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Programmable control of the inductive electric field enables advanced operations of reversed-field pinch (RFP) plasmas in the Madison Symmetric Torus (MST) device and further develops the technical basis for ohmically heated fusion RFP plasmas. MST’s poloidal and toroidal magnetic fields (B p and B t) can be sourced by programmable power supplies (PPSs) based on integrated-gate bipolar transistors (IGBT). In order to provide real-time simultaneous control of both B p and B t circuits, a time-independent integrated model is developed. The actuators considered for the control are the B p and B t primary currents produced by the PPSs. The control system goal will be tracking two particular demand quantities that can be measured at the plasma surface (r = a): the plasma current, I p ∼ B p(a), and the RFP reversal parameter, F ∼ B t(a)/Φ, where Φ is the toroidal flux in the plasma. The edge safety factor, q(a) ∝ B t (a), tends to track F but not identically. To understand the responses of I p and F to the actuators and to enable systematic design of control algorithms, dedicated experiments are run in which the actuators are modulated, and a linearized dynamic data-driven model is generated using a system identification method. We perform a series of initial real-time experiments to test the designed feedback controllers and validate the derived model predictions. The feedback controllers show systematic improvements over simpler feedforward controllers.

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.

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.

Recent technological developments make a laser Thomson scattering diagnostic, operating in the incoherent electron scattering regime, a good candidate to accomplish measurements of fast electron dynamics (i.e., fast equilibrium changes, turbulence, and electron fluctuations) in high temperature plasmas. Pulse repetition rates for current generation Thomson scattering diagnostics have been limited to about 100 Hz, with the possible exception of “burst” modes in which multiple laser systems provide a limited sequence of closely spaced pulses. To overcome this limitation in laser capability, we propose that recent advances in compact, high power, diode-pumped solid state lasers be applied to a fast Thomson scattering diagnostic for fusion research. To illustrate the possibilities, we present an overview of a diagnostic system designed for the Madison Symmetry Torus reversed field pinch. The operational goal for this single-laser system is to measure Te, ne, and pe with a measurement rate of at least 10 kHz...

The far-infrared laser interferometer on the Madison Symmetric Torus has recently been upgraded to allow simultaneous interferometry and polarimetry measurements. Centered around a heterodyne laser system that operates at λ≃432 μm, the diagnostic has a frequency response of 1 kHz for the polarimeter and up to 1 MHz for the interferometer. The distribution of laser power through the 11 chord system is managed with wire mesh beamsplitters. Concurrent with the system upgrade to polarimetry, a detailed study of the effect of the wire mesh beamsplitters on beam polarization was conducted. The study shows that the mesh characteristics must be included to extract the polarimetry phase measurement from the raw data. The modifications to the data analysis as well as preliminary five chord polarimetry profile data are presented.

The Thomson scattering diagnostic on MST records both equilibrium and fluctuating electron temperature with a range capability of 10 eV-5 keV. Standard operation with two modified commercial Nd:YAG lasers allows measurements at rates of 1 kHz-25 kHz. Several subsystems of the diagnostic are being improved. The power supplies for the avalanche photodiode detectors (APDs) that record the scattered light are being replaced to improve usability, reliability, and maintainability. Each of the 144 APDs will have an individual rack mounted switching supply, with bias voltage adjustable to match the APD. Long-wavelength filters (1140 nm center, 80 nm bandwidth) have been added to the polychromators to improve capability to resolve non-Maxwellian distributions and to enable directed electron flow measurements. A supercontinuum (SC) pulsed white light source has replaced the tungsten halogen lamp previously used for spectral calibration of the polychromators. The SC source combines substantial brightness produced in nanosecond pulses with a spectrum that covers the entire range of the polychromators.

Two types of fast bolometers are described for the plasma energy transport study in the Madison symmetric torus plasma confinement device. Both types use pyrocrystals of LiTaO3 or LiNbO3 as the sensors. One type is used for measurements of the radiated heat losses and is situated at the vacuum shell inner surface. Another type is insertable in the plasma and measures the plasma particle heat flux. The frequency response of the bolometers is measured to be in the 150–200 kHz range. The range of the measured power fluxes is 0.1 W/cm2–10 kW/cm2 and can be adjusted by changing the size of the entrance aperture. The lower limit is determined by the amplifier noise and the frequency bandwidth, the higher limit by destruction of the bolometer sensor.

