High Power Neutral Beam Heating Research Articles

Significance of MHD Effects in Stellarator Confinement

Substantial progress has been achieved in raising the plasma beta in stellarators and helical systems by high-power neutral beam heating, approaching reactor-relevant values. The achievement of high-beta operation is closely linked with configuration effects on the confinement and with magnetohydrodynamic (MHD) stability.The magnetic configurations of the Wendelstein 7-AS (W7-AS) stellarator and of the Large Helical Device (LHD) and their optimization for high-beta operation within the flexibility of the devices are characterized. A comparative description of the accessible operational regimes in W7-AS and LHD is given. The finite-beta effects on the flux surfaces depend on the degree of configuration optimization. In particular, a large Shafranov shift is accompanied by formation of islands and stochastic field regions as found by numerical equilibrium studies. However, the observed pressure gradients indicate some mitigation of the effects on the plasma confinement, presumably because of the high collisionality of high-beta plasmas and island healing effects (LHD). As far as operational limits by pressure-driven MHD instabilities are concerned, only weak confinement degradation effects are usually observed, even in linearly unstable regimes.The impact of the results concerning high-beta operation in W7-AS and LHD on the future stellarator program will be discussed, including the relationship to tokamak research. Some of the future key issues appear to be the following: the control of the magnetic configuration (including toroidal current control), the modification of confinement and MHD properties toward the low-collisional regime, and the compatibility of high-beta regimes with power and particle exhaust requirements to achieve steady-state operation.

Development of Powerful RF Plasma Sources for Present and Future NBI Systems

This paper deals with the topic of RF plasma sources and their application in high-power neutral beam heating systems for nuclear fusion devices. RF sources represent an interesting alternative to the conventional arc discharge sources. Due to the absence of hot filaments they exhibit an inherent simplicity both in mechanical and electrical aspects and consequently offer advantages in terms of cost savings, gain in availability and reliability and reduced maintenance. This renders the RF plasma source attractive for any long pulse (> 10 sec) NBI system and in particular for the ITER NBI system. The latter, however, requires that the RF plasma source is also capable of delivering negative rather than positive hydrogen ions. In the first part of the paper the types, characteristics and operation experience of RF plasma sources for positive ions in operation are described. The second part is devoted to the development for ITER NBI: The basic requirements, physics and technology issues and the present status are discussed.

Charge-exchange measurements of d-d triton distribution functions in high-power neutral beam heating on JT-60U

We present the first results of the fusion triton measurements in JT-60U using the MeV energy range neutral particle analyser (NPA). An energy distribution of slowing-down fusion tritons has been obtained during the experiments of high-power neutral beam injection (NBI). We associate a dominant mechanism of triton neutralization with charge-exchange (CX) on the plasma residual neutral deuterium and on the hydrogen-like carbon impurity ions. To determine a contribution of these processes in triton neutralization the simplified steady-state balance of carbon impurity ions is considered. The spectrum of slowing-down triton is modelled using the analytical solution of the simplified zero-dimensional Fokker-Planck equation. A good agreement between the experiment with the model is demonstrated.

Phenomenology of intense electron cyclotron emission bursts during high power neutral beam heating on TFTR (abstract)

A 20-channel grating polychromator has been used to study intense bursts of electron cyclotron emission (ECE) from TFTR deuterium plasmas predominantly heated by 90–110-keV neutral beams (Pinj/Poh≳30). The ECE bursts have a duration of 20–150 μs and are usually seen 300–500 ms after the start of neutral beam injection, when the stored energy and neutron production are collapsing or rolling over. In most cases the ECE bursts have Δf/f∼0.2–0.5, if this frequency spread is due entirely to relativistic broadening it implies an electron energy of 10–100 keV (Core electron temperatures in these plasmas are typically 7–12 keV). The ECE bursts are often correlated with ELM activity during limiter H modes and appear to occur at the beginning of the rise in the Dα signal. In some instances the spectral width of the ECE burst is narrow enough (Δf/f∼0.1) to allow identification of the origin of the emission, in these cases the source appears to be within 0.2 m of the plasma edge and the ECE burst exhibits a delay characteristic of an outwardly directed velocity of 2–3×103 m/s. This work is supported by U.S. Department of Energy Contract No. DE-AC02-76-CHO-3073.

