Thomas W. Lovell

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.

Tokamak-like confinement at a high beta and low toroidal field in the MST reversed field pinch

Energy confinement comparable with tokamak quality is achieved in the Madison Symmetric Torus (MST) reversed field pinch (RFP) at a high beta and low toroidal magnetic field. Magnetic fluctuations normally present in the RFP are reduced via parallel current drive in the outer region of the plasma. In response, the electron temperature nearly triples and beta doubles. The confinement time increases ten-fold (to ~10 ms), which is comparable with L- and H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Runaway electron confinement is evidenced by a 100-fold increase in hard x-ray bremsstrahlung. Fokker–Planck modelling of the x-ray energy spectrum reveals that the high energy electron diffusion is independent of the parallel velocity, uncharacteristic of magnetic transport and more like that for electrostatic turbulence. The high core electron temperature correlates strongly with a broadband reduction of resonant modes at mid-radius where the stochasticity is normally most intense. To extend profile control and add auxiliary heating, rf current drive and neutral beam heating are in development. Low power lower-hybrid and electron Bernstein wave injection experiments are underway. Dc current sustainment via ac helicity injection (sinusoidal inductive loop voltages) is also being tested. Low power neutral beam injection shows that fast ions are well-confined, even in the presence of relatively large magnetic fluctuations.

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.

Dynamo-free plasma in the reversed-field pinch : Advances in understanding the reversed-field pinch improved confinement mode

Generation and sustainment of the reversed field pinch (RFP) magnetic configuration normally relies on dynamo activity. The externally applied electric field tends to drive the equilibrium away from the relaxed, minimum energy state which is roughly described by a flat normalized parallel current density profile and is at marginal stability to tearing modes. Correlated fluctuations of magnetic field and velocity create a dynamo electric field which broadens the parallel current density profile, supplying the necessary edge current drive. These pervasive magnetic fluctuations are also responsible for destruction of flux surfaces, relegating the standard RFP to a stochastic-magnetic transport-limited device. Application of a tailored electric field profile (which matches the relaxed current density profile) allows sustainment of the RFP configuration without dynamo-driven edge current. The method used to ascertain that a dynamo-free RFP plasma has been created is reported here in detail. Several confinement...

Overview of results in the MST reversed field pinch experiment

Confinement in the reversed field pinch (RFP) has been shown to increase strongly with current profile control. The MST RFP can operate in two regimes: the standard regime with a naturally occurring current density profile, robust reconnection and dynamo activity; and the improved confinement regime with strong reduction in reconnection, dynamo and transport. New results in standard plasmas include the observation of a strong two-fluid Hall effect in reconnection and dynamo, the determination that the m = 0 edge resonant mode is nonlinearly driven, and the determination that tearing modes can lock to the wall via eddy currents in the shell. New results in improved confinement plasmas include observations that such plasmas are essentially dynamo-free, contain several isolated magnetic islands (as opposed to a stochastic field) and contain reduced high frequency turbulence. Auxiliary current drive and heating is now critical to RFP research. In MST, a programme to apply auxiliary systems to the RFP is underway and progress has accrued in several techniques, including lower hybrid and electron Bernstein wave injection, ac helicity injection current drive, pellet injection and neutral beam injection.

Madison symmetric torus far‐infrared interferometer

A far‐infrared (FIR) interferometer plasma density diagnostic has been installed on the Madison Symmetric Torus (MST) at the University of Wisconsin at Madison. This collaborative experiment between University of California at Los Angeles and the University of Wisconsin at Madison has been undertaken to study the physics of the reversed field pinch magnetic confinement configuration. The density profile will be measured via an 11 channel FIR Interferometer. Since MST does not possess slotted plasma viewing ports, each channel will have an individual 4‐cm‐diam port in the vacuum vessel wall. In order to provide sufficient spatial sampling while minimizing field perturbations, the ports are positioned along two radial arms (separated by 5°) with five ports along one arm and six along the other. The FIR system consists of a tunable twin‐frequency FIR laser pumped by a tunable CO2 laser. The system will initially operate at 743 μm but can be discretely tuned from 185 to 1222 μm. This will allow the system to ...

Recent improvements in confinement and beta in the MST reversed-field pinch

In the general area of confinement improvement and concept advancement, recent results in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) include good confinement of both thermal and large-orbit ions and near doubling of total beta to 26% with deuterium pellet injection. Current profile control enables MST to reduce stochastic transport and achieve tokamak-like confinement. In standard RFP operation, substantial MHD tearing mode activity results in stochastic transport and an energy confinement time of about 1 ms in MST. Application of inductive current profile control reduces MHD activity and accompanying stochasticity, improving confinement by about a factor of ten. Previous work concentrated on electron confinement in improved-confinement RFP operation. Recent work confirms that ions are also well confined, and that high beta and improved confinement can be achieved simultaneously. PACS numbers: 52.55.Hc, 52.55.Dy (Some figures in this article are in colour only in the electronic version)

Protection of large capacitor banks

Large capacitor banks, as used in many pulsed plasma experiments, are subject to catastrophic failure in the event of a short in the output or in an individual capacitor. Methods are described for minimizing the damage and down time caused by such a failure.

Study of a triode-dynode amplifier for low intensity near-infrared detection with a photodiode

Summary form only given, as follows. This study is motivated by the observation that PIN photodiodes have excellent quantum efficiency for photon detection well into the near-infrared where the quantum efficiency of photocathodes falls below 1%. Coupling the high quantum efficiency of solid state diodes with the low noise, high gain, and fast time response of dynode chain electron multipliers, we hope to produce a design for a near-infrared detector capable of very low signal detection with fast time response (>10 MHz bandwidth) in the near-infrared (/spl sim/1 micron). Such a detector would be useful not only for plasma diagnostics, but also in other areas of optical sensing such as communications. In order to couple electrons into a dynode chain, we are examining triode assemblies based on field emission tips, thermionic cathodes with energy filters, and photoemissive cathodes. The advantages and difficulties associated with each technique will be reported.

High current plasma electron emitter

Summary form only given. A high current plasma electron emitter based on a miniature plasma source has been developed. The source is characterized by a high electron emission current density and emission current, small size, and low impurity content. The emitting plasma is created by a pulsed high current gas discharge. The source is biased negatively to extract electrons. Electron currents of the order of 1 kA and the emission current density of 1 kA/cm/sup 2/ at a bias voltage of about 100 V are obtained. The source has a simple design and has proven to be very reliable in operation. Extensive studies of the effect of the source geometry and materials have been conducted. The gas feed through, power dissipation, and impurity content were measured. In particular, spectroscopic measurements revealed that the impurities generated by the source electrodes are trapped inside by the source plasma. A high emission current, small size (3-4 cm in diameter), and low impurity generation make the source attractive for a variety of fusion, general science, and technological applications.