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Lower hybrid current-drive experiments have been carried out on the PLT Tokamak. Steady currents up to 175 kA have been maintained for three seconds and 400 kA for 0.3 sec by the rf power alone. The principal current carrier appears to be a high energy (approx. 100 keV) electron tail, concentrated in the central 20 to 40 cm diameter core of the 80-cm PLT discharge. Effective current drive is observed only for anti n/sub e/ less than or equal to 8 x 10/sup 12/ cm/sup -3/. This limitation may be a wave propagation phenomenon and not a fundamental plasma physics effects.

To investigate the effects of a light-element limiter on plasma parameters, aluminium rail limiters interchangeable with Mo rails were installed top, bottom, and outside directions in the ST tokamak. The inside limiter remained a fixed Mo rail. Compared with discharges produced immediately before and after with the usual Mo limiters, the "aluminium" discharges showed an increase of Te (by factors of 1.4 – 2 near the centre) and of energy confinement (by factors of 2 to 3 in electron energy/power input, depending on time of observation). H2 and He discharges showed practically identical effects. In plasma composition, the Mo concentration dropped significantly, but the Fe concentration decreased only slighly if at all; the A1 concentration was about 3 – 5% (i.e. high compared to the heavier metals), whereas oxygen, about 4 to 8% to start with, dropped to insignificance, probably as a result of A1 evaporation. The Zeff from resistivity increased 20 – 30% although the resistance dropped because of the higher Te. The improved Te and energy confinement are thought to be the result of cumulative effects of more favourable radial current and power input distributions rather than direct energy losses by radiation.

Radiation and charge-exchange losses in the PLT tokamak are compared for discharges with Ohmic heating only (OH), and with additional heating by neutral beams (NB) or RF in the ion cyclotron frequency range (ICRF). Spectroscopic, bolometric and soft-X-ray diagnostics were used. The effects of discharge cleaning, vacuum wall gettering, and rate of gas inlet on radiation losses from OH plasmas and the correlation between radiation from plasma core and edge temperatures are discussed. – For discharges with neutral-beam injection the radiation dependence on type of injection (e.g. co-injection versus counter- and co- plus counter-injection) was investigated. Radial profiles of radiation loss were compared with profiles of power deposition. Although total radiation was in the range of 30–60% of total input power into relatively clean plasma, nevertheless only 10–20% of the total central input power to ions and electrons was radiated from the plasma core. The radiated power was increased mainly by increased influx of impurities, however, a fraction of this radiation was due to the change in charge-state distribution associated with charge-exchange recombination. – During ICRF heating radiation losses were higher than or comparable to those experienced during co- plus counter-injection at similar power levels. At these low power levels of ICRF heating the total radiated power was ~ 80% of auxiliary-heating power. Radiation losses changed somewhat less rapidly than linearly with ICRF power input up to the maximum available at the time of these measurements (0.65 MW).

Internal disruptions occurring in an inductively driven discharge are suppressed by lower hybrid waves that drive the plasma current in the same direction as the inductively driven current. Upon suppression, in the central region a finite-amplitude, non-growing m = 1 oscillation is observed and strong electron heating is measured.

The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

During a discharge a puff of hydrogen is admitted, sufficient to more than triple the plasma density, and the resulting changes in various plasma parameters are determined. The absolute densities of various wall and limiter (carbon) materials are found to decrease by a substantial fraction, probably as a result of lowered peripheral temperature. The radiation pattern deduced from spectroscopically determined plasma composition is in good quantitative agreement with direct bolometric measurements. In the interior of the discharge, radiation constitutes only a small part of the power input. Neither the radiated power nor the power input changes very markedly as a result of the density rise, since the effects of temperature and plasma composition changes tend to compensate each other.

This paper gives a brief description of the geometry and parameters of the PLT tokamak, reviews some of the last four years' results that are particularly relevant to plasma-boundary interactions, and then concentrates on two specific problems.

Experiments with the PLT 2.45 GHz system show that lower hybrid waves launched from the top of the tokamak torus are about half as effective for current drive as an identical spectrum of waves launched from the outside of the torus at intermediate densities (1 − 2 × 1013 cm−3)

Lower-hybrid current drive requires the generation of a high-energy electron tail anisotropic in velocity. Measurements of bremsstrahlung emission produced by this tail are compared with the calculated emission from reasonable model distributions. The physical basis and the sensitivity of this modelling process are described, and the plasma properties of current-driven discharges which can be derived from the model are discussed.

