Ohmic Phase Research Articles

Heat pulse analysis in JET limiter and X-point plasmas

Measurements of the electron heat diffusivity χ of the JET plasma in a wide variety of discharge conditions are reported. χ is deduced from heat pulse propagation analysis, using the sawtooth collapse as the perturbation of the temperature profile. The measurements of the electron temperature are performed with a 12-channel ECE polychromator. In limiter discharges, scans of plasma current, density, input power and Zeff have been studied. It is shown that χ is independent of density and input power, while a clear dependence on Zeff and on the electron temperature Te is demonstrated. The scaling provides a good fit to the JET data. xhp denotes the normalized radius where χ is measured. This radius is coupled to qa. As a result, the xhp-dependence only parameterizes the combined effect of the plasma current and the profile shapes of temperature, density, safety factor, etc. Various transport models are discussed in the light of this scaling. Also, χ measurements in the Ohmic, L-mode and H-mode phases of X-point plasmas are reported. The χ values are on average 20% higher in the H-mode than in the Ohmic phase. In one discharge where χ was measured in all three phases, no significant changes could be demonstrated.

Electron cyclotron wave propagation and absorption in the Compact Ignition Tokamak

The possibility of heating the Compact Ignition Tokamak (CIT) with electron cyclotron power at 280 GHz (fundamental cyclotron resonance) is examined by simple analytic estimates and by detailed computational means. Efficient single-pass absorption of the ordinary polarization is accomplished by scanning the angle of injection of the microwave power from 60° to 90° relative to the toroidal magnetic field direction as the field intensity is ramped from 7 or 8 T to 10 T and the volume average beta increases from 0.4% to 2.8%. Efficient absorption occurs for the ranges of central density of (2-8) × 1020 m−3 and central temperatures of 5-20 keV. Thus, electron cyclotron resonance heating, using either gyrotrons or free-electron lasers, offers a reasonable heating scenario from the Ohmic phase to the ignition phase, even with a single-frequency radiofrequency source.

Turbulent heating experiment in the STOR-1M tokamak

In this paper the STOR-1M tokamak and its diagnostics are described. The plasma behavior before, during, and after turbulent heating is also discussed. Anomalously rapid current penetration has been observed during both the ohmic current-rise phase and the turbulent heating phase. The critical streaming parameter for the onset of current driven turbulence has been measured. Efficient electron and ion heating during turbulent heating has been observed. Plasma confinement properties show significant improvement, following the turbulent heating pulse, for a period much longer than the duration of the turbulent heating pulse.

Power balance analysis of ion Bernstein wave heating experiments in the Alcator C tokamak

Energy and particle transport processes in Alcator C [Phys. Rev. Lett. 60, 298, (1988)] discharges heated by directly launched ion Bernstein waves (IBW) are investigated using the onetwo transport code [Nucl. Fusion 26, 329 (1986)]. The dependence of the observed ion heating rate (ΔTH/Prf) on plasma density is shown to result mainly from the inherent ion energy confinement that is characteristic of these discharges in the Ohmic phase and not on IBW propagation and absorption characteristics. The inferred value of the Ohmic ion thermal conduction exhibits an increasing anomaly with increasing plasma density. At a plasma density of n̄e ≲1×1020 m−3, the inferred ion thermal conduction is nearly equal to the Chang–Hinton neoclassical prediction. However, at a plasma density of n̄e ≳2×1020 m−3, the inferred ion thermal conduction increases to about 5–8 times the Chang–Hinton neoclassical value. This increasing anomaly, which may result from the ion temperature gradient driven instability, can essentially account for the observed ion heating rate behavior during IBW heating. The effect of edge turbulence on IBW propagation is modeled using a Monte Carlo direct sampling simulation. The result is mainly a radial broadening of the calculated power deposition profile with increasing plasma density.

Reduction of thermal expansion in Z-pinches by electron beam assisted magnetic field generation

Weak radial expansion of a Z-pinch plasma column during its strong initial ohmic heating phase is expected when the generation of a confining magnetic field is assisted by a correctly formed electron beam pulse. Appropriate one-dimensional magnetohydrodynamic equations are numerically solved, and the observed increase of plasma radius as a function of time for various discharge parameters is compared to a normal Z-pinch discharge initiation.

Transport analysis of JT-60 plasma with the drift wave turbulence model

Characteristics of the JT-60 plasma in both the Ohmic heating phase and the neutral beam heating phase have been studied numerically by using a one-dimensional tokamak transport code, with the thermal conductivities based on the drift wave turbulence (DWT) model. The numerical results for the central electron temperature Te(0) and the energy confinement time τE show good agreement with the experimental data in the medium line averaged electron density range n̄e ∼ 4 × 1019 m−3, for both Ohmic heating and neutral beam'heating cases. The absorbed power degradation of is also reproduced with this model. On the other hand, the DWT model underestimates the plasma temperatures at low line averaged electron density, especially in the neutral beam heating phase, because of the strong dependence of the thermal conductivity on the electron temperature, and high temperature plasmas cannot be reproduced. This leads to a discrepancy in the n̄e dependence of τE between experiment and calculation. The current dependence of τE is not reproduced either, even when the Venetian blind model is used.

Probe measurements of TFTR edge plasmas at neutral beam heating powers up to 20 MW

This paper presents radial profiles in the scrape-off layer of neutral beam heated plasmas in TFTR at powers up to 20 MW. The measurements have been made with, among other, a novel “fast probe” which moves quickly through the scrape-off during the discharges to provide radial profiles of electron density, electron temperature, and power flux parallel to field lines. The dependence of the edge on changes in plasma current and density during beam heating has been measured. A number of heating scenarios have been investigated, including current ramping, power ramping, and variation in beam injection direction. Scrape-off scale lengths of density, temperature and power during ohmic and beam phases are found to be similar: λ n = 4.5 cm, λ T = 5 cm, λ P = 2.5 cm . The power to the ion side is consistently a factor 2–4 higher than that to the electron side, and there are differences in scale-length of up to 1 cm between the two sides. At low core electron densities, the edge electron density during counter-injection is found to exceed that during co-injection by more than a factor of ten.

The effect of ecrh on the dite plasma boundary

During the past year electron cyclotron resonance heating (ECRH) has been applied for the first time to the DITE tokamak. A new edge diagnostic, operable both as a Langmuir probe and a retarding field analyser, has been used extensively to investigate the variation of plasma boundary parameters during ECRH heated discharges. Arrays of Langmuir probes embedded in a pumped and inner bumper limiter have also been operated during additionally heated discharges. Measurements have been made over a wide range of ECRH input powers, as a function of global plasma density and for many values of the toroidal magnetic field. The behaviour of the edge plasma in response to the additional heating is observed to be strongly dependent on the major radial position at which resonant absorption of the ECRH pulse occurs. Radial profiles of electron density and temperature have been obtained, together with observations of the time response of these edge parameters to the ECRH pulse. Characteristic lengths in the boundary are observed to remain approximately unchanged throughout heated discharges. In general, the edge particle flux exhibits a well defined, reproducible behaviour, increasing during ECRH to ~1.5 times its ohmic value with rise times ≤ 2 ms. The percentage increase in the edge temperature is smaller with rise times ≤ 10 ms. Reasonable global power balance is obtained using data from the limiter Langmuir probes in both ohmic and ECRH phases of a typical discharge.

Nonthermal X-Ray Spectra from the JIPPT-IIU Tokamak

Titanium full Kα spectra have been measured using a wide-band crystal spectrometer in lower-hybrid current drive experiments and increase in the total Kα counts has been observed at electron densities below 7×1012cm−3. Highly resolved titanium He-like spectra have been also measured using a high-resolution crystal spectrometer to obtain electron temperature with intensity ratio of IS/Iwand ion temperature with w line broadening. The results show that the intensity ratio has remarkably high values and the narrowing of the w line is occured at ohmic phase. The production of suprathermal electrons in JIPPT-IIU is described with nonthermal x-ray spectra.