Attainment of high plasma densities in TFTR with injection of multiple deuterium pellets

Sequential injection of multiple deuterium pellets combined with high power neutral beam heating in TFTR has produced stable plasmas with very high central densities, up to 5×1020 m-3. The line averaged densities achieved in these experiments correspond to Murakami parameters neR/BT up to 12×1019 m-2.T-1. This value, which does not appear to represent an intrinsic density limit, surpasses the values previously obtained in TFTR and exceeds, by more than a factor of two, the density limit predicted by the scaling of Greenwald et al. (Nuclear Fusion 28 (1988) 2199). The highest densities obtained with both pellet and gas fuelling have been achieved since boronization was applied to the first wall in TFTR. The characteristics and energy balance of the highest density plasma are discussed

The present status of impurity transport investigations

The present status of impurity transport studies in tokamaks is reviewed. Ohmically heated and neutral-beam heated L-mode discharges exhibit anomalous impurity transport. The values of particle diffusivity range from 0.5 to 2 m2/s. With improved energy confinement such as in the H-mode the impurity transport is suppressed, which results in a radiation collapse of the H-mode state in the absence of the edge-localized mode (ELM). With high power neutral-beam heating, ELMs frequently expel particles and heat out of the separatrix surface, which results in long (≈ 5 seconds) quasi-stationary H-mode operation.

Peaked density profiles in circular-limiter H modes on the TFTR tokamak.

Circular-limiter H modes are obtained on the TFTR tokamak during high-power neutral-beam heating. The transition is usually from the supershot to the H mode rather than the usual L to H transition, and thus is obtained in a low-recycling environment with core fueling mainly from the heating beams. As a result, the density and pressure profiles are highly peaked at the center. The global confinement time τE is enhanced over L-mode scaling by up to ≊2.5 times. The onset of edge-localized MHD modes shortly after the H-mode transition appears to limit τE. Limiter H modes of up to 1.5 s duration have been realized.Received 2 April 1990DOI:https://doi.org/10.1103/PhysRevLett.65.424©1990 American Physical Society

Charge exchange recombination spectroscopy measurements in the extreme ultraviolet region of central carbon concentrations during high power neutral beam heating in TFTR

The carbon concentration in the central region of TFTR discharges with high power neutral beam heating has been measured by charge exchange recombination spectroscopy (CXRS) of the C5+ n = 3–4 transition in the extreme ultraviolet region. The carbon concentrations were deduced from absolute measurements of the line brightness using a calculation of the beam attenuation and the appropriate cascade corrected line excitation rates. As a result of the high ion temperatures (20–30 keV) in most of the discharges, the contribution of beam halo neutrals to the line brightness was significant and therefore had to be included in the modelling of the data. Carbon concentrations have been measured in discharges with plasma currents Ip in the range 1.0-1.6 MA and beam power in the range 2.6–30 MW, including a number of supershots. The results are in good agreement with carbon concentrations deduced from the visible bremsstrahlung Zeff and with metallic impurity concentrations measured by X-ray pulse height analysis, demonstrating the reliability of the atomic rates used in the beam attenuation and line excitation calculations. Carbon is the dominant impurity species in these discharges; the oxygen concentration measured via CXRS in a high beam power case was 0.0006 of ne, compared to 0.04 for carbon. Trends with plasma current and beam power in the carbon concentration and the inferred deuteron concentration are presented. The carbon concentration is independent of plasma current and decreases from 0.13 at 2.6 MW beam power to 0.04 at 30 MW, while the deuteron concentration increases from 0.25 to 0.75 over the same range of beam power. These changes are primarily the result of beam particle fuelling, as the carbon density did not vary significantly with beam power. The time evolutions of the carbon and deuteron concentrations during two high power beam pulses, one which exhibited a carbon bloom (a sudden influx of carbon due to local heating of the limiter) and one which did not, are compared. In both types of discharge, the carbon concentration decreases early in the beam pulse as a result of beam particle fuelling, and the carbon density rises slowly during the beam pulse until the start of the bloom. The electron density rise during the bloom is primarily due to the increase in the carbon density.

Present status of impurity transport study.

Present status of impurity transport study in tokamaks is reviewed. Ohmically-heated and neutral-beam heated L-mode discharges exhibit anomalous impurity transport. The values of particle diffusivity range from 0.5 to 2m2/s. With improved energy confinement such as in H-mode, impurity transport is suppressed, which results in a radiation collapse of H-mode state in the absence of edge-localized mode (ELM). With high power neutral beam heating, frequent ELMs expel particles and heat out of the separatrix surface, which results in long (-5seconds), quasi-stationary H-mode.

Characteristics of the JT-60 divertor and limiter plasmas with high power auxiliary heating

Essential divertor functions — density and energy control — have been investigated in the JT-60 divertor with metal walls. Neutral pressure in the divertor chamber increases in proportion to n ̄ 2 e and high recycling state for particle exhaust is realized. At n e ≤ 6 × 10 19 m −3 , wall pumping by the divertor plates is dominant in particle exhaust. However, at n ̄ e ≥ 6 × 10 19 m −3 , particle exhaust by active divertor pumping systems becomes essential. Because of the effectiveness of the divertor, radiation loss in the main plasma is reduced to 5–10% of the absorbed power and impurity concentrations are significantly suppressed at very low level ( Z eff = 1.5-2.0). In the experiments with graphite walls, short periods of H-mode phases were found in the outside X-point divertor discharges although improvement in energy confinement is limited to within 10%. In limiter discharges, combination of high vessel temperature (210–300 °C) and low current limiter discharges (1.5 MA) without gas puffing is effective as a wall conditioning in obtaining reproducible high density discharges. The maximum parameters, I p = 3.2 MA, n ̄ e = 1.2 × 10 20 m −3 , q eff = 2.2 at B t = 4.8T, a Murakami parameter of 7.5, and a stored energy of 3.1 MJ have been attained. The maximum density is limited by the occurence of the MARFE. During high power neutral beam heating, enhanced carbon influx was observed possibly due to chemical sputtering. The wall pumping by the inner wall is still effective in decreasing the electron density from the high density region of n ̄ e ~ 1 × 10 20 m −3 . An empirical energy confinement scaling of JT-60 has been drawn in terms of an offset linear function as τ E = 0.19 I 1.9 P P abs + 0.062 a 1.8 p

High ion temperatures and transport characteristics of high Zeff hydrogen plasmas in JT-60 limiter discharges

Ion temperatures around 10 keV were obtained in JT-60 limiter plasmas by high power neutral beam heating (H0 → H+), which is much higher than the maximum value of 6.6 keV obtained in JT-60 diverted plasmas. The experiment was performed in the parameter region of Ip = 1−2 MA, n̄e (before heating) = (0.7−1.5)×1019 m−3, n̄e (end of heating) = (2.5−4.1)×1019 m−3 and = 19−21.8 MW. The Zeff values of the limiter plasmas were high (i.e. 4 to 7) compared with those of the diverted plasmas (i.e. 1 to 2). Transport analysis indicates that, since the deposition power to electrons, Pbe, increases with Zeff, high Zeff is effective in raising the electron temperature. Furthermore, it is shown that since the equipartition loss is reduced by both the increase in electron temperature and the decrease in the effective ion density, np + nzZ2mp/mz, with increasing Zeff, high ion temperatures are obtained in limiter plasmas with high Zeff.

High power neutral beam heating experimentson TFTR with balanced and unbalanced momemtum input

New long-pulse ion sources have been employed to extend the neutral beam pulse on TFTR from 0.5sec to 2.0 sec. This made it possible to study the long-term evolution of supershots at constantcurrent and to perform experiments in which the plasma current was ramped up during the heatingpulse. Experiments were conducted with co and counter injection as well as with nearly balancedinjection of deuterium beams up to a total power of 20 MW. The best results, i.e., central iontemperatures Tio > 25 keV and neo τE Tio values of 3 × 1020 keV sec m-3, were obtained withnearly balanced injection. The central toroidal plasma rotation velocity scales in a linear-offset fashion with beam power and density. The scaling of the inferred global momentumconfinement time with plasma parameters is inconsistent with the predictions of the neoclassicaltheory of gyroviscous damping. An interesting plasma regime with properties similar to the H-modehas been observed for limiter plasmas with edge qa just above 3 and 2.5.

The Poloidal Divertor Experiment(PDX) and the Princeton Beta Experiment(PBX)

A review of high-power neutral-beam heating, impurity studies, and high-beta research on the PDX/PBX tokamak is presented.

Spectroscopic Diagnostics of Tokamak Plasmas

Spectroscopic observations of charge exchange recombination in tokamak plasmas during high power neutral beam heating experiments and effect of such recombination as well as transport processes on ionization equilibrium are presented. Application of charge exchange recombination for measurements of totally stripped ion densities is also discussed. Special attention is devoted to spectroscopic measurements of ion temperature and impurity concentration by using long-wavelength forbidden lines (from magnetic dipole transitions) of highly ionized atoms and Kα lines of helium- and lithium-like Fe and Ti ions in the X-ray region (crystal spectroscopy). Fluorescense spectroscopy applications for ion density measurements in the plasma periphery is also presented. All these presentations are related to requirements of accuracy and of new calculations and measurements of rate coefficients for ionization, recombination (radiative, dielectronic and charge exchange) and excitation for the CTR program.

Charge exchange as a recombination mechanism in high-temperature plasmas

Charge exchange with neutral hydrogen is examined as a recombination mechanism for multi-charged impurity ions present in high-temperature fusion plasmas. At sufficiently low electron densities, fluxes of atomic hydrogen produced by either the injection of neutral heating beams or the background of thermal neutrals can yield an important or even dominant recombination process for such ions. Equilibrium results are given for selected impurity elements showing the altered ionization balance and radiative cooling rate produced by the presence of various neutral populations. A notable result is that the stripping of impurities to relatively non-radiative ionization states with increasing electron temperature can be postponed or entirely prevented by the application of intense neutral beam heating power. A time dependent calculation modelling the behavior of iron in recent PLT tokamak high power neutral beam heating experiments is also presented.