Magnetic Probe Array Research Articles

Movable magnetic probe array for toroidal Alfven eigenmodes study in the compact helical system heliotron/torsatron

A movable magnetic probe array is installed on the compact helical system (CHS) heliotron/torsatron to study Alfven eigenmodes exited by energetic beam ions. The array consists of six sets of magnetic probes for measurements of radial and poloidal magnetic field fluctuations. This probe array is put inside a thin stainless pipe protected with a c/c-composite carbon sleeve containing 20% boron. The carbon sleeve also acts to minimize deleterious effects on a plasma, evaporating boron during a plasma shot. By using this probe array, radial profiles of magnetic fluctuations in the range of f<200 kHz can be obtained even inside the last closed flux surface up to 〈r〉/〈a〉∼0.8 without serious disturbances to the neutral beam injection (NBI) heated CHS plasmas. High frequency magnetic fluctuations less than 200 kHz related to the toroidal Alfven eigenmodes (TAE) are detected with these magnetic probe arrays in NBI heated plasmas. The mode numbers of the observed TAE can be successfully determined by combination of the radial profiles of fluctuations obtained with the movable probe array and a set of poloidal and toroidal arrays of Mirnov probes.

Particle end loss in the edge plasma of a field-reversed configuration

Plasma parameters, particle end loss flux, flow velocity, and pressure are measured using a radial array of magnetic probes and directional electrostatic probes, in order to investigate particle loss processes in the edge layer of a field-reversed configuration (FRC). A plasma flow toward the end region is detected outside the separatrix between the axial midplane and the end region. The exhaust flow is also found in the end region. These results imply that particles are lost radially across the separatrix and then axially to the end. Measured flow velocity in the end region agrees within an error of 20% with the fluid-theory prediction, in which isentropy and axial momentum balance along magnetic flux tubes are assumed. The existence of the sonic condition in the end region is also suggested, analogous to ordinary fluid flow in a nozzle. The magnetic flux embedded in the edge layer of the confinement region and in the end region agrees within an error of 30%. These results indicate the applicability of the magnetohydrodynamics (MHD) theory for particle end loss. The end loss time along the open field agrees with the MHD prediction within an error of 20%. The measured particle loss flux from the end region is explained by the MHD theory within an error of 20%. The plasma outside the separatrix is considered to behave as hydrodynamic flow through the magnetic loss channel, contrary to the previous work [L. C. Steinhaur, Phys. Fluids 29, 3379 (1986)]. It seems that the magnetic mirror field improves the particle confinement in the edge plasma of the FRC and thus assist the FRC confinement as previously predicted [Slough et al., Nucl. Fusion 24, 1537 (1984)].

Scaling studies of spheromak formation and equilibrium

Formation and equilibrium studies have been performed on the Swarthmore Spheromak Experiment (SSX). Spheromaks are formed with a magnetized coaxial plasma gun and equilibrium is established in both small (dsmall=0.16 m) and large (dlarge=3dsmall=0.50 m) copper flux conservers. Using magnetic probe arrays it has been verified that spheromak formation is governed solely by gun physics (in particular the ratio of gun current to flux, μ0Igun/Φgun) and is independent of the flux conserver dimensions. It has also been verified that equilibrium is well described by the force free condition ∇×B=λB (λ=constant), particularly early in decay. Departures from the force-free state are due to current profile effects described by a quadratic function λ=λ(ψ). Force-free SSX spheromaks will be merged to study magnetic reconnection in simple magnetofluid structures.

Comparison between measurements of the poloidal distribution of magnetic fluctuations and predictions of theoretical models during TAE activity

Fluctuations produced by beam-driven toroidicity-induced Alfven eigenmode (TAE) activity in the DIII-D tokamak are measured by a poloidal array of magnetic probes and compared with the wavefields computed by two theoretical models. Fluid resistive models compute continuum damped TAEs. A kinetic plasma model that retains Landau damping and finite Larmor radius effects computes global drift-kinetic Alfven eigenmodes. The phases of the probes disagree with both theoretical predictions, while the amplitudes agree best with the kinetic model

Determination of microturbulence enhanced electron collisionality in magnetized coaxial accelerator channels by direct magnetic field measurement

A miniature magnetic probe array, consisting of 10 spatially separated coils, has been used to obtain profile information on the time varying magnetic field within the 2.54 cm wide flow channel of the coaxial plasma source experiment (CPS-1) [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)]. The magnetic field data have been used, together with a resistive, Hall magnetohydrodynamic (MHD) model of applied field distortion by the flowing plasma, to obtain estimates of the microturbulent enhancement to electron collisionality within the CPS-1 flow channel. These measurements provide direct experimental evidence of anomalous electron collisionality, a previously predicted effect in these devices. The anomaly parameter, a=νan/νcl, determined both from the distortion of contours of constant magnetic flux, and from local Bθ and Bz measurements scales with the classical electron magnetization parameter (Ωcl=ωce/νecl), indicating that collisionality plays a strong role in determining the level of anomalous transport in the plasma. When this anomaly parameter scaling is cast in terms of the ratio νecl/ωlh, it is found that the resistivity enhancement scales with νecl/ωlh, and becomes significant at νecl/ωlh⩽1, suggesting that a lower hybrid drift instability may be the responsible mechanism for enhanced transport.

