S.A. Hokin

Global confinement and discrete dynamo activity in the MST reversed field pinch

Results obtained on the Madison Symmetric Torus (MST) reversed‐field pinch [Fusion Technol. 19, 131 (1991)] after installation of the design poloidal field winding are presented. Values of βθe0≡2μ0ne0Te0/B2θ(a)∼12% are achieved in low‐current (I=220 kA) operation; here, ne0 and Te0 are central electron density and temperature, and Bθ(a) is the poloidal magnetic field at the plasma edge. An observed decrease in βθe0 with increasing plasma current may be due to inadequate fueling, enhanced wall interaction, and the growth of a radial field error at the vertical cut in the shell at high current. Energy confinement time varies little with plasma current, lying in the range of 0.5–1.0 msec. Strong discrete dynamo activity is present, characterized by the coupling of m=1, n=5–7 modes leading to an m=0, n=0 crash (m and n are poloidal and toroidal mode numbers). The m=0 crash generates toroidal flux and produces a small (2.5%) increase in plasma current.

Ion heating and magnetohydrodynamic dynamo fluctuations in the reversed‐field pinch

Ion temperatures have been measured in the Madison Symmetric Torus (MST) [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed‐field pinch (RFP) with a five channel charge exchange analyzer. The characteristic anomalously high ion temperature of RFP discharges has been observed in the MST. The ion heating expected from ion–electron collisions is calculated and shown to be too small to explain the measured ion temperatures. The charge exchange determined ion temperature is also compared to measurements of the thermally broadened Cv 227.1 nm line. The ion temperature, Ti≊250 eV for I=360 kA, increases by more than 100% during discrete dynamo bursts in MST discharges. Magnetic field fluctuations in the range 0.5–5 MHz were also measured during the dynamo bursts. Structure in the fluctuation frequency spectrum at the ion cyclotron frequency suggests that the mechanism of ion heating involves the dissipation of dynamo fluctuations at ion cyclotron frequencies.

Design and calibration of a fast‐time resolution charge exchange analyzer

A five channel, fast time resolution, scanning charge exchange analyzer has been developed for the Madison Symmetric Torus (MST). The analyzer consists of an iron vacuum vessel, a gas stripping cell, an electrostatic bending field, and five continuous electron multiplier detectors. The incident neutral flux and operation of the detectors in current mode limits the time resolution of the analyzer to 10 μs. The analyzer was absolutely calibrated over the energy range of interest (500–2000 eV) with a H+ beam, so that the charge exchange power loss could also be measured. The analyzer can be swiveled on a shot‐to‐shot basis for measurements of Ti(r), where 0.3<r/a<0.7. The mechanical design was driven by the need for a low cost, expandable ion temperature diagnostic.

Turbulent transport in the Madison Symmetric Torus reversed-field pinch

Measurements of edge turbulence and the associated transport are ongoing in the Madison Symmetric Torus (MST) reversed‐field pinch [Fusion Technol. 19, 131 (1991)] using magnetic and electrostatic probes. Magnetic fluctuations are dominated by m=1 and n ∼2R/a, tearing modes. Particle losses induced by magnetic field fluctuations have been found to be ambipolar (〈J∥ Br〉/B0=0). Electrostatic fluctuations are broadband and turbulent, with mode widths Δm∼3–7 and Δn∼70–150. Particle, parallel current, and energy transport arising from coherent motion with the fluctuating Ẽ×B drift have been measured. Particle transport via this channel is comparable to the total particle loss from MST. Energy transport (from 〈PEφ 〉/B0) due to electrostatic fluctuations is relatively small, and parallel current transport (from 〈J∥ Eφ〉/B0) may be small as well.

First results from the Madison Symmetric Torus reversed field pinch

The first period of physics operation of the Madison Symmetric Torus (MST) reversed field pinch [Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol 2, p. 757] has produced information on sawtooth oscillations, edge magnetic and electrostatic fluctuations, and equilibrium parameters at large plasma size. Sawtooth oscillations are prevalent at all values of pinch parameter and might constitute discrete dynamo events. Both electrostatic and magnetic fluctuations are of sufficient magnitude to be relevant to transport in the reversed field pinch. In the plasmas studied to date (up to a plasma current of 0.5 MA) the poloidal beta value is about 10% or greater.

