R.A. Moyer

A fast scanning probe for DIII–D

A fast reciprocating probe has been developed for DIII–D which can penetrate the separatrix during H mode with up to 5 MW of NBI heating. The probe has been designed to carry various sensor tips into the scrape‐off layer at a velocity of 3 m/s and dwell motionless for a programmed period of time. The driving force is provided by a pneumatic cylinder charged with helium to facilitate greater mass flow. The first series of experiments have been done using a Langmuir probe head with five graphite tips to measure radial profiles of ne, Te, φf, ne, and φf. The amplitude and phase of the fluctuating quantities are measured by using specially constructed vacuum compatible 5‐kV coaxial transmission lines which allow us to extend the measurements into the MHz range. TTZ ceramic bearings and fast stroke bellows were also specially designed for the DIII–D probe. Initial measurements will be presented.

Observation of infrared synchrotron radiation from tokamak runaway electrons in TEXTOR

During runaway discharges in TEXTOR, intense infrared (IR) radiation is-emitted in the electron flow direction. This can only be explained by synchrotron radiation of fast electrons. The observed spectral dependence is consistent with electrons of 25-30 MeV energy; the intensity corresponds to about 1016 electrons or to an electrical current of 40 kA. From the spatial structure of the observed IR pattern, new insight into the spatial distribution of the runaway electrons and their perpendicular momentum can be gained. The runaway electrons populate a torus with a diameter of 0.5-0.6 m, which is slightly larger than the plasma radius; the perpendicular momentum is determined from the vertical extent of the IR pattern and amounts to about 5 m0c. The transformation rate of electrons to runaways can be estimated from the time delay of the IR signal as 2 × 10−4 s−1; this agrees with theoretical expectations derived from the ratio of the electrical field strength to the critical field strength. In TEXTOR, runaways are confined up to energies of 50 MeV, which is just below the limit where a phase should exist in which runaways radiate as much energy as they gain per turn.

Experimental evidence for self-organized criticality in tokamak plasma turbulence

Abstract Measurements of plasma turbulence spectra and particle flux from the DIII-D tokamak exhibit significant agreement with predictions of self-organized criticality (SOC) modeling. Power spectra of density n , potential gf, and particle flux Γ, are observed to have three regions of frequency dependence: f0, f−1 and f−4. In addition, the particle flux probability distribution displays a Γ−1 scaling over two decades in Γ. These results provide the first evidence that the plasma is in a state consistent with SOC models and place a constraint on plasma transport models.

Development of a radiative divertor for DIII-D

Abstract We have used experiments and modeling to develop a new radiative divertor configuration for DIII-D. Gas puffing experiments with the existing open divertor have shown the creation of a localized (∼ 10 cm diameter) radiation zone which results in substantial reduction (3–10) in the divertor heat flux while τ E remains ∼ 2 times ITER-89P scaling. However, n e increases with D 2 puffing, and Z eff increases with neon puffing. Divertor structures are required to minimize the effects on the core plasma. The UEDGE fluid code, benchmarked with DIII-D data, and the DEGAS neutrals transport code are used to estimate the effectiveness of divertor configurations; slots reduce the core ionization more than baffles. The overall divertor shape is set by confinement studies which indicate that high triangularity (δ ≈ 0.8) is important for high τ E VH-modes. Results from engineering feasibility studies, including diagnostic access, will be presented.

Electrostatic biasing of the ALT-II pump limiter

Electrostatic biasing experiments using the Advanced Limiter Test (ALT-II) pump limiter in the TEXTOR tokamak have been carried out with the dual goals of: (a) improving the core plasma confinement in the tokamak and (b) enhancing the performance of the pump limiter. The fully toroidal belt limiter has been biased during both ohmic and neutral beam heated discharges. Both polarities of bias have been applied up to a maximum of +or-500 V with no evidence of impurity accumulation in the central plasma, although applying either polarity of bias to the limiter increases recycling from both the limiter face and the vacuum vessel liner. This in turn results in an increase of the central density. The application of a negative bias to the limiter produces a barrier to radial particle transport in the region between the limiter and the wall. This barrier is not observed in either the no bias or the positive bias case. Neither polarity of limiter bias affects the central plasma energy confinement, apparently because the electric field structure producing the radial barrier is outside the limiter tangency radius. The enhanced recycling, coupled with high edge density, increases the radiated power from the plasma edge and may lower the power flux to the plasma facing surface of the limiter blade. In the case of positive limiter biasing, the pressure in the pumped plasma collection scoops of the limiter increases by approximately 20%, corresponding to a similar increase in the particle removal rare of the pump limiter. The increase in the particle removal rate appears to result from a lower edge electron temperature. This is consistent with the observation of an increase in edge radiated power

