F. Tenney

Attainment of high confinement in neutral beam heated divertor discharges in the PDX tokamak

Abstract The PDX divertor configuration has recently been converted from an open to a closed geometry to inhibit the return of neutral gas from the divertor region to the main chamber. Since then, operation in a regime with high energy confinement in neutral beam heated discharges (ASDEX H-mode) has been routine over a wide range of operating conditions. These H-mode discharges are characterized by a sudden drop in divertor density and H α emission and a spontaneous rise in main chamber plasma density during neutral beam injection. The confinement time is found to scale nearly linearly with plasma current, but can be degraded due either to the presence of edge instabilities or heavy gas puffing. Detailed Thomson scattering temperature profiles show high values of T c near the plasma edge (∼ 450 eV) with sharp radial gradients (∼ 400 eV/cm) near the separatrix. Density profiles are broad and also exhibit steep gradients close to the separatrix.

Impurity levels and power loading in the pdx tokamak with high power neutral beam injection

Abstract 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.

Initial limiter and getter operation in TFTR

Abstract During the recent ohmic heating experiments on TFTR, the movable limiter array, preliminary inner bumper limiter, and prototype ZrAl alloy bulk getter surface pumping system were brought into operation. This paper summarizes the operational experience and plasma characteristics obtained with these components. The near-term upgrades of these systems are also discussed.

Initial results from the scoop limiter experiment in PDX

Abstract 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.

Tokamak Fusion Test Reactor horizontal high‐resolution Bragg x‐ray spectrometer

A bent quartz‐crystal spectrometer of the Johann type with a spectral resolution of λ/Δλ= 10000–25000 is used on the Tokamak Fusion Test Reactor (TFTR) to determine central plasma parameters from the spectra of helium‐like and lithium‐like metal impurity ions (Ti, Cr, Fe, and Ni). The spectra are observed along a central radial chord and are recorded by a position‐sensitive multiwire proportional counter with a spatial resolution of 250 μ. Standard delay‐line time‐difference readout is employed. The data are histogrammed and stored in 64 K of memory providing 128 time groups of 512‐channel spectra. The central ion temperature and the toroidal plasma rotation are inferred from the Doppler broadening and Doppler shift of the Kα lines. The central electron temperature, the distribution of ionization states, and dielectronic recombination rates are obtained from satellite‐to‐resonance line ratios. The performance of the spectrometer is demonstrated by measurements of the Ti xxi Kα radiation.

Tokamak Fusion Test Reactor prototype x‐ray pulse‐height analyzer diagnostic

The x‐ray pulse‐height analysis (PHA) diagnostic uses a liquid‐nitrogen cooled array of 5 Si(Li) and one HpGe detectors to do time‐resolved (5–100 ms) x‐ray spectroscopy of the central horizontal chord of the Tokamak Fusion Test Reactor (TFTR) plasmas in the 1–50‐keV range. Central electron temperature Te and concentration of medium and low‐Z impurities are derived from the spectra. Remotely selectable absorber‐foil arrays provide selection of the energy range. Fixed and movable aperture arrays allow approximate equalization of count rates in different energy bands and extend dynamic range. Amplifier pulse shapes are approximately triangular. Main amplifier peaking time is 4 μs yielding 230‐eV FWHM at 5.9 keV. Pileup inspection times are selectable at 0.13, 0.4, or 0.9 μs. Throughput is up to 42 kHz. The PHA has been used to study temperature and impurities over a wide range of TFTR operational parameters. Dramatic variations in metal impurities with density, plasma current, and major radius have been obs...

Particle fueling and impurity control in PDX

Abstract 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.

Fast multichannel electron temperature diagnostic for TFTR using x‐ray imaging

The TFTR x‐ray imaging system (XIS) is an array of 64‐Si surface barrier diodes which image the plasma poloidally. Special absorber foils have been installed to permit measurement of electron temperature Te with <100‐μs time resolution along 10–30 chords. The technique uses the ratio of x‐ray fluxes transmitted through two different foils, which depend mainly on Te. Simulations show that strong line radiation can change this ratio. To correct for these effects, special beryllium–scandium filters are employed to select the line‐free region between 2 and 4.5 keV. Separate filter pairs allow correction for strong L line radiation as well as Ti or Ni Kα emission. The Te determination is based on simulations. Comparison of results with Te values from the electron–cyclotron emission and x‐ray pulse height analysis diagnostics is presented.

Preliminary results of electron cyclotron heating experiments on the PDX Tokamak

ECH experiments on PDX have been carried out with two 60GHz pulsed gyrotrons at about l00kW each. The electron cyclotron (EC) waves were launched in narrow beams from both outside and inside the plasma torus. In the best heating results obtained, Thomson scattering data recorded a central temperature increase from ≤1.5keV to ≤2.5keV at about 1O13cm-3 density with approximately 80kW outside ordinary mode heating.

Measurement of the wall radiation in soft x‐ray region in PDX

A detector setup with three LN‐cooled Si(Li) diodes is used to measure soft x‐ray spectra (0.8–20 keV) emitted from the inside walls of the PDX vessel during the plasma discharge. The setup is part of a pulse‐height‐analysis system, which is used to measure the plasma and wall radiation simultaneously at five different radial positions. The wall and the plasma radiation are measured under different plasma conditions (e.g., OH and neutral beam heating). The wall radiation is very much increased during the neutral beam heating, with an enhancement factor of at least 10 over the OH wall radiation. Since we measure the plasma and the wall radiation at the same time, the measurements allow the conclusion that the wall radiation can be attributed essentially to fluorescence (line radiation, e.g., Ti‐Kα) and scattering (continuum part of the wall radiation spectrum). The fluorescence and the scattering are both caused by soft x‐ray radiation flux coming from the plasma. There seems to be no need to invoke other,...