Direct magnetic field measurement of flow dynamics in magnetized coaxial accelerator channels

A miniature magnetic probe array, consisting of ten spatially separated coils, has been used to obtain profile information on the time-varying magnetic field within the 2.54 cm wide flow channel of the Coaxial Plasma Source experiment (CPS-1) [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)] at the North Carolina State University. Two-dimensional (2-D) current profiles within the annular flow channel, which were constructed from the time-varying magnetic field data, reveal several complex features reflecting the influence of gun inductance, the Hall effect, and the applied magnetic field. When an external, electrode linking magnetic field is applied, the evolution of the 2-D current profile shows evidence of an ionizing shock front identified by a narrow current sheet propagating through the channel during the first few microseconds of the discharge. The thickness of this current sheet is on the same order as both the collisional mean-free path and the ion electromagnetic skin depth. In this applied field case, the plasma is prevented from advancing ahead of the current sheet by the applied magnetic field, which turns the ions and electrons without collisions. In the absence of an applied field, plasma is able to advance ahead of the current sheet, where it may initiate ionization downstream before the advance of the ionization front.

Measurement of magnetic field fluctuations near plasma edge with movable magnetic probe array in CHS heliotron/torsatron

Magnetic fluctuations are measured with a newly developed movable probe array in the region from the scrape-off layer to just inside the last closed flux surface of neutral-beam-heated plasmas in a CHS. Radial profiles of magnetic fluctuation amplitude and their correlation length are derived for low frequency (less than 40 kHz) coherent components with low mode number and for relatively high frequency (greater than 60 kHz) coherent components are inferred to an ideal interchange mode excited in the plasma interior and have a large radial correlation length L r _>; (average minor radius). The latter components, whose instability is not yet identified, have a relatively short correlation length r ∼ < a>/7.

Measurement of magnetic field fluctuations within last closed flux surface with movable magnetic probe array in the JIPP T-IIU tokamak

Poloidal and radial magnetic field fluctuations less than 100 kHz are measured with a newly developed movable magnetic probe array in the JIPP T-IIU tokamak. The probe array protected by a carbon-carbon composite with 20% boron is inserted beyond the last closed flux surface (LFCS) up to r/a 0.77 without deleterious effect on ohmically and neutra beam heated plasmas. From the radial variation in the fluctuation amplitude just outside the LCFS the poloidal mode number m is estimated to be 4 or less for the low frequency coherent part (f > 30 kHz) and about 10 for the relatively high frequency incoherent part (f > 50 kHz).

Current Sheath Studies in a Small Plasma Focus Operating in Hydrogen–argon Mixtures

We present preliminary time and space resolved studies of current sheath formation in Plasma Focus discharges, using a novel array of non-invasive magnetic field probes. The experiments are performed in a Mather type plasma focus, operating at 2 kV. The discharge is formed between a hollowed anode and six symmetrically arranged cathode rods. The array of small magnetic probes is located along the cathode rods. The probes are of millimeter size. They are shielded behind the rods, as to minimize capacitive coupling to the anode electrode, and allow non-perturbing measurements to be made. A simple analytical model of current sheath evolution is used to analyze the probe signals. The experiments have been performed in pure Hydrogen and Hydrogen with Argon mixture, at pressures from below 0.2 Torr upwards. The effect of the Argon mixture on the current sheath structure is investigated with the probe array. It is found that at constant mass density operation, the increase in the percentage of Argon results in a thinner sheath, with steeper current profile.

Magnetic field mapping with an array of nuclear magnetic resonance probes

Magnetic field maps can rapidly and accurately be acquired on the surface of a cylinder or sphere with an array of nuclear magnetic resonance (NMR) coils multiplexed to a single radiofrequency receiver. Using an array of NMR probes significantly decreases the time necessary to map B0 and improves field map accuracy because the probes do not move. The spatial variation of the Larmor frequency is used to determine the spherical harmonic components present in the field. A limitation of this approach is that NMR resonant circuits are inherently narrow band which limits the apparatus to a specific field strength and only points in space defined by the array layout. This apparatus can characterize the magnetic fields of high homogeneity dipole magnets, quadrupole magnets, gradient coils, and shims used in NMR imaging systems.

High sensitivity, inductively coupled miniature magnetic probe array for detailed measurement of time varying magnetic field profiles in plasma flows

A modified Ḃ circuit design has been implemented as part of a miniature magnetic probe array for the Coaxial Plasma Source experiment [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)] at the North Carolina State University. This facility is currently being used for the generation of energetic plasma flows to allow laboratory study of magnetogasdynamics with particular emphasis on the importance of the Hall effect [D. C. Black et al., Phys. Plasma 1, 3115 (1994)] and plasma microinstabilities [R. M. Mayo et al., Phys. Plasma 2, 337 (1995)] to plasma transport in coaxial plasma sources. The miniature magnetic probe array consists of ten spatially separated coils wound on an Acetal form of dimensions 2.38 cm by 0.32 cm by 0.32 cm. At five positions, with roughly 0.32 cm separation, two mutually perpendicular coils are wound to measure the magnetic field in the θ̂ and ẑ directions. The modification to the signal processing circuitry consists of the use of a step-up transformer to boost the probe signal prior to filtering and acquiring the signal at the data acquisition system. This additional means of amplifying the signal allows for reduction in the size of the probe, and thus helps minimize the perturbing effect of the magnetic probe on the plasma. An additional advantage of using a signal transformer is that it provides electrical isolation between the experiment and the data acquisition system.

