Cary Forest

Techniques for using emitting probes for potential measurement in rf plasmas

Investigations of the effects of rf on plasma potential measurements with electron emitting probes and methods for interpreting data are presented. Techniques correspond to the floating and inflection point methods of single‐emitting and differential emitting probes, respectively. A simple method of measurement of plasma potential fluctuations is given which makes use of time‐averaged emitting probe I–V characteristics.

Laser polarimetric measurement of equilibrium and fluctuating magnetic fields in a reversed field pinch (invited)

New developments in Faraday rotation polarimetry have provided the first measurements of current density profile and core magnetic fluctuations in the core of a high-temperature reversed field pinch. This has been achieved by a fast-polarimeter system with time response up to 1 μs and phase resolution <1 mrad. Recent experiments on Madison Symmetric Torus have directly measured radial magnetic field fluctuations in the plasma interior with amplitude 33 G, ∼1%. A broad spectrum of magnetic fluctuations is observed up to 100 kHz. Relaxation of the current density profile at the sawtooth crash occurs on the timescale of 100 μs. Reversed-field pinch behavior is determined in large part by magnetic fluctuations driven by the radial gradient in the parallel current density. Hence, measurement of magnetic fluctuations and the current density profile is essential to understand the link between the current density profile, fluctuations, and transport.

High confinement plasmas in the Madison Symmetric Torus reversed-field pinch

Reduction of core-resonant m=1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus [Dexter et al., 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 ...

Interaction of energetic particles with large and small scale instabilities

Beyond a certain heating power, measured and predicted distributions of NBI driven currents deviate from each other, in a form that can be explained by the assumption of a modest diffusion of fast particles. Direct numerical simulation of fast test particles in a given field of electrostatic turbulence indicates that for reasonable parameters fast and thermal particle diffusion indeed are similar. High quality plasma edge plasma profiles on ASDEX Upgrade, used in the linear, gyrokinetic, global stability code LIGKA give excellent agreement with the eigenfunction measured by a newly extended reflectometry system for ICRH-excited TAE-modes. They support the hypothesis of TAE-frequency crossing of the continuum in the edge region as explanation of the high TAE-damping rates measured on JET.A new fast ion loss detector with 1MHz time resolution allows frequency and phase resolved correlation between low frequency magnetic perturbation, giving, together with modelling of the particle orbits, new insights into the mechanism of fast particle losses during NBI and ICRH due to helical perturbations.

Equilibrium reconstruction in the Madison Symmetric Torus reversed field pinch

A non-linear Grad–Shafranov toroidal equilibrium reconstruction code (MSTFit) has been developed for the Madison Symmetric Torus. This is the first such code applied to the unique magnetohydrodynamic (MHD) equilibrium of the reversed field pinch. A new set of toroidal Greens tables have been computed to impose the boundary condition of the close-fitting conducting shell. The non-linear fitting routine is sufficiently versatile for incorporating data from a variety of internal and external diagnostics, including a novel constraint based on orbits from a heavy ion beam probe diagnostic. Utilizing the full complement of internal and external magnetic and pressure diagnostics, MSTFit resolves accurately subtle changes in internal magnetic structure with implications on MHD stability. We show example equilibria that confirm conservation of magnetic helicity during relaxation and two-dimensional equilibrium effects.

Observation of a Turbulence-Induced Large Scale Magnetic Field

An axisymmetric magnetic field is applied to a spherical, turbulent flow of liquid sodium. An induced magnetic dipole moment is measured which cannot be generated by the interaction of the axisymmetric mean flow with the applied field, indicating the presence of a turbulent electromotive force. It is shown that the induced dipole moment should vanish for any axisymmetric laminar flow. Also observed is the production of toroidal magnetic field from applied poloidal magnetic field (the omega effect). Its potential role in the production of the induced dipole is discussed.

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.

Numerical simulations of current generation and dynamo excitation in a mechanically forced turbulent flow.

The role of turbulence in current generation and self-excitation of magnetic fields has been studied in the geometry of a mechanically driven, spherical dynamo experiment, using a three-dimensional numerical computation. A simple impeller model drives a flow that can generate a growing magnetic field, depending on the magnetic Reynolds number Rm=micro0sigmaVa and the fluid Reynolds number Re=Vanu of the flow. For Re<420, the flow is laminar and the dynamo transition is governed by a threshold of Rmcrit=100, above which a growing magnetic eigenmode is observed that is primarily a dipole field transverse to the axis of symmetry of the flow. In saturation, the Lorentz force slows the flow such that the magnetic eigenmode becomes marginally stable. For Re>420 and Rm approximately 100 the flow becomes turbulent and the dynamo eigenmode is suppressed. The mechanism of suppression is a combination of a time varying large-scale field and the presence of fluctuation driven currents (such as those predicted by the mean-field theory), which effectively enhance the magnetic diffusivity. For higher Rm, a dynamo reappears; however, the structure of the magnetic field is often different from the laminar dynamo. It is dominated by a dipolar magnetic field aligned with the axis of symmetry of the mean-flow, which is apparently generated by fluctuation-driven currents. The magnitude and structure of the fluctuation-driven currents have been studied by applying a weak, axisymmetric seed magnetic field to laminar and turbulent flows. An Ohms law analysis of the axisymmetric currents allows the fluctuation-driven currents to be identified. The magnetic fields generated by the fluctuations are significant: a dipole moment aligned with the symmetry axis of the mean-flow is generated similar to those observed in the experiment, and both toroidal and poloidal flux expulsion are observed.

Multichannel far-infrared polarimeter-interferometer system on the MST reversed field pinch

The multichannel far-infrared (FIR) heterodyne polarimeter-interferometer system on the Madison Symmetric Torus (MST) is now operational. The combined system consists of 11 channels with variable radial and toroidal spacing. Poloidal magnetic field is determined by measuring the Faraday rotation of the FIR laser beam after propagation through the plasma by use of a phase technique. The polarimeter has 3 mrad rms noise level and 1 ms temporal resolution while the interferometer resolution is nedl=1×1012 cm−2 with time response of 1 μs. Absolute calibration of the polarimeter system is achieved by use of a rotating quartz half-wave plate. The first 11-channel polarimeter measurements from MST indicate a Faraday rotation profile in good agreement with expectations from the MSTFIT equilibrium code. Future plans to reduce the polarimeter time response from 1 ms to 10 μs will allow direct measurement of magnetic fluctuations associated with global resistive tearing modes on MST. The effect of these modes on den...

Off-midplane launch of electron Bernstein waves for current drive in overdense plasmas

Numerical modeling shows that localized, efficient current drive is possible in overdense toroidal plasmas (such as reversed field pinches and spherical tokamaks) using perpendicular launch of electron Bernstein waves. The wave directionality required for driving current can be obtained by launching the waves above or below the midplane of the torus and is a geometric effect related to the poloidal magnetic field. Wave absorption is strong, a result of the electrostatic nature of the waves, giving efficient suprathermal tail formation and current drive.