MST Reversed Field Pinch Research Articles

Reduced edge instability and improved confinement in the MST reversed-field pinch.

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.

Initial Measurements of Plasma Potential in the Core of the MST Reversed Field Pinch with a Heavy Ion Beam Probe

Measurement of the plasma potential in the core of MST marks both the first interior potential measurements in an RFP, as well as the first measurements by a Heavy Ion Beam Probe (HIBP) in an RFP. The HIBP has operated with (20-110) keV sodium beams in plasmas with toroidal currents of (200-480) kA over a wide range of densities and magnetic equilibrium conditions. A positive plasma potential is measured in the core, consistent with the expectation of rapid electron transport by magnetic fluctuations and the formation of an outwardly directed ambipolar radial electric field. Comparison between the radial electric field and plasma flow is underway to determine the extent to which equilibrium flow is governed by E×B. Measurements of potential and density fluctuations are also in progress.

A diagnostic neutral beam system for the MST reversed-field pinch: Charge-exchange recombination spectroscopy and Rutherford scattering (abstract)

A diagnostic neutral beam system is being purchased for the MST reversed-field pinch. Initially, this beam will be used for charge-exchange recombination spectroscopy (CHERS) to measure impurity ion velocity and temperature, both equilibrium and fluctuating. This work is an extension of an existing MST spectroscopic diagnostic which has successfully measured chord-averaged flow with 10 μs time resolution [D. J. Den Hartog and R. J. Fonck, Rev. Sci. Instrum. 65, 3238 (1994)]. CHERS will provide high spatial resolution, critically needed for the investigation of fluctuations associated with the dynamo and transport. This beam will also be applied to Rutherford scattering for local measurements of majority ion velocity and temperature. Ion velocity is determined by the shift of the energy spectrum of the scattered neutral beam; ion temperature by the width of the energy spectrum. The scattered spectrum will be detected by two multichannel neutral particle analyzers. In order to achieve a time resolution of about 10 μs, the neutral beam equivalent current density and current must be high; both of these characteristics are hallmarks of the DINA beam design.

Enhanced Confinement with Plasma Biasing in the MST Reversed Field Pinch

We report an increase in particle confinement with plasma biasing in a reversed field pinch. Miniature plasma sources are used as electrodes to negatively bias the plasma at the edge $(r/a\ensuremath{\sim}0.9)$. Particle content increases and ${\mathrm{H}}_{\ensuremath{\alpha}}$ radiation decreases upon application of bias and global particle confinement roughly doubles as a result. Energy confinement is not significantly affected by biasing the edge. Measurements of plasma potential, impurity flow, and floating potential fluctuations indicate that strong flows are produced and that electrostatic fluctuations are reduced.

Calibration techniques for a fast duo spectrometer

We have completed the upgrade and calibration of the ion dynamics spectrometer (IDS), a high-speed Doppler duo spectrometer which measures ion flow and temperature in the MST reversed-field pinch. This paper describes an in situ calibration of the diagnostic’s phase and frequency response. A single clock was employed to generate both a digital test signal and a digitizer trigger thus avoiding frequency drift and providing a highly resolved measurement over the system bandwidth. Additionally, we review the measurement of the spectrometer instrument function and absolute intensity response. This calibration and subsequent performance demonstrate the IDS to be one of the fastest, highest throughput diagnostics of its kind. Typical measurements are presented.

Solid target boronization of the MST reversed-field pinch during pulsed discharge cleaning

A solid rod of hot-pressed boron carbide is currently being used as the source of boron during boronization of MST. In previous work we have demonstrated that boronization can be effectively accomplished by insertion of a low apparent density B 4C rod into the edge hydrogen plasma of normal high-power RFP discharges. We have now extended that technique and can boronize MST by inserting a negatively biased B 4C rod into pulsed discharge cleaning (PDC) helium plasmas. The same positive results of reduced impurity contamination and particle reflux are achieved with this new boronization method. The bias for the target is provided by the ohmic heating transformer which is pulsed to produce the PDC discharges. Current flow through the B 4C rod is limited by an inductor. The amount of hydrogen in the amorphous boron carbide films is minimal because the only hydrogen in the PDC plasma is that left over from RFP discharges.

Analysis methods for fast impurity ion dynamics data

A high-resolution spectrometer has been developed and used on the MST reversed-field pinch (RFP) to measure passively impurity ion temperatures and flow velocities with 10 μs temporal resolution. Such measurements of MHD-scale fluctuations are particularly relevant in the RFP because the flow velocity fluctuation-induced transport of current (the ‘‘MHD dynamo’’) may produce the magnetic field reversal characteristic of an RFP. This instrument will also be used to measure rapid changes in the equilibrium flow velocity, such as occur during locking and H-mode transition. The precision of measurements made to date is ≤0.6 km/s. We are developing accurate analysis techniques appropriate to the reduction of this fast ion dynamics data. Moment analysis and curve-fitting routines have been evaluated for noise sensitivity and robustness. Also presented is an analysis method which correctly separates the flux-surface average of the correlated fluctuations in u and B from the fluctuations due to rigid shifts of the plasma column.

Magnetic fluctuation induced transport in MST (abstract)a)

We made local measurements of the magnetic fluctuation induced transport of particles and energy in the outer region (r/a≳0.75) of the MST reversed field pinch plasma. These measurements allow comparison with the Rochester–Rosenbluth stochastic diffusion coefficients. An electrostatic electron energy analyzer and fast pyrobolometer were employed in combination with magnetic pickup coils to directly measure the correlated products 〈J̃θB̃r〉 and 〈q̃∥B̃r〉, respectively. Magnetic fluctuation induced transport of both particles and energy is negligible at the extreme edge of MST (r/a≳0.9), but increases to a level consistent with estimates of the entire particle and energy flux as the core is approached (5×1021 m−2 s−1 and 105 W m−2). Stochastic particle transport is limited by an apparent ambipolar electric field to the ion stochastic diffusion rate (D=50 m2 s−1), the electron transport being dominated by the energetic component of the distribution (E≳50 eV). The measured heat transport is consistent with the convective energy transport carried by the fast electron component (Teffective=100 eV).

Particle transport due to magnetic fluctuations.

Electron current fluctuations are measured with an electrostatic energy analyzer at the edge of the MST reversed-field pinch plasma. The radial flux of fast electrons ($E>{T}_{\mathrm{ea}}$) due to parallel streaming along a fluctuating magnetic field is determined locally by measuring the correlated product $〈{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{J}}_{e}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}}_{r}〉$. Particle transport is small just inside the last closed flux surface (${\ensuremath{\Gamma}}_{e,\mathrm{mag}}<0.1{\ensuremath{\Gamma}}_{e,\mathrm{total}}$), but can account for all observed particle losses inside $\frac{r}{a}=0.85$. Electron diffusion is found to increase with parallel velocity, as expected for diffusion in a region of field stochasticity.

Measurement of magnetic fluctuation induced energy transport.

The local electron energy flux produced by magnetic fluctuations has been measured directly in the MST reversed field pinch (over the radial range r/a>0.75).The flux, produced by electrons traveling parallel to a fluctuating magentic field, is obtained from correlation between the fluctuations in the parallel heat flux and the radial magnetic field. The fluctuation induced flux is large (100 kW/${\mathrm{cm}}^{2}$) in the ``core'' (r/a0.85) and small (10--30 kW/${\mathrm{cm}}^{2}$) in the edge.

An infrared vibration compensated interferometer for the MST experiment

The paper describes a two-color CO2/HeNe laser (10.6/3.4 μm) interferometer which has been designed and built at the IGI laboratory in Italy and installed on the MST Reversed Field Pinch (R=2 m, a=0.52 m, I=0.6 MA) at University of Wisconsin Madison. The system is very compact and is mounted on a 1.2×0.6 m wood board for a total weight of ≊60 kg. The infrared vibration compensation system allows us to use a one-window access (the beam is reflected back by a mirror mounted in the vacuum vessel) and to operate without a stiff mechanical structure embracing the machine. Several new features, such as the use of an interferometric local oscillator, the on-line calibration algorithm, and the four-passage scheme, allow a line-integral density accuracy of 1×1018 m−2.

Measurement of nonlinear mode coupling of tearing fluctuations.

Three-wave nonlinear coupling of spatial Fourier modes is measured in the MST reversed field pinch by applying bispectral analysis to magnetic fluctuations measured at the plasma edge at 64 toroidal locations and 16 poloidal locations, permitting observation of coupling over 8 polodial modes and 32 toroidal modes. Comparison to bispectra predicted by resistive MHD computation indicates reasonably good agreement. However, during the crash phase of the sawtooth oscillation the nonlinear coupling is strongly enhanced, concomitant with a broadened (presumably nonlinearly generated) k spectrum.