The discrete Alfvén wave spectrum observed in the TORTUS tokamak

In this paper, results are presented concerning the excitation of discrete Alfvén wave (DAW) modes in the TORTUS tokamak. The experiments involved sweeping a single antenna in frequency to generate a number of DAW modes which were identified using magnetic probe arrays. With reliable mode identification, and a good number of modes observed, it was possible to estimate the central value of the safety factor for the discharges. It was found that the results could be explained by discharges with sharp profiles of density and central q values below unity. Results are also presented for deuterium and helium discharges, in which the spectrum up to the cyclotron frequency was investigated.

Edge turbulence spectrum alterations driven by resonant fields

An ergodic magnetic structure is generated in the TBR-1 Tokamak boundary to handle heat flux and minimize impurity generation. In this work, the effect of this structure on the plasma edge turbulence is investigated. Thus, spectral alterations driven by perturbing magnetic fields created by resonant helical windings are observed in the small Tokamak TBR-1. The investigated oscillations are detected with an array of Langmuir and magnetic probes. Spectral and bispectral analyses revealed significant changes induced by the resonant helical windings, mainly in the MHD frequency band. Thus, whereas the known Mimov oscillation amplitude reduction is observed, the density and potential oscillation spectra are slightly affected. However, the bispectral analysis shows a clear reduction of the wave-wave mode coupling in the MHD frequency range, while the high frequency turbulent coupling is steadily reduced

Two-dimensional magnetic field evolution measurements and plasma flow speed estimates from the coaxial thruster experiment

Local, time-dependent magnetic field measurements have been made in the Los Alamos coaxial thruster experiment (CTX) [C. W. Barnes et al., Phys. Fluids B 2, 1871 (1990); J. C. Fernández et al., Nucl. Fusion 28, 1555 (1988)] using a 24 coil magnetic probe array (eight spatial positions, three axis probes). The CTX is a magnetized, coaxial plasma gun presently being used to investigate the viability of high pulsed power plasma thrusters for advanced electric propulsion. Previous efforts on this device have indicated that high pulsed power plasma guns are attractive candidates for advanced propulsion that employ ideal magnetohydrodynamic (MHD) plasma stream flow through self-formed magnetic nozzles. Indirect evidence of magnetic nozzle formation was obtained from plasma gun performance and measurements of directed axial velocities up to vz∼107 cm/s. The purpose of this work is to make direct measurement of the time evolving magnetic field topology. The intent is to both identify that applied magnetic field distortion by the highly conductive plasma is occurring, and to provide insight into the details of discharge evolution. Data from a magnetic fluctuation probe array have been used to investigate the details of applied magnetic field deformation through the reconstruction of time-dependent flux profiles. Experimentally observed magnetic field line distortion has been compared to that predicted by a simple one-dimensional (1-D) model of the discharge channel. Such a comparison is utilized to estimate the axial plasma velocity in the thruster. Velocities determined in this manner are in approximate agreement with the predicted self-field magnetosonic speed and those measured by a time-of-flight spectrometer.

Compact toroid formation, compression, and acceleration

Research on forming, compressing, and accelerating milligram-range compact toroids using a meter diameter, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is described. The compact toroids that are studied are similar to spheromaks, but they are threaded by an inner conductor. This research effort, named marauder (Magnetically Accelerated Ring to Achieve Ultra-high Directed Energy and Radiation), is not a magnetic confinement fusion program like most spheromak efforts. Rather, the ultimate goal of the present program is to compress toroids to high mass density and magnetic field intensity, and to accelerate the toroids to high speed. There are a variety of applications for compressed, accelerated toroids including fast opening switches, x-radiation production, radio frequency (rf) compression, as well as charge-neutral ion beam and inertial confinement fusion studies. Experiments performed to date to form and accelerate toroids have been diagnosed with magnetic probe arrays, laser interferometry, time and space resolved optical spectroscopy, and fast photography. Parts of the experiment have been designed by, and experimental results are interpreted with, the help of two-dimensional (2-D), time-dependent magnetohydrodynamic (MHD) numerical simulations. When not driven by a second discharge, the toroids relax to a Woltjer–Taylor equilibrium state that compares favorably to the results of 2-D equilibrium calculations and to 2-D time-dependent MHD simulations. Current, voltage, and magnetic probe data from toroids that are driven by an acceleration discharge are compared to 2-D MHD and to circuit solver/slug model predictions. Results suggest that compact toroids are formed in 7–15 μsec, and can be accelerated intact with material species the same as injected gas species and entrained mass ≥1/2 the injected mass.

Ion ring propagation in a magnