R. N. Dexter

The Madison Symmetric Torus

AbstractThe Madison Symmetric Torus (MST) is the newest and largest reversed-field pinch (RFP) currently in operation. It incorporates a number of design features that set it apart from other pinches, including the use of the conducting shell as both a vacuum vessel and single-turn toroidal field coil. Specially insulated voltage gaps are exposed to the plasma. Magnetic field errors at these gaps as well as at the various diagnostic and pumping ports are minimized through a variety of techniques. The physics goals of MST include study of the effect of large plasma size on confinement and the detailed investigation of RFP turbulence, dynamo, and transport. Details of the design and initial operation of the device are presented.

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.

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.

Initial results from the Tokapole-II poloidal divertor device

The.latest in a series of internal-ring devices, called Tokapole II, has recently begun operation at the University of Wisconsin. Its purpose is to permit the study of the production and confinement of hot, dense plasmas in either a toroidal-octupole (with or without toroidal field) or a tokamak with a four-node poloidal divertor. The characteristics of the device and the results of its initial operation are described here. Quantitative measurements of Impurity concentration and radiated power have been made. Poloidal divertor equilibria of square and dee shapes have been produced, and an axisymmetric instability has been observed with the inverse dee. Electron cyclotron resonance heating is used to initiate the breakdown near the axis and to control the initial influx of impurities. A 2-MW RF source at the second harmonic of the ion cyclotron frequency is available and has been used to double the ion temperature when operated at low power with an unoptimized antenna. Initial results of operation as a pure octupole with poloidal Ohmic heating suggest a tokamak-like scaling of density (n <* Bp) and confinement time (T.<* n).

q-Dependence of magnetic turbulence in a tokamak

Low‐frequency (ω≪ωci) radial magnetic field turbulence has been measured over the full minor radius for discharges in the Tokapole II tokamak [Nucl. Fusion 9, 1509 (1979)], and scaled over the range of edge safety factor 0.6≤qa≤5.0. It was found that qa served as a control parameter for both the magnetic fluctuation amplitude and the global confinement time. As qa is reduced from 5 to 0.6 the turbulence level increases by a factor of 50 while the confinement time decreases by more than a factor of 10. At qa≤1, the full fluctuation amplitude is roughly large enough to account for the global confinement using simple estimates of collisionless stochastic magnetic transport. At high qa, the turbulence is too small to account for transport using these estimates. Frequency spectra have been obtained from 10 to 400 kHz. For all qa most of the fluctuation power appears below 100 kHz. This low‐frequency structure changes as the safety factor is varied. Although broadband in frequency, the long radial coherence len...

Trapping of gun‐injected plasma by a tokamak

It has been seen that a plasma produced by a Marshall gun can be injected into and trapped by a tokamak plasma. This trapping of a gun‐injected plasma is explained in terms of a depolarization current mechanism. A model is developed that describes the slowing of a plasma beam crossing into the magnetic field of a tokamak. The slowing down time is shown to go as τs∝T3/2eL2/nbα20, where nb and Te are the density and temperature of the plasma beam and α0/L is the pitch of the field lines per unit length in the direction in which the beam is traveling. Experimental tests of this model are consistent with the scaling predictions.

q measurements during sawtooth oscillations in a low q tokamak

The central safety factor during a sawtooth oscillation in material limiter tokamak discharges (scrapeoff plates inserted to the separatrix) in the TOKAPOLE II poloidal divertor tokamak [Phys. Rev. Lett. 49, 734 (1982)] is measured to be significantly less than 1 (approximately 0.7) during a sawtooth oscillation. This result is identical to that observed earlier in the same device in magnetic limiter discharges. Thus the presence of scrapeoff plasma beyond the divertor separatrix is not responsible for the absence of total reconnection.

STUDIES OF A REVERSED FIELD PINCH IN A POLOIDAL DIVERTOR CONFIGURATION

An attempt has been made to form a reversed field pinch (RFP) in a poloidal divertor configuration which positions the plasma far from a conducting wall. In this configuration, the plasma is localized within a magnetic separatrix formed by a combination of toroidal currents in the plasma and four internal aluminium rings. Plasmas were formed with a plasma current of ~ 135 kA, toroidal field reversal lasting ~1 ms, line averaged density of ~(1-2) × 1013 cm−3 and central electron temperature of ~50 eV, but a large asymmetry in the magnetic field (δB/B ~ 40%) set in at about the time when the toroidal field reversed at the wall. This behaviour might be expected on the basis of MHD stability analysis of a cylindrical plasma bounded by a large vacuum region and a distant conducting wall. The symmetric equilibrium before the asymmetry develops and the asymmetry itself are described.

Plasma wake field acceleration: A proposed experimental test

The prospect of achieving very high accelerating fields has led to proposals for using electrostatic plasma waves to accelerate charged particles for high energy physics. It has been predicted theoretically that these plasma waves can be driven by the wake fields of short bunches, or trains of bunches, of charged particles, to accelerate a subsequent bunch; the longitudinal electric fields possible could be of the order of a few GV/m. This note presents an outline of a proposed experimental test of this principle.

Studies of large, non-circular, reversed field pinch discharges

Reversed field pinch (RFP) discharges have been produced in a large (1.39 metre major radius, 0.56 metre average minor radius), thick walled (5 cm), aluminium vacuum vessel with indented sides. The discharges are self-reversed and ramped up to a current of 300 kA over a time of 10 ms. Reversal is sustained for 10 resistive diffusion times, despite the presence of large magnetic fluctuations. The influence of the bad poloidal magnetic curvature on RFP stability is examined by measurement of magnetic fluctuations near the plasma edge in the separate bad and good curvature regions of the non-circular plasma for RFP and non-reversed discharges with an edge safety factor, qa, of 0.4 and 1.4. For qa ~ 1.4 discharges, the poloidal field curvature is small. The large device size permits RFP startup at a low toroidal loop voltage (200 V), which is applied to a gap exposed to plasma, but successfully protected against arcing (up to 300 V). RFP plasmas have also been obtained with a toroidal limiter.