion rates are estimated.

Experimental results from high-power neutral-beam--injection experiments on the Princeton Large Torus tokamak are reported. At the highest beam powers (2.4 MW) and lowest plasma densities (n/sub e/(0) = 5 x 10/sup 13/cm/sup -3/), ion temperatures of 6.5 keV are achieved. The ion collisionality v/sub i/* drops below 0.1 over much of the radial profile. Electron heating of ..delta..T/sub e//T/sub e/ approx. = 50% has also been observed, consistent with the gross energy-confinement time of the Ohmically heated plasma, but indicative of enhanced electron-energy confinement in the core of the plasma.

This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (≤ 2.2 MA) ohmically heated, and high-power (∼ 10 MW) neutral-beam heated plasmas. The conditioning procedures which are applied routinely to the first-wall hardware are reviewed. Fueling characteristics during gas, pellet, and neutral-beam fueling are described. Recycling coefficients near unity are observed for most gas fueled discharges. Gas fueled discharges after helium discharge conditioning of the toroidal bumper limiter, and discharges fueled by neutral beams and pellets, show R

A model for the time evolution of the particle inventories within the device components of TFTR has been developed. The DIFFUSE code is used to calculate the one-dimensional diffusion and trapping of particles in the wall and limiters. The DEGAS neutral transport code computes the charge-exchange fluxes onto these components. This combined model will be useful in determining the concentrations of isotopes in the device components, overall pumping by the wall and limiter during discharges, degassing rates between discharges, and the rate of isotope exchange in the machine. Example results for a 1.4 MA, 80 cm, ohmically heated discharge shaped by the TFTR moveable limiter are given. One preliminary finding was that the hydrogen inventory in the wall is comparable to that in the limiter. A bound on the diffusivity of hydrogen in graphite was also suggested by these calculations. Pumping of this discharge by the Zr/Al getter system was also simulated, and was found to be negligible.

The paper describes the present (end of February 1985) status of the plasma confinement studies in the TFTR tokamak with emphasis on those with neutral beam injection (NBI). Recent improvements in the device capabilities have substantially extended operating parameters: B /sub T/ increased to 4.0 T, I /sub p/ to 2.0 MA, injection power (P /sub b/ ) to /sup 5/ MW with H/sup 0/ or D/sup 0/ beams, n /sub e/ to 5 x 10/sup 19/ m/sup -3/ and Z /sub eff/ reduced to 1.4. With ohmic heating (OH) alone, the previously established scaling for gross energy confinement time (T /sub E/ /sup 0/ n /sub e/ q) has been confirmed at higher I /sub p/ and B /sub T/ , and the maximum T /sub E/ of 0.4 sec has been achieved. With NBI at P /sub b/ substantially (by factor > 2) higher than P /sub OH/ , excellent power and particle accountability have been established. This suggest that the less-than-expected increase in stored energy with NBI is not due to problems of power delivery, but due to problems of confinement deterioration. T /sub E/ is observed to scale approximately as I /sub p/ P /sub b/more » /sup 0.5/ (independent of n /sub e/ ), consistent with previous L-mode scalings. With NBI we have achieved the maximum T /sub E/ of 0.2 s and the maximum T /sub i/ (o) of 4.4 keV in the normal operating regime, and even higher T /sub i/ (o) in the energetic-ion regime with low-n /sub e/ and low I /sub p/ operation.« less

A strong pumping effect was observed with plasma operation on the toroidal graphite bumper limiter in TFTR. The pumping effect was induced by conditioning the limiter with a short series (10-20) of low density deuterium or helium initiated discharges. The decay constant for gas fuelled Ohmic discharges was reduced from before conditioning to a minimum of after conditioning, which corresponds to a reduction in the global recycling coefficient from about 100% to less than 50%. Coincident with the low recycling conditions, low current neutral beam fuelled discharges showed global energy confinement times which were enhanced by a factor of two over the values obtained with an unconditioned limiter. Two models are proposed for the observed pumping effects: (1) a depletion model, based on pumping of hydrogenic species in the near-surface region of the limiter after depletion of the normally saturated surface layer by (carbon and helium) ion induced desorption; and (2) a co-deposition model, based on adsorption of hydrogenic species in carbon films formed by material sputtered from the limiter during conditioning.

The neutral beam injection experiment on TFTR has now been underway for over two years, during which time we have injected as much as about 20 MW of beam power at energies up to 110 keV. We have obtained qualitative evidence of direct current drive by the injected beam ions. Recently we have produced plasmas in which the energy confinement time, βp, neutron emission, stored energy (both total and electron), and ion temperature exhibit a pronounced enhancement over previous behavior. Under these conditions we have obtained preliminary indications that significant bootstrap current is being driven. Operation will continue with new sources with which we should be able to increase the beam pulse length fourfold.

The PDX tokamak provides an experimental facility for the direct comparison of various impurity control techniques under reactor-like conditions. Four neutral beam lines inject > 6 MW for 300 ms. Carbon rail limiter discharges have been used to test the effectiveness of perpendicular injection, but non-disruptive full power operation for > 100 ms is difficult without extensive conditioning. Initial tests of a toroidal bumper limiter indicate reduced power loading and roughly similar impurity levels compared to the carbon rail limiter discharges. Poloidal divertor discharges with up to 5 MW of injected power are cleaner than similar circular discharges, and the power is deposited in a remote divertor chamber. High density divertor operation indicates a reduction of impurity flow velocity in the divertor and enhanced recycling in the divertor region during neutral injection.

Carbon probes were exposed to the plasma edge in PLT at various distances from the plasma and to different numbers [1–3, 6, 12] of overlayed discharges. The type and areal density of impurities retained were determined using ion backscattering. The principal impurities were found to be O, Fe, Cr, Ti, and Cu which had fluxes of 14, 2.8, 0.8, 0.6 and 0.2 × 10 15 /cm 2 (±25%), respectively, near the radius of the wall. At the same position the deuterium flux was 2.2 ± 0.5 × 10 17 D/cm 2 s . Lateral spreading of the impurity deposition profile behind the probe slit allowed a lower limit to be placed on the impurity mean ion energy. Between the wall and radius of the ring limiter the impurity fluxes increased by a factor of ~2 compared to an increase in the D flux by a factor of ~5. The observed nonlinear impurity retention versus the number of discharges is consistent with the values determined for the impurity and D fluxes and the expected effects of sputtering. The observed large flux of iron at the radius of the ring limiter suggests that this may be a primary source of metal impurities in PLT.

Helium leak detection systems for magnetic fusion devices present several unique design problems because of the large dynamic range required and the high partial pressures of D2 encountered. We describe the design and operation of a He leak detector system for the PDX tokamak. The system consists of a differentially‐pumped, low resolution, He mass spectrometer which is interfaced to the foreline of one of the torus turbomolecular pump lines. The He detector has a minimum throughput sensitivity of 10−10 Torr⋅l/s, and the torus‐integrated system has been designed for detection of torus leaks over the range of 10−7 to 10 Torr⋅l/s. Minimum leak‐rates on the 38 m3 PDX vessel which have been quantified using this system are ∠3×10−8 Torr⋅l/s. When PDX is operated with D2 plasmas it is necessary to reduce the partial pressure of D2 by a factor of 100 within the mass spectrometer to maintain this sensitivity in the presence of the torus D2 outgassing. We have designed and incorporated a D2 filter which employs a Z...

This paper briefly reviews the use of ion beam techniques to study surface modification of samples exposed to tokamak plasma discharges. Primary emphasis is given to a continuing study of samples exposed at the first wall of the Princeton Large Torus (PLT) tokamak throughout its operating life. Both impurity flow and hydrogen recycle effects are investigated. Laboratory hydrogen implantations at 200-400 eV suggest a method to determine the mean ion temperature at the wall from the hydrogen saturation trapping dependences in Si samples.

Special limiter conditioning techniques using low density deuterium or helium discharges have produced enhanced plasma confinement in TFTR. Measurements with a rotatable collector probe have been made to increase our understanding of the boundary layer during these conditioning sequences. A set of silicon films behind slits facing in the ion and electron drift directions was exposed to four different D + and He 2+ discharge sequences. The amounts of deuterium and impurities trapped in the surface regions of the samples have been measured by different analytical techniques, including nuclear reaction analysis for retained deuterium, Rutherford backscattering spectroscopy for carbon and metals, and Auger electron spectroscopy for carbon, oxygen and metals. Up to 1.9 × 10 −17 cm −2 of deuterium was detected in codeposited carbon layers with D/C generally in the range of the bulk saturation limit. Radial profiles and ion drift/electron drift asymmetries are discussed.

Fueling requirements and impurity levels in neutral-beam-heated discharges in the PDX tokamak have been compared for plasmas formed with conventional graphite rail limiters, a particle scoop limiter, and an open or closed poloidal divertor. Gas flows necessary to obtain a given density are highest for diverted discharges and lowest for the scoop limiter. Hydrogen pellet injection provides an efficient alternative fueling technique, and a multiple pellet injector has produced high density discharges for an absorbed neutral beam power of up to 600 kW, above which higher speeds or more massive pellets are required for penetration to the plasma core. Power balance studies indicate that 30–40% of the total input power is radiated while ~15% is absorbed by the limiting surface, except in the open divertor case, where 60% flows to the neutralizer plate. In all operating configurations, Z eff usually rises at the onset of neutral beam injection. Both open divertor pl;asmas and those formed on a well conditioned water-cooled limiter have Z eff ⪅ 2 at the end of neutral injection. A definitive comparison of divertors and limiters for impurity control purposes requires longer beam pulses or higher power levels than available on present machines.

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 25–100°C. In situ measurements of kr, 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 kr 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 kr, at 25°C are in the range of (0.4−4)×10−27 cm4 s−1 assuming a value of the hydrogenic diffusivity of 2 × 10−12 cm2 s−1 at room temperature. No significant isotopic dependence was observed and the temperature dependence of kr is consistent with the literature value (0.5 eV) of the activation energy. The implications of this range of values of kr, for the estimation of the in-vessel tritium inventory following DT operation in TFTR are discussed.

The important impurities in ohmically heated TFTR plasmas are carbon and oxygen. The concentration of O is independent of the mass or charge of the fuel gas; C is slightly less abundant in D plasmas than in He plasmas. C is more abundant than O by a factor of 10 at low densities ( n c ≅ 2 × 10 19 m −3 ); at the high density limit ( n e ≅ 4 × 10 19 m −3 for H, n e ≅ 8 × 10 19 m −3 for He) the O concentration is equal to or slightly greater than that of C. The impurity concentration as a function of edge electron temperature is compared with a simple sputtering model, which shows the importance of C-C and O-C sputtering at the limiter in addition to fuel gas sputtering in explaining the observed carbon concentration.

Initial operation of the tokamak fusion test reactor has concentrated upon confinement studies of Ohmically heated hydrogen and deuterium plasmas. Total energy confinement times (${\ensuremath{\tau}}_{E}$) are 0.1-0.2 s for a line-average density range (${\overline{n}}_{e}$) of (1-2.5)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{m}}^{\ensuremath{-}3}$ with electron temperatures of ${T}_{e}(0)\ensuremath{\sim}1.2\ensuremath{-}2.2$ keV, ion temperatures of ${T}_{i}(0)\ensuremath{\sim}0.9\ensuremath{-}1.5$ keV, and ${Z}_{\mathrm{eff}}\ensuremath{\sim}3$. A comparison of Princeton large torus, poloidal divertor experiment, and tokamak fusion test reactor plasma confinement supports a dimension-cubed scaling law.

The characteristics of plasma operation on the axisymmetric inner toroidal limiter in TFTR are described. After conditioning, plasmas with low metal content and low zeff are obtained with this limiter. There is no substantial increase in zeff with total input power during neutral beam injection. Compared to operation on the outer blade limiter, additional gas is required to fuel plasmas on the inner limiter. Injection of D pellets increased the plasma density substantially and produced energy confinement times up to 0.5 s in ohmically heated plasmas. The four neutral beam lines have injected up to 13.5 MW total power into the plasma for 0.5 s with up to 90 kV accelerating voltage. The scaling of the plasma stored energy was studied as a function of the input power, plasma current and plasma density. In the range 1.4 to 2.2 MA, the overall and incremental confinement times for both the total and thermal stored energies increase with plasma current at fixed density. There appears to be a weak negative scaling of the total stored energy with density at high injection power.

New long-pulse ion sources have been employed to extend the neutral beam pulse on TFTR from 0.5 sec to 2.0 sec. This made it possible to study the long-term evolution of supershots at constant current and to perform experiments in which the plasma current was ramped up during the heating pulse. Experiments were conducted with co and counter injection as well as with nearly balanced injection of deuterium beams up to a total power of 20 MW. The best results, i.e., central ion temperatures Tio > 25 keV and neo τE Tio values of 3 × 1020 keV sec m-3, were obtained with nearly balanced injection. The central toroidal plasma rotation velocity scales in a linear-offset fashion with beam power and density. The scaling of the inferred global momentum confinement time with plasma parameters is inconsistent with the predictions of the neoclassical theory of gyroviscous damping. An interesting plasma regime with properties similar to the H-mode has been observed for limiter plasmas with edge qa just above 3 and 2.5.