B4C solid target boronization of the MST reversed-field pinch

Abstract A solid rod of hot-pressed boron carbide is being used as the source of boron during boronization of MST. The most striking result of this procedure is the reduction in oxygen contamination of the plasma (OIII radiation, characteristic of oxygen at the edge, falls by about a factor of 3 after boronization.). The radiated power fraction drops to about half its initial value. Particle reflux from the wall is also lowered, making density control simpler. The rod (12.7 mm diameter) is inserted into the edge plasma of normal high-power RFP discharges. B 4 C is ablated from the surface of the rod and deposited in a thin film (a-B/C:H) on the walls and limiters. The energy flux carried by “superthermal” (not “runaway”) electrons at the edge of MST appears to enhance the efficient, nondestructive ablation of the boron carbide rod.

Nonlinear coupling of tearing fluctuations in the Madison Symmetric Torus

Three‐wave, nonlinear, tearing mode coupling has been measured in the Madison Symmetric Torus (MST) reversed‐field pinch (RFP) [Fusion Technol. 19, 131 (1991)] using bispectral analysis of edge magnetic fluctuations resolved in ‘‘k‐space.’’ The strength of nonlinear three‐wave interactions satisfying the sum rules m1+m2=m3 and n1+n2=n3 is measured by the bicoherency. In the RFP, m=1, n∼2R/a (6 for MST) internally resonant modes are linearly unstable and grow to large amplitude. Large values of bicoherency occur for two m=1 modes coupled to an m=2 mode and the coupling of intermediate toroidal modes, e.g., n=6 and 7 coupled to n=13. These experimental bispectral features agree with predicted bispectral features derived from magnetohydrodynamic (MHD) computation. However, in the experiment, enhanced coupling occurs in the ‘‘crash’’ phase of a sawtooth oscillation concomitant with a broadened mode spectrum suggesting the onset of a nonlinear cascade.

Transport reduction by current profile control in the reversed‐field pinch

An auxiliary poloidal inductive electric field applied to a reversed‐field pinch (RFP) plasma reduces the current density gradient, slows the growth of m=1 tearing fluctuations, suppresses their associated sawteeth, and doubles the energy confinement time. This experiment attacks the dominant RFP plasma loss mechanism of parallel streaming in a stochastic magnetic field. The auxiliary electric field flattens the current profile and reduces the magnetic fluctuation level. Since a toroidal flux change linking the plasma is required to generate the inductive poloidal electric field, the current drive is transient to avoid excessive perturbation of the equilibrium. To sustain and enhance the improved state, noninductive current drivers are being developed. A novel electrostatic current drive scheme uses a plasma source for electron injection, and the lower‐hybrid wave is a good candidate for radio‐frequency current drive.

Reversed-field pinch studies in the Madison Symmetric Torus

Studies of large-size (R=1.5 m,a=0.5 m), moderate current (I <750 kA) reversed-field pinch (RFP) plasmas are carried out in the Madison Symmetric Torus in order to evaluate and improve RFP confinement, study general toroidal plasma MHD issues, determine the mechanism of the RFP dynamo, and measure fluctuation-induced transport and anomalous ion heating. MST confinement scaling falls short of the RFP scaling trends observed in smaller RFPs, although the plasma resistance is classical. MHD tearing modes with poloidal mode numberm=1 and toroidal mode numbersn=5–7 are prevalent and nonlinearly couple to produce sudden relaxations akin to tokamak sawteeth. Edge fluctuation-induced transport has been measured with a variety of insertable probes. Ions exhibit anomalous heating, with increases of ion temperature occurring during strong MHD relaxation. The anomalous heating fraction decreases with increasing density, such that ion temperatures approach the lower limit given by electron-ion friction. The RFP dynamo has been studied with attention to various possible mechanisms, including motion-EMF drive, the Hall effect, and superthermal electrons. The toroidal field capacity of MST will be upgraded during Summer 1993 to allow low-current tokamak operation as well as improved RFP operation.

Three-dimensional neutral atom imaging of tokamak plasmas

Recent technological advances in neutral atom imaging of space plasmas can be applied to the next generation of long pulse, high density fusion experiments. In particular, the small size of these neutral atom imagers makes them ideal for measurements of charge exchange neutrals in the edge and divertor region of tokamaks. In the edge of tokamak plasmas, the lower energy neutrals are created locally and the higher energy neutrals are generated within a few mean free paths of the edge. This charge exchange neutral flux can be imaged and its energy spectrum analyzed. The energy spectrum measurements provide the radial information necessary for a three‐dimensional (3D) image of the convolved ion and neutral populations.