Results of pellet fuelled discharges on the Doublet III tokamak

Results are presented for multiple deuterium pellet injection into both the Ohmic phase and the beam heated phase of Doublet III discharges. Pellet injection experiments were carried out using an injector, built at Oak Ridge National Laboratory, which has the capability of firing up to 40 hydrogen or deuterium pellets per second at a speed of 800 m·s−1. For the deuterium pellets used in this experiment, one pellet raises the line average density to 1.3 × 1013 cm−3. In the Ohmic phase, the pellets penetrate deep into the plasma, but with 2.2 MW of neutral beam heating they penetrate only half as deep. This difference is mostly due to the increased ablation in the outer 25% of the plasma during beam heating. Ablation calculations using the neutral shielding model agree well with experimental results during Ohmic heating, but they predict a shallower pellet penetration than is actually observed during neutral beam heating. About 90–100% of the pellet mass can be accounted for after injection. Electron temperature profile relaxation accompanies pellet injection, but on a time-scale that is shorter than the pellet lifetime. A transient increase in the energy confinement time τE is possible, provided pellet penetration is sufficient to sustain a peaked density profile. In discharges with one or more multifractured pellets, which ablate rapidly at the plasma edge and thus broaden the density profile, no increase in τE is seen. These results indicate that continuous τE improvement may be possible if a peaked density profile can be maintained by sustained deep pellet penetration.

Correlation between low frequency turbulence and energy confinement in TFR

A close correlation between density fluctuations detected with the help of a coherent scattering experiment and the electron transport confinement time deduced from a zero-dimensional code valid at the centre of the discharge has been observed. This correlation suggests that turbulence is responsible for the confinement degradation observed in tokamak experiments. The correlation is similar in the Ohmic phases and the auxiliary heating phases, which suggests a continuity in this turbulent process. It is shown that the relative density fluctuation can vary much more than the density gradient length, in contradiction to the often quoted ‘mixing length’ relationship.

Plasma heating in JET

Heating the JET plasma well above temperatures reached in the ohmic phase is the aim of the two additional heating systems planned for JET: ion cyclotron resonance heating (ICRF) and neutral beam injection (NBI). Operations with the latter started in February 1986, initially with hydrogen injection, up to a power level of 7 MW. ICRF power has been delivered to the plasma by three antennae and has reached power levels of 6 MW for 2 s. In most experiments, the frequency of the waves is adjusted to position the minority ion resonance layer close to the centre of the discharge, resulting in a centrally peaked power deposition profile. Results have also shown that the increase of the volume averaged electron temperature was much less dependent on the radial position of the ion resonance layer than were the central temperatures of both ions and electrons which can both approach 5 keV. In cases of 'on axis' heating, large increases in the sawtooth activity is observed with sawtooth periods exceeding 0.3 s. Initial results with NBI have shown a decrease of the global energy confinement time with additional power similar to the one observed with ICRF heating.

Electron temperature measurements during electron cyclotron heating on PDX using a ten-channel grating polychromator

First-harmonic electron cyclotron heating (ECH) experiments (PRF ≈ 80 kW, f ≅ 60 GHz) have been conducted on the Princeton Divertor Experiment (PDX) (R =137 cm, a ≈ 40 cm). Off-axis (≈ 4 cm) location of the resonance zone resulted in heating without sawtooth or m = 1 activity. However, heating with the resonance zone at the plasma centre caused very-large-amplitude sawteeth accompanied by strong m = 1 activity: ΔT/TMAX ≈ 0.41, sawtooth period ≈ 4 ms, m = 1 period ≅ 90 μs (11 kHz). This is the first time such intense MHD activity driven by low power (PRF/POH ⪅ 0.35) ECH has been observed. (For both cases, there was no sawtooth activity in the Ohmic phase of the discharge before ECH.) At very low densities, there is a clear indication that a superthermal electron population is created during ECH. – The electron temperature evolution was monitored using a grating polychromator which measured second-harmonic electron cyclotron emission from the low field side of the tokamak. Interference from the high-power heating pulse on the broadband detectors in the grating instrument was eliminated by using a waveguide filter in the transmission line which brought the emission signal to the grating instrument.

Plasma ion fluxes in the ASDEX edge plasma during neutral-beam-heated discharges

Deuterium fluxes parallel to the toroidal magnetic field in the plasma boundary of the midplane of ASDEX were collected with graphite collector probes positioned at a distance of about 5 to 10 cm from the separatrix, and then quantitatively analysed by the nuclear reaction D(3He, p)4He. The time resolution of about 0.1 s of the measuring setup allows the fluxes to be collected during the ohmic heating plateau phase (about 1 s) of the ASDEX discharge (typically 2 to 3 s) and during the neutral injection phase (about 0.2 s). The measurements yield fluxes of 1016 to 1017 D/cm2 s in the plateau phase, and fluxes one order of magnitude larger during the neutral injection phase. In the ohmic heating phase the e-folding length of the curve described by the measured points is observed to be about 1.5 cm. During neutral injection in discharges of the H-type the e-folding length was found to be about 8 cm. A series of such measurements is presented and discussed, with particular regard to the plasma-wall behavior during L-type and H-type discharges.

Impurity origin during ICRF heating in the JIPP T-IIU tokamak

Replacing stainless steel limiters by graphite limiters, we found that radiations from iron and titanium ions were significantly reduced. Total radiation and loop voltage also decreased. This indicates that the limiters are the major impurity sources both in the ohmic and RF heating phases. Although titanium radiations increased with RF power injected by an antenna with a titanium Faraday shield, the maximum intensity was much smaller than in the experiment where the titanium-flashed stainless steel limiters were used. Thus it has been found that the Faraday shield is less important as an impurity source than limiters. Toroidal asymmetry observed for O II radiation suggest that the energetic charge-exchange neutrals play a role in releasing oxygen from the wall and that those energetic particles are relatively abundant in the toroidal sections near the antenna.The Hα + Dα radiation decreases during the RF pulse around the limiter, which may be due to the change in hydrogen/ deuterium recycling at the limiter. The reduction of Hα + Dα is greater with graphite limiters than with stainless steel limiters. The relation between recycling and impurity release is briefly discussed.

Temporal behavior of the electron density profile in the scrape-off layer of neutral-beam-heated ASDEX plasmas

Density measurements in the peripheral region of a NI heated discharge are presented. Most of the results are obtained via a new technique utilizing a 60 keV neutral lithium beam which yields profiles — on a continuous basis — that can be related to the density profile. The initial consequence of NI is a deterioration of the particle confinement time (L-phase) which manifests itself as an ∼ 40% increase in the density e-folding length λ n outside the separatrix; possibly, there is a slight decrease in density inside the separatrix. Attainment of the H-phase with its better confinement properties is accompanied by an abrupt decrease in λ n to a value typical of the Ohmic phase; there are indications that the density gradient near the separatrix is even steeper. The subsequent increase in edge density is then intermittently interrupted by bursts which, at the conclusion of each burst, lead to a reduction in n e of the scrape-off layer and at least the region immediately inside the separatrix.

Hydromagnetic Stability of Force-Free Toroidal Fields

Stellerators can be made hydromagnetically stable during the ohmic heating phase of their operation when the pressure is zero. It was shown that for force-free systems curvature does not affect the stability criteria which were obtained for a large class of equilibria in which small multipolar fields and axial currents distort a zeropressure plasma in a uniform axial field. (M.C.G.)