Bare POCO AXF‐5Q graphite tiles were installed as the plasma‐facing surface of the Tokamak Fusion Test Reactor (TFTR) movable limiter for a series of ∼2700 high power discharges (600 with up to 6 MW neutral beams). During this operating phase, erosion and deposition processes modified the surface of the limiter. In the regions of the most intense plasma contact, which reached temperatures over 2400 °C, only small amounts of metallic impurities (

It is pointed out that the accidentally resonant charge exchange reaction, O/sup +/ + H/sup 0/ reverse arrows O/sup 0/ + H/sup +/, is an important mechanism for causing the loss of singly charged oxygen ions from oxygen contaminated hydrogen plasmas. Results of a Monte Carlo simulation are presented which show that the fraction of oxygen lost because of charge exchange exceeds /sup 1///sub 3/ when the parameters n/sub e/ approx. 10/sup 13/cm/sup -3/, n/sub H//sup o/ approx. 10/sup 11/cm/sup -3/ and T/sub e/ approx. 3 eV are attained.

Plasma particle pumping effects with carbon limiters have been seen in Tokamak Fusion Test Reactor (TFTR), Joint European Torus (JET), and Torus Experiment for Technology Oriented Research (TEXTOR). The pumping effects in TFTR were induced by conditioning the large‐area axisymmetric bumper limiter with low‐density, helium initiated discharges. As a result of the conditioning ∼150 Torr l of gas are removed from the carbon limiter tiles. The conditioning has reduced the particle recycling coefficient from near unity to values as low as 0.5. The low recycling conditions are not permanent. A pumping capacity of ∼100 Torr l has been determined from wall loading experiments. The total particle input has been compared to the total outgassing after a discharge for Ohmic plasmas in both helium and deuterium, and for deuterium neutral beam heated discharges. Ohmic deuterium plasmas have less outgassing than input. Ohmic helium plasmas have equal or greater outgassing compared to gas input. Neutral beam heated disch...

The application of glow discharge cleaning techniques for the surface conditioning of electron synchrotron vaccum vessels is discussed. Glow discharge techniques can be applied either as a surface treatment prior to vessel assembly or as an in‐situ conditioning technique. In either application glow discharge treatments with a variety of reactant gases (Ar, Ar/O2, He, and H2) are shown to reduce particle‐induced desorption coefficients, and thus reduce beam conditioning time. Specific methods of applying glow discharge treatments to electron synchrotrons are discussed, and a comparison of glow discharge with photoelectron‐induced beam conditioning is presented.

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.

During the first eighteen months of PLT operation, measurements on impurity-related effects have been obtained using Thomson scattering, visible, vacuum ultra-violet and soft x-ray spectroscopy, mass spectrometry, bolometry, and surface analysis. A brief review is presented of these diagnostic techniques and the present characterization of the discharge cleaning process and impurity related effects in high power discharges.

H/sub ..cap alpha../ emission is a useful indicator of hydrogen ionization since the energy required to excite hydrogen to the n = 3 level is approximately the same as to ionize the hydrogen atom. Other aspects of recycling, such as plasma scrape-off parameters, wall particle fluxes, and wall retention rates can be inferred from spatial distributions of H/sub ..cap alpha../ emission. There are two H/sub ..cap alpha../ diagnostics on TFTF observing the inner limiter: a relatively calibrated wide-angle TV camera, and the absolutely calibrated HAIFA diagnostic, both viewing the inner limiter. H/sub ..cap alpha../ observations by these instruments have been analyzed using the DEGAS neutral transport code. The core recycling rate, and ion and neutral particle limiter fluxes, have been deduced from the calculations. The results are very sensitive to the ion flux distribution, and therefore also provide information on the plasma scrape-off conditions. A survey was made of over 500 TFTR discharges. H/sub ..cap alpha../ emission appears to increase in proportion to /bar n//sub e//sup 2/. The ratio of core ionization to average H/sub ..cap alpha../ emission was calculated to be roughly constant over a large range of discharges. 9 refs., 4 figs., 1 tab.