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.

Certain applications of laser blow‐off impurity injection, such as ion temperature measurement and spectral line identification, may require the ratio of injected ions‐to‐plasma electron density to exceed 0.001. To achieve these concentrations we have relied on injection of micron‐sized clusters as well as monatomic species. In these cases the injected impurity can no longer be regarded as a nonperturbing trace sourced at the plasma edge. We therefore have studied the variation of plasma parameters in PLT and PDX as a function of the amount of injected impurities, the species mix, and the type of impurity. We find that for ohmically‐heated plasmas, impurity injection can increase ?e more than 4×1012 cm−3 without causing disruptions. During Ge‐injection experiments where Δne/ne ∠0.05, Te initially rises ∠5% on axis and decreases ∠20% at the edge. The plasma current drops 1–3 % and the loop voltage and radiated power both double. Varying the amount of injected impurities changes the peaking and decay times ...

Helium discharges have been produced in the ST Tokamak with electron densities exceeding 1014 cm−3, with less than 1% oxygen (and other light impurities) and less than 0.05% metallic impurities. In such discharges, the effect of the impurities on the plasma resistivity is relatively unimportant, and, consequently, the usual ambiguities about the radial current distribution are unusually small. We find, consistently with previous experiments and in spite of various quantitative differences, that the plasma resistance is determined by Spitzer resistivity, that the rise of electron temperature is limited by the development of the q = 1 surface and that the electron kinetic energy confinement time is equal to the average particle confinement time. Energy loss by electrons constitutes about three quarters of the total and is roughly divided between line radiation and kinetic energy transport.

Hydrogen discharge cleaning of the PLT vacuum vessel has been studied by mass spectroscopy of desorbed gases and surface analysis of exposed samples. Several modes of vessel conditioning have been studied to date: (1) a high‐power discharge cleaning (PDC) mode, with a peak power density to the vessel wall Ps∠0.6 W/cm2 and a peak electron temperature Te∠100 eV; (2) low‐power (Taylor‐type) discharge cleaning (TDC) with Ps∠0.05 W/cm2 and Te?5 eV. The predominant residual gases produced during PDC are CH4 (1–5×10−6 Torr) and CO (1–10×10−7 Torr), whereas TDC produced primarily H2O (1–2×10−6 Torr) and CH4 (1‐10×10−7 Torr). The transient behavior of the residual gases is not simply related to pumping time constants, and provides evidence for the role of surface chemical reactions and diffusion on the formation of these gases. In situ surface analysis of hydrocarbon‐covered stainless steel has shown significant decreases in carbon coverage occurring after 103–104 pulses of either cleaning mode. Observed changes i...

The characteristics of neutral-beam-heated ASDEX discharges exhibiting either low (L)- or high (H)-confinement are described. H-mode discharges, which are by now observed over a wide range of operating conditions, show a spontaneous improvement in particle and energy confinement after a short L-phase at the beginning of neutral injection. H-discharges yield high βp values comparable to the aspect ratio A (βp≤2.65~0.65 A). The most important parameter for transition to the H-mode seems to be a high edge electron temperature: Te values of -600 eV only a few centimeters inside the separatrix with radial gradients of ~300 eV/cm are measured. This requirement of high edge temperatures explains the lack of success in obtaining the H-regime in limiter discharges. Numerical simulation of the broad ne and Te profiles typical of H-mode plasmas indicates a reduction in electron thermal diffusivity by a factor of typically 2 over the entire plasma. H-mode energy confinement times are found to scale linearly with current, but to have little dependence on plasma density and absorbed beam power (PNI≤3.4MW). The confinement is degraded by a fast growing mode localized at the plasma edge that may be identified as a kink or tearing mode driven unstable by the high current densities at the edge.

Order of magnitude improvements in the level and duration of current driven by lower hybrid waves have been achieved in the PLT tokamak. Steady currents up to 175 kA have been maintained for three seconds and 400 kA for 0.3 sec by the rf power alone. The principal current carrier appears to be a high energy (approx. 100 keV) electron component, concentrated in the central 20 to 40 cm diameter core of the 80 cm PLT discharge.