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.

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.

The multipoint Thomson scattering diagnostic on the Madison Symmetric Torus (MST) is now fully operational with 21 spatial points, which cover the entire minor radius. Four full electron temperature profiles can be obtained during each MST discharge, with a variable delay between each profile. This system overcomes challenges that arise from the unique machine design, location, and plasma characteristics of MST. The machine design limits the maximum porthole diameter to 11.4 cm, requiring a compact, re-entrant, seven element lens for scattered light collection. Limited space near MST necessitates a long beam path for the two Nd:YAG lasers requiring a remote beam line adjustment system to suppress drift in the beam position due to thermal expansion of the building. Due to the remote location of the laser head, substantial design effort was put into the creation of a set of safety interlocks for the laser system. The dynamic nature of MST plasmas and the wide range of operating space require a versatile scattered light detection system consisting of filter polychromators with temperature controlled avalanche photodiode detectors. We also implement an insertable integrating sphere, which travels along the laser beam path through the vacuum vessel, for the alignment of both the fiber optics and the lasers.

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)

To aid in diagnosis of 3D equilibria in the Madison Symmetric Torus, it has become necessary to control the orientation of the equilibria. In reversed field pinch experiments a transition to a 3D equilibrium is common with sufficiently large plasma current (and Lundquist number). Diagnosis of this state is hampered by the fact that the helical structure is stationary but with an orientation that varies shot-to-shot. A resonant magnetic perturbation (RMP) technique has been developed to vary controllably the orientation of the 3D equilibria and optimized to minimize the plasma wall interaction due to its use. Application of an RMP now allows alignment of the structure with key diagnostics, including Thomson scattering and an interferometer-polarimeter.

We have constructed a simple, low-cost charge-coupled device (CCD) spectrometer capable of both high resolution (Δλ⩽0.015 nm) and large bandpass (110 nm with Δλ∼0.3 nm). These two modes of operation provide two broad areas of capability for plasma spectroscopy. The first major application is measurement of emission line broadening; the second is emission line surveys from the ultraviolet to the near infrared. Measurements have been made on a low-temperature plasma produced by a miniature electrostatic plasma source and the high-temperature plasma in the Madison Symmetric Torus reversed-field pinch. The spectrometer is a modified Jarrell–Ash 0.5 m Ebert–Fastie monochromator. Light is coupled into the entrance slit with a fused silica fiber optic bundle. The exposure time (2 ms minimum) is controlled by a fast electro-mechanical shutter. The exit plane detector is a compact and robust CCD detector developed for amateur astronomy by Santa Barbara Instrument Group. The CCD detector is controlled and read out ...

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 improvements during the accompanying periods of low magnetic fluctuations are observed. Namely, the magnetic fluctuation level is reduced to the point where stochastic-magnetic transport is no longer the dominant process in the core and nested flux surfaces are restored in the core of the dynamo-free RFP. (c) 2005 American Institute of Physics.

The stabilizing effect of an axial flow on the m=1 kink instability in z-pinches has been studied numerically by reducing the linearized ideal MHD equations to a one-dimensional eigenvalue equation for the radial displacement. A diffuse z-pinch equilibrium is chosen that is made marginally stable to the m=0 sausage mode by tailoring the pressure profile. The principal result reveals that a sheared axial flow does stabilize the kink mode when the shear exceeds a threshold value which is inversely proportional to the wavelength of the mode. This threshold value can be satisfied with a peak flow which is less than the Alfven speed for certain wavelengths. Additionally, the m=0 sausage mode is driven from marginal stability into the stable regime which suggests that the equilibrium pressure profile control can be relaxed. The flow stabilization agrees with experimental observations. The details of the theoretical development will be presented. The implications of this stabilizing effect are to be investigated with a flow-through Z-pinch experiment, ZaP.

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.

Reduction of core-resonant m=1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus [Dexter , Fusion Technol. 19, 131 (1991)] reversed-field pinch have been routinely achieved through control of the surface poloidal electric field, but it is now known that the achieved confinement has been limited in part by edge-resonant m=0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the m=0 and m=1 fluctuations. This has allowed confinement of high-energy runaway electrons, possibly indicative of flux-surface restoration in the usually stochastic plasma core. The electron temperature profile steepens in the outer region of the plasma, and the central electron temperature increases substantially, reaching nearly 1.3 keV at high toroidal plasma current (500 kA). At low current (200 kA), the total beta reaches 15% with an estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the constant-beta confinement scaling that has characterized most reversed-field-pinch plasmas. (C) 2002 American Institute of Physics.