ALT-II toroidal belt pump limiter performance in TEXTOR☆

The Advanced Limiter Test (ALT-II) is a toroidal belt pump limiter in the TEXTOR tokamak. ALT-II is composed of 8 blade segments which form an axisymmetric toroidal belt of 3.4 m2 exposed surface area, located on the outside of the torus at 45° below the horizontal midplane. Ohmic plasma operation with ALT-II as the main limiter is characterized by a line-averaged density range of 5 × 1012 to 5.5 × 1013cm−3 at BT = 2 T and IP = 340 kA, Zeff = 1.1 to 2 and typically 40 to 95% of the power radiated depending on the plasma density. ICRH heating of the plasma with up to 2.6 MW of incident power has been achieved, which modifies the scrape-off layer (SOL) and the pump limiter performance. The recycling coefficient in TEXTOR is normally close to one, but helium RG conditioning and baking of the limiter at 400 ° C is found to lower the recycling coefficient to 0.8 for the order of 10 shots. Measurements by arrays of probes in the SOL and thermocouples in the limiter tiles indicate the flow to the limiter is toroidally symmetric (taking field ripple into account) and poloidally asymmetric. The asymmetries result in different power and particle fluxes to the ion and electron drift sides of the limiter. The density and power scrape-off lengths are on the order of 1 cm and significantly longer on the outside of the torus (electron drift side). In spite of the flow asymmetry favoring the ion drift side near the tangency point, the longer e-folding lengths on the electron side in the SOL result in equal or higher particle collection by the electron side. The probe arrays indicate that during ohmic heating a total of 15 to 20% of the core efflux is incident on the neutralizer plates located in scoops beneath the blades. More particles are collected during ICRH auxiliary heating due to changes in the SOL profiles and shorter particle confinement times. Based on particle re moval experiments with pumping on one blade, the total exhaust efficiency of the limiter if pumped at all eight blades is 5 to 10%.

The effect of ELMs on edge plasma scaling in DIII-D

In this paper we report results of scaling studies aimed at determining how the divertor conditions vary with plasma current, toroidal field, and neutral beam heating power in H-mode discharges with ELMs in the DIII-D tokamak. We find that ELMs produce relatively more direct particle losses (50% or more of the total) than energy losses (≤20%). The time-average peak divertor heat flux in these plasmas is found to scale as d α ( P NBI I p )( B p,mp / B p,div ). The linear power dependence suggests that the plasma sheath at the targets is primarily responsible for limiting the parallel energy flow, while the I p variation may mean that the radial energy transport in the SOL decreases with increasing plasma current, just as it does in the core.

E×B transport in the DIII-D boundary plasma

We have measured the electrostatic turbulence and associated particle transport in the DIII-D boundary plasma using a fast reciprocating Langmuir probe array located on the outboard midplane. Both the normalized rms fluctuation levels (density and floating potential) and the fluctuation-driven particle transport are altered by the L-H transition in the SOL. At the separatrix, the density fluctuation level is reduced a factor of 2, consistent with reflectometry results. There is a corresponding decrease in the turbulent particle flux. Deeper in the SOL, the turbulent particle transport in H-mode exceeds the L-mode value. The perpendicular diffusion coefficient D ⊥ and particle confinement time τ p have been estimated, assuming that the transport is purely turbulent and uniform on a flux surface. We find D ⊥ =0.7 D B (L) and 0.04 D B (ELM-free H), and τ p =54 ms (L) and 480 ms (ELM-free H).

Scrape-off layer measurements in DIII-D

In this paper, scrape-off layer measurements in DIII-D are presented as a function of the main discharge plasma parameters. A systematic study is under way to understand and predict the behavior of the edge and divertor plasma in DIII-D and this scaling behavior will be crucial for the design of ITER. To facilitate the studies, a fast reciprocating Langmuir probe incorporating five graphite tips was installed at the midplane of DIII-D which has the capability of performing multiple plunges 1 cm inside the separatrix during 5 MW of NBI. Density and temperature profiles in the midplane (reciprocating probe), near the top (Thomson scattering) and at the lower divertor plate (fixed Langmuir probe array) are compared by mapping the measurements into magnetic flux coordinates. Local pressure measurements are compared on different parts of a flux surface. The three different local measurements also indicate the spatial evolution of plasma conditions as plasma approaches the divertor plate. Ohmic and L-mode discharges exhibit similar (exponential) density and temperature decay in the scrape-off layer. H-mode discharges, however, display a faster spatial decay reflecting at least a factor of 3 decrease in the perpendicular diffusion coefficient. Consistency of the magnitude and scaling behavior of the edge profile parameters with models of the scrape-off layer is examined.

Helium exhaust studies with the ALT-II pump limiter in TEXTOR

Abstract In TEXTOR helium removal experiments with the pump-limiter ALT-II have started. To simulate the presence of helium ash in the plasma, helium is injected into the discharge (e.g. at t = 0.7s) as a short pulse of Δt = 20ms. It is found that the He is removed from the discharge in an e-folding time of about half a second for neutral beam heated plasmas and in an e-folding time of about 1.5 s in an OH plasma. The exhaust efficiency of helium amounts to about 8% and is close to the one for deuterium. The fuelling efficiency for the injected helium is found to be in the range of 50–100%; the remaining part seems to be stored in the TEXTOR walls. An estimate of the surface density leads to a value of several times 1013 cm−2. This helium can easily be liberated in succeeding discharges and can be removed efficiently when ALT is pumping.