Concentrations and influx rates of oxygen and carbon ions have been determined in a variety of discharges in the PLT Tokamak. Oxygen, and to a lesser extent carbon are found to affect strongly the plasma behavior, mostly by changes in resistivity. The principal effect appears to be peripheral cooling by oxygen radiation, which changes the radial distribution of current density and temperature, and simultaneously inhibits the influx of high-Z limiter and wall material.

Dielectronic satellite spectra of Fe XXV near 1.8500 A have been observed from PLT (Princeton Large Torus) tokamak plasma discharges for electron temperatures in the range from 1.5 to 3 keV and an electron density of 2 x 10/sup 13/ cm/sup -3/. The electron temperature was independently determined from the electron cyclotron radiation emitted by the plasma. The quality of the spectra allows a detailed comparison with theoretical prediction, which is of importance in view of diagnostic applications.

Introduction of large amounts of neon into Ohmically heated deuterium discharges in the PLT tokamakl results in higher central electron temperature (Te(0) 3 keV) and values of electron energy containment time that are larger than in regular discharges at the same electron density (τEe = 44 ms at e = 2 × 1019 m−3). For steady-state discharges with high effective Z (~ 5–8) the conductance is larger than that predicted by neoclassical theory by as much as a factor of two. Transport rates of hydrogen-like and helium-like ions can be fairly well approximated by assuming a diffusion constant of between 0.4 and 1 m2s−1. Within experimental uncertainties the diffusion of hydrogen-like neon is the same for co- and counter-directed high-power neutral beams.

In ST Tokamak discharges with hydrogen or helium as the working gas the measured plasma resistance is usually found to be slight but consistently larger (1.1-1.5 times) than the resistance calculated from measured electron temperature and ionic charge radial profiles. The discrepancy has been ascribed tentatively to undetected high-Z material (Mo or W) from the aperture limiter. Recent experiments with neon, with effective ionic charge Z=9-10 in the discharge, do show good agreement between measured and calculated resistance. In other respects the behaviour of the neon discharge differs only slightly and in the expected direction from hydrogen or helium discharges: larger power input, higher electron temperature, and higher radiation losses. Also, measurement of the intensities of the recently discovered resonance lines of Mo XIII and Mo XIV do indicate a molybdenum concentration in the discharges of the ST Tokamak that is adequate to account for the above mentioned discrepancies in the resistance of H and He plasmas.

A detailed description of the time behaviour of a hydrogen discharge in the ST-Tokamak is based on measured radial electron temperature and density profiles at 12 different times, together with measurements of the Ohmic-heating current and voltage, the temporal, spatial, and spectral distributions of hydrogen light, the ion temperatures, and impurity concentrations. Early in the discharge the electron temperature profiles show evidence of a skin effect that develops on a time-scale of several milliseconds into a peaked profile of about 2.2 keV maximum. Thereafter the peak temperature stops growing and develops into a flat plateau, the width of which appears to be determined by the Kruskal-Shafranov limit. The average particle confinement time scales with , and reaches a maximum of 13-14 ms. The power balance is dominated by electron loss and re-cycling, rather than ion loss or radiation. The recycling process at the aperture limiter appears to involve sufficiently energetic neutral atoms to provide a fairly flat radial source function for particles, and hence to influence directly the development of the radial distribution of power input and energy balance.

Bands of radiation in the 30–70 A wavelength range, ascribed to unresolved resonance lines of various tungsten ions, have been observed in a variety of discharges in the PLT tokamak. The amount of tungsten present in the discharge appears to depend sensitively on the peripheral plasma temperature, and it may be dropped significantly by adding small amounts of neon or oxygen, and also by appropriate programming of hydrogen influx. In high-tungsten discharges the observed bands account for a large fraction (~ 0.5 or more) of the power input near the centre of the plasma.

An interpretation of the x‐ray bremsstrahlung emission by suprathermal electrons perpendicular to a magnetic field is given in terms of the parallel and perpendicular temperature of a three‐temperature distribution function. The slope (i.e., the temperature) of the distribution can be determined relatively well. Factor‐of‐two uncertainties remain for the number of electrons.

Two distinct conditions where the rf phase velocity is directed opposite to the ohmically induced electron drift have been studied experimentally: (1) rf current ramp-up (f/sub p/ > 0), where an induced electric field opposes the current increase; (2) rf current ramp-down, where waves are launched opposite the electron drift direction for a decaying plasma current. The time behavior of the electron tail is inferred from hard x-ray (30 to 750 keV) emission as a function of angle to B. In cases (1) and (2) we find that the emission amplitude in the reverse (opposite the LH phase velocity) direction increases throughout the rf pulse, while it is constant in the forward direction. This increase indicates that some high energy electrons are accelerated or are even running away in the direction opposite to the main rf produced tail. It also indicates for case (1) that the electric field in the plasma center has been reversed by the rf current drive.

Recent lower-hybrid current-drive experiments have clearly demonstrated that the current in a tokamak discharge can be maintained by rf drive alone. We have extended the operating regime of such plasmas to include ramping up of the current. We find that at densities of \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ approximately 20% of the launched rf power is converted to magnetic field energy.

For identical 90° settings the side coupler of the PLT 2.45 GHz, 1 MW lower hybrid system is more efficient at driving current than the top coupler at densities between 1 and 2×1013 cm−3. We interpret this to be a reflection of the toroidal n∥ upshift introduced by top launch. Measurements with 90°/60° (side/top) phase setting produces identical ΔI, as predicted theoretically from the ray tracing code.

In a tokamak fusion reactor, as in all tokamaks, the confinement of the reacting plasma will be provided by two orthogonal magnetic fields, the toroidal and poloidal fields. The toroidal field is generated by a system of coils external to the plasma. Whereas in most present day tokamaks these coils are resistive, in a reactor they will presumably be superconducting. For present devices, the duration of the toroidal field, at least at the highest fields these machines can produce, is limited by coil heating; for the reactor, on the other hand, the field will be on for unlimited duration. Because of the pulsed nature of the toroidal field, among other reasons, present tokamaks are pulsed devices, and thus rapid start-up is a necessary part of their operation.

Sawteeth oscillations, which exist in inductively driven discharges in tokamaks, are eliminated by launching additional lower hybrid wave power. At rf power levels beyond that required to eliminate the sawteeth oscillations, electron heating in the central region of the plasma is observed.

In order to investigate the effects of a light-element limiter on plasma parameters, aluminum rail limiters interchangeable with Mo rails were installed top, bottom, and outside directions in the ST tokamak. The inside limiter remained a fixed Mo rail. Compared with discharges produced immediately before and after with the usual Mo limiters, the ''aluminum'' discharges showed an increase of T/sub e/ (by factors of 1.4-2 near the center) and of energy confinement (by factors of 2 to 3 in el. energy/power input, depending on time of observation). H/sub 2/ and He discharges showed practically identical effects. In plasma composition, the Mo concentration dropped significantly, but Fe only slightly if at all; the Al concentration was about 3-5 percent (i.e., large compared to the heavier metals), whereas oxygen, about 4 to 8 percent to start with, dropped to insignificance, probably as a result of Al evaporation. The z/sub eff/ from resistivity increased 20-30 percent although the resistance dropped because of the higher T/sub e/. The improved T/sub e/ and energy confinement are thought to be the result of cumulative effects of more favorable radial current and power input distributions rather than direct energy losses by radiation.

Two different discharges in the ST Tokamak are described: A discharge dominated by high-Z (iron) impurity (Case A), and one dominated by low-Z (oxygen) impurity (Case B). The oxygen-dominated discharge shows higher electron temperature and longer containment time than the iron-dominated discharge. Suppression of Fe and Mo impurities has been achieved by pulsing in air for a few hundred shots. (auth)

The RF current ramp‐up efficiency versus RF power in tokamak is studied theoretically.(AIP)

Quantitative vacuum ultraviolet (UV) measurements (lambda greater than or equal to 100 A) of various ST-Tokamak discharges show that the atom density of metal impurities is about 0.1 to 1.5 percent of the electron density and that the atom density of oxygen is about 1 to 10 percent. These concentrations yield an effective ionic charge, anti Z, consistent with the measured current, voltage, and profiles of electron temperature. The radial distribution of high-Z material is obtained from x-ray measurements in the range 1 A less than or equal to lambda less than or equal to 4 A; from these one concludes that there is no extreme concentration of high-Z material in the center, but cannot exclude variations within a factor of 2. The x-ray spectrum is dominated by recombination; using bremsstrahlung and recombination calculations the value of anti Z derived from x- ray intensity is comparable with the value from conductivity. An instrument is described for making measurements in the extreme UV region (5 less than lambda less than 20 A). In this range strong emission has been observed (radiated power about 10 percent of power input in the center), which is attributed to line emission from heavy-metal (Fe ormore » Mo) ions which are produced only at an electron temperature above about 1200 eV; the very narrow radial profile of observed intensity is consistent with measured temperature and density profiles. Rough estimates of the concentration of heavy metals required to account for the radiated power are consistent with vacuum UV and x-ray data. (auth)« less

Steady currents up to 165 kA for 3.5 seconds and 420 kA for 0.3 seconds have been maintained by 800 MHz lower hybrid waves. For line-averaged densities up to 7 x 10/sup 12/ cm/sup -3/ the current is maintained with no input power from the ohmic heating transformer. The waves are launched with an array of six waveguides. Measurements of X rays and electron cyclotron radiation show that the rf power produces and maintains a suprathermal tail of electrons apparently independent of the number of fast electrons in the plasma prior to turning on the rf power. Measurements of current-drive efficiency and the electron tail provide direct evidence for a resonant wave-particle interaction. The radial profile of the rf-sustained current inferred from x-ray measurements is peaked in the center of the plasma and appears to obey the same q-value restraints as the inductively driven ohmic heating current. Current drive is observed to be accompanied always by radiation at frequencies greater than or equal to ..omega../sub ce/ and less than or equal to ..omega../sub pe/. The connection between this radiation and the current-drive mechanism is under study.

PLT discharges are categorized in terms of energy balance into three types: radiation dominated (RD); transport dominated (TD), and an intermediate type (TR). Control of plasma edge conditions is essential for the formation of these types. Measurements and calculations show the relative importance of radiation, transport, and electron-ion coupling in different regions of the discharge. Ion heating for neutral beam injection is found to be effective in all types. Electron heating by neutral beam injection is only consistently obtained for TD discharges.

Neutral beam injection experiments or PLT and PDX have been conducted over a wider range in parameter space than previously. On PLT H O beams have been injected into well-confined high toroidal field, high density Ohmic plasmas, giving n e (0) T Ee products during injection of up to 5 × 10 12 sec cm −3 . T Ee is found to rise slowly with increasing density in these experiments. Comparing these results with earlier (1979) discharges, which showed much lower heating efficiency, the importance of starting with a hot Ohmic plasma and a peaked density profile is striking. On PDX high power injection experiments over a range in plasma current have shown a significant variation with current of both ion heating and total stored plasma energy. Transport analysis of these results indicates that global confinement drops little when I p is varied from 480 to 320 kA, but as I p falls to 200 kA, T E deteriorates significantly. Attention has recently focused on low q operation, for the purpose of obtaining high beta plasmas. PLT and PDX have both been operated with cylindrical q in the range of 2.0 or below, with absorbed beam power of 1.8 MW (D O ). Operating in this regime, the two machines have obtained similar results, with PDX reaching a toroidal β of 1.3 − 1.4%, at B t = 12.6 kG. These results suggest that parallel and perpendicular neutral beam injection are about equally well suited tor low q operation. Strong sawtooth oscillations have been observed to travel to the plasma edge in PDX, but confinement is not substantially degraded relative to the higher toroidal field PDX results reported here.

Recent experimental results for ICRF heating in PLT are presented. For the two-ion regime in D-H or D-/sup 3/He plasmas minority H and /sup 3/He ions are found to absorb the rf power and transfer it to the deuterons and electrons in accordance with Fokker-Planck theory. The deuteron heating rate is approx. 3 eV x 10/sup 13/ cm/sup -3//kW for H and approx. 6 eV x 10/sup 13/ cm/sup -3//kW for /sup 3/He minorities. Neutron fluxes of approx. 3 x 10/sup 11/ sec/sup -1/ corresponding to a T/sub d/ approx. 2 keV (..delta..T/sub d/ approx. 1.2 keV) have been produced with P/sub rf/ approx. = 620 kW at anti n/sub e/ approx. = 2.9 x 10/sup 13/ cm/sup -3/. Neutron energy spectra and mass sensitive charge exchange spectra indicate Maxwellian deuteron distributions. In addition, D-/sup 3/He fusion reaction rates greater than or equal to 10/sup 12/ sec/sup -1/ have been produced by the energetic /sup 3/He ions. For the second harmonic regime, initial heating results for an H plasma at P/sub rf/ approx. = 140 kW are consistent with the Fokker-Planck theory and the bulk heating rate is comparable to that of D heating in the D-H minority regime.