The Big Dee device will be the first major tokamak to incorporate both divertor and pump limiter particle control techniques. The Big Dee is an upgrade of the Doublet III facility designed to reach reactor grade plasma parameters. The device, presently under construction, is designed to operate at plasma current of Ip ≤ 5 MA (3.5 MA if diverted), input power of 20 MW and pulse duration of several seconds with either divertor or pump limiters. The diverted plasmas have a nominal major radius of 167 cm, minor radius of 55 cm, and elongation of 2. The pump limiters have a radial stroke of 50 cm and are compatible with a variety of plasma cross-sections from full size highly elongated dee-shaped plasmas (R0 = 167 cm, a = 67 cm, k = 2) to circular plasmas of reduced minor and major radii.

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 m2·s−1. Within experimental uncertainties the diffusion of hydrogen-like neon is the same for co- and counter-directed high-power neutral beams.

Using a neutral-beam injection power of 3.4 M W, volume-averaged toroidal betas of up to ⟨βT⟩ = 4.5% have been obtained in low-toroidal-field, low-qψ, vertically elongated discharges in the Doublet III tokamak. This level of ⟨βT⟩ is above the minimum level required for a tokamak reactor, thus demonstrating that reactor level values of ⟨βT⟩ are possible in a tokamak device. The observed enhancement of ⟨βT⟩ with vertical elongation lends confidence in the design of future devices which rely on vertical elongation.

A general overview of the Doublet III laser spectroscopy program is presented. This program includes tokamak diagnostic experiments and laboratory development work. Measurements of titanium and deuterium in Doublet III are presented along with recent laboratory results. Sputtering of the vacuum walls by energetic charge-exchange neutrals from the plasma center appears to be responsible for the observed Ti impurities in Doublet III. Resonance fluorescence scattering from deuterium atoms near the plasma center has been observed using a broadband dye laser and a narrow band detector. The feasibility of determining ion temperatures using this technique is discussed. Using the measured deuterium densities we have calculated a local electron particle confinement time. Laboratory measurements of carbon and oxygen densities using two-photon laser excitation and of Fe i speed distributions (temperatures) using a laser double-pulse excitation technique are shown to be applicable to the tokamak conditions of Doublet III.

Measurements of edge-region TiI impurity densities and central-region deuterium (n = 2) densities have been obtained in the Doublet III tokamak using laser-induced fluorescence spectroscopy. TiI densities range from 1012 to 1013 m−3 in ohmically heated D2 discharges and a factor of 2–5 times less in H2 discharges. Edge-region TiI equilibration times following neutral-beam injection are in good agreement with central-region ion confinement times suggesting that energetic charge-exchange neutrals from the plasma center are responsible for TiI impurities. Resonance fluorescence scattering from deuterium atoms in the plasma center has been observed using a broadband (8 nm) dye laser source and a narrow band detector to investigate the feasibility of determining ion temperatures.

Summary The first Neutral Injection System Beamline became operational on the Doublet III tokamak in September 1981. Power delivery tests and system debugging proceeded through December and initial experiments on plasma heating were started in January 1982. This first beamline has been running well and as of February 1982 is routinely delivering about 2.0 MW of neutral hydrogen power to various circular and elongated dee-shaped deuterium plasmas. The second beamline is now being brought on-line and we expect to have 5–6 MW of heating power available for definitive β studies in mid 1982. We find that the change in the total plasma stored energy caused by the beams is essentially independent of toroidal field and plasma current, but increases weakly with density. The total stored energy increases with current and density, but is independent of toroidal field. A volume-average βT= 2.2% is obtained in low toroidal field discharges.

Temporally and spatially resolved profiles of the PDX soft x-ray spectra have been measured during single tokamak pulses of circular divertor plasmas with a recently developed pulse-height analyzer. This detection system incorporates an array of five vertically displaced sets of lithium-drifted silicon detectors, each consisting of three independent channels optimized for rapid data collection in adjacent energy regions. Simultaneous measurements of x-ray emission integrated along five chords of the plasma cross section can, thereby, be achieved. Abel inversion of these data yields temporally resolved radial profiles of the local electron temperature from the slope of the continuum, concentrations of high-Z impurities from the characteristic line intensities, and a measure of Zeff from the continuum intensity. The techniques of x-ray pulse-height analysis, with illustrations featuring the results from the initial PDX circular plasma experiments, are discussed in detail. In addition, comparisons between circular and divertor plasmas on PDX, derived from the x-ray measurements, are also presented.

Analysis of the expanded boundary divertor in Doublet III reveals a medium density operating regime (n̄e ≲ 4 × 1013 cm-3), in which (1) the divertor density ne(Div) rises with the cube of the main plasma line average density, n̄e, and (2) the neutral pressure at the exhaust end of the divertor has a quadratic dependence on n̄e (with divertor plasma temperatures ≲ 15 eV). In this regime, injection of argon into the discharge results in a steady state concentration of argon, which is two orders of magnitude higher in the divertor plasma than in the main discharge. This trapping effect suggests that the argon concentration in the divertor region may be useful as an externally controllable parameter; that is, injected argon effectively cools the plasma boundary, yet is not deleterious to the plasma as a whole. A model which successfully describes the behavior of the edge and divertor plasmas in ohmically heated discharges suggests that these desirable trapping and radiative features may be maintained in the high powered neutral beam heating phase.

Fusion neutron emission of 1.5 × 1014 neutrons · s−1 and 2 × 1013 neutrons/pulse has been observed for PLT deuterium discharges with up to 2.5 MW of deuterium neutral-beam injection. The neutron time evolution and magnitude are consistent with theoretical calculations of the fusion reactions caused by energetic injected ions which are confined and slow down classically. The factor-of-two accuracy in the absolute neutron calibration is the major uncertainty in the comparison with theory. Neutron sawtooth oscillations (⪅ 3%) are observed which can also be explained classically.

Complex, transient, spatially inhomogeneous tokamak plasmas require careful diagnosis. As the reactor regime is approched, soft x rays become more important as a versatile diagnostic tool and an energy‐loss mechanism. Continuum emission provides a measure of electron temperatures and light impurity content. Impurity lines serve as a probe for ion and electron temperature, impurity behavior, and radiative cooling. The entire spectrum yields vital information on instabilities and disruptions. The importance of impurities is illustrated by the extensive efforts toward understanding impurity production, effects, and control. Minute heavy impurity concentrations can prevent reactor ignition. Si(Li)‐detector arrays give a broad overview of continuum and line x‐ray emission (.3−50 keV) with moderate energy (200 eV) and time (50 ms) resolution. Bragg crystal and grating spectrometers provide detailed informaion on impurity lines with moderate to excellent (E/ΔE=100−23,000) resolving power and 1−50 ms time resolution. Imaging detector arrays measure rapid (∼10 μs) fluctuations due to MHD instabilities and probe impurity behavior and radiative cooling. Future tokamaks require more diagnostic channels to avoid spatial scanning; higher throughput for fast, single‐shot diagnosis; increased spectral information per sample period via fast scanning or use of multi‐element detectors with dispersive elements; and radiation shielding and hardening of detectors.

In this paper we report on the observation of characteristic "steps" in the continuum spectrum of high-temperature tokamak plasmas associated with recombination radiation from impurity ions. During special argon-seeded discharges on the Princeton Large Torus tokamak the x-ray spectrum exhibited large enhancements over the bremsstrahlung continuum beginning with energies of 4.1 keV. This corresponds to the radiative capture of free electrons by hydrogenlike argon into the ground state of heliumlike argon. The size of these edges increased to unexpectedly large values with minor radius (decreasing electron temperature), consistent with a departure of the hydrogenlike species from the predictions of corona equilibrium. Hence, the coronal equilibrium equations must be modified to account for the radial transport of argon. A simple particle diffusion model is proposed, with the Ar XVIII radial profiles evaluated from the size of the recombination edges. For the case of moderate density ($〈{n}_{e}〉\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) and temperature (${T}_{e}(0)\ensuremath{\sim}1.5$ keV) discharges the outward radial transport velocity is found to be approximately 10 m/sec.

In the four years of operation of the PLT tokamak (1976–1979), plasma parameters and machine operating conditions have been importantly affected by changes in the choice of limiter material and by the mode of vacuum vessel conditioning. Tungsten, stainless steel, and graphite limiters have been used. The fractional power lost by radiation and the source distribution of the radiation vary strongly with limiter material, wall treatment, and gas programming. With tungsten limiters, radiation from partially ionized tungsten atoms can strongly limit the temperature in the center of the plasma; with graphite limiters, radiation is primarily from carbon and oxygen in the outer cold region and not so much from the center, since in the hot core these atoms are totally ionized. With both tungsten and stainless steel limiters intermediate states exist which have rather uniform radiation source distributions; the characteristics of these distributions are affected by gas programming and by wall conditioning.

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.

Experiments conducted on the PLT tokamak have shown that both plasma-limiter interaction and the gross energy confinement time are functions of the gas influx during the discharge. By suitably controlling the gas influx, it is possible to contract the current channel, decrease impurity radiation from the core of the discharge, and increase the gross energy confinement time, whether the aperture limiters are of tungsten, stainless steel or carbon.