Improved confinement has been achieved in the MST through control of the poloidal electric field, but it is now known that the improvement has been limited by bursts of an edge-resonant instability. Through refined poloidal electric field control, plus control of the toroidal electric field, we have suppressed these bursts. This has led to a total beta of 15% and a reversed-field-pinch-record estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the confinement scaling that has characterized most reversed-field-pinch plasmas.

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 measure a wide range of density parameters. This paper will report and discuss the installation of the system and initial single‐chord data from the interferometer.

Summary form only given. The energy loss in the reversed field pinch (RFP) predominantly results from parallel streaming in a stochastic magnetic field. This stochasticity results from B/spl tilde//B/spl sim/1% magnetic fluctuations which accompany m=1, n/spl sim/2R/a tearing (or resistive kink) instabilities in the plasma core. Major research goals in the MST are to understand fluctuation induced transport and to improve plasma confinement using this understanding. Magnetic fluctuation induced transport in the plasma core is studied during a period of high magnetic activity preceding sawteeth events. The flow dynamics of bulk plasma rotation is examined by measuring Doppler shifts of impurity ions spectral lines. Both, spontaneous and actively driven confinement improvement regimes have been observed. After machine conditioning with solid-target boronization, a high confinement regime, characterized by the absence of sawteeth, spontaneously appears during low-density discharges. Similar improvements result by actively applying a transient auxiliary inductive electric field to the MST plasma. The current density gradient is reduced, the growth of the m=1 tearing fluctuations slows, and the energy confinement time doubles. To sustain and enhance the improved plasma, electrostatic and Rf current drivers are being developed.

Summary form only given. The Madison Symmetric Torus (MST) reversed-field pinch is a toroidal, ohmically-heated, magnetic fusion research device capable of plasma currents of I 700 kA and plasma temperatures of several hundred eV. Recent MST discharges have exhibited energy confinement times, /spl tau//sub E/, that are 2-3 times greater than normal. An important benchmark of the MST's performance, /spl tau//sub E/=(total plasma energy)/(input ohmic heating power) and is about 1 ms in normal discharges. /spl tau//sub E/ is limited by transport of energy and particles out of the plasma core due primarily to fluctuations in the structure and magnitude of the internal magnetic field. The dominant magnetic fluctuations are sawtooth events, so-named for the shape of their waveform in many plasma parameters. Sawteeth arise due to overpeaking of the current-density profile on the magnetic axis. During each sawtooth, the total plasma energy decreases, and the ohmic heating power increases, resulting in a sharp drop in /spl tau//sub E/. Recently, discharges with sawtooth-free periods have been achieved, resulting in enhanced confinement. One method of preventing sawteeth is to actively drive current in the edge plasma. This flattens the normally-peaked current-density profile. The best results with this method have been achieved with careful conditioning by solid-target boronization of the plasma-facing inner wall of the vacuum vessel.

Detailed calibrations of the Madison Symmetric Torus polychromator Thomson scattering system have been made suitable for electron temperature fluctuation measurements. All calibrations have taken place focusing on accuracy, ease of use and repeatability, and in situ measurements wherever possible. Novel calibration processes have been made possible with an insertable integrating sphere (ISIS), using an avalanche photodiode (APD) as a reference detector and optical parametric oscillator (OPO). Discussed are a novel in situ spatial calibration with the use of the ISIS, the use of an APD as a reference detector to streamline the APD calibration process, a standard dc spectral calibration, and in situ pulsed spectral calibration made possible with a combination of an OPO as a light source, the ISIS, and an APD used as a reference detector. In addition a relative quantum efficiency curve for the APDs is obtained to aid in uncertainty analysis.

The Madison Symmetric Torus (MST) presents challenging conditions for Thomson scattering (TS) measurements. The MST plasmas are reversed-field pinches (RFPs) with electron density n(e)3x10(13) cm(-3), typically 1x10(13) cm(-3). The TS system was designed to measure from 10 eV to 2 keV; however, six polychromators were upgraded from four to eight spectral channels to resolve to 10 keV. There is no diverter or vertical field, so wall interaction results in high background light both from ion and neutral bremsstrahlungs and from line radiation. Also during standard plasmas, the RFP exhibits regular reconnection sawteeth events during which the plasma current, density, and temperature profiles are flattened. These events are of interest both due to the reconnection physics and to their contribution to the MST equilibrium and confinement. These events occur over 100 microS and exhibit large changes in background light and fast changes in temperature. During improved confinement plasmas, there are no sawteeth; the background is low but the temperature can be over an order of magnitude higher. Data analysis of the system has been developed to accommodate both the large dynamic range of the temperature, the fast dynamics, and the fast changing, high amplitude background. Special attention has been paid to the sources of error, in particular, the contribution of the background. A response-function method reduces the measured uncertainty by a factor of 2. Numerical techniques have been developed which are extremely robust. Two methods are used, a conventional chi(2) minimization using a Levenberg-Marquardt algorithm coupled with Monte Carlo modeling for the error bar and a Bayesian statistics method. The Bayesian method computes the probability distribution for the measured photons and electron temperature and this information can be used to ensemble data and will allow future integrated data analysis efforts.

Innovative charge-exchange recombination spectroscopy (CHERS), motional Stark effect (MSE), and Rutherford scattering diagnostics are now in operation on the Madison Symmetric Torus (MST) reversed-field pinch (RFP). The CHERS diagnostic measures impurity ion flow and temperature, localized to 2cm with high time resolution (∼100kHz). A spectral MSE diagnostic has been in use for five years, measuring ∣B∣ down to 0.2T with high precision (∼2%) and good time resolution (10kHz). The Rutherford scattering diagnostic has demonstrated the robustness of this technique for reliable measurement of majority (D) ion temperature, also with high time resolution. MST is a large RFP (R=1.5m, a=0.52m) operated at moderate current (Ip⩽600kA), with ne typically (1–2)×1019m−3 and Te, Ti⩽2keV. Two compact and reliable diagnostic neutral beams are installed on MST. These beams are short pulse, intense, monoenergetic, and low divergence. The first, a neutral H beam, is used in combination with ultraviolet and visible spectrosco...

We are building a new Thomson scattering diagnostic system to measure electron temperature and density on the Madison Symmetric Torus (MST) reversed-field pinch experiment. This system has been designed to produce accurate single-shot measurements for 10 eV

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

A device is described for measuring the time dependent, volume averaged density of a decaying plasma in a large multimode microwave cavity such as is typically used for plasma confinement studies. The technique consists of filling the cavity with microwave radiation and digitally counting the modes observed with a detector as the plasma decays. Since each mode corresponds statistically to a constant density increment, the density at a given time is proportional to the number of modes that appear as the density decays to zero. Experiments in a toroidal octopole show good agreement with density as measured by other methods.

The increase in the toroidal magnetic field of a tokamak due to plasma presence is measured experimentally and compared with predictions of an equilibrium code.

The feasibility of using poloidal currents to heat plasmas confined by a multipole field has been examined experimentally in Tokapole II, operating the machine as a toroidal octupole. The plasma resistivity ranges from Spitzer to about 1500 times Spitzer resistivity, as predicted by mirror-enhanced resistivity theory. This allows large powers (approx. 2 MW) to be coupled to the plasma at modest current levels. However, the confinement time is reduced by the heating, apparently due to a combination of the input power location (near the walls of the vacuum tank) and fluctuation-enhanced transport. Current-driven drift instabilities and resistive MHD instabilities appear to be the most likely causes for the fluctuations.

The feasibility of using strong poloidal currents to heat plasmas has been examined experimentally in Tokapole II, operating as a toroidal octupole. The plasma resistivity ranges from that of Spitzer to about 1500 times Spitzer resistivity, as predicted by mirror‐enhanced resistivity theory. This allows large powers (approximately 2 MW) to be coupled to the plasma at modest current levels. However, the confinement time is reduced by the heating, apparently due to a combination of the input power location (near the walls of the vacuum tank) and fluctuation‐enhanced transport.

The increase in the toroidal magnetic field of a tokamak due to plasma presence is measured experimentally and compared with predictions of an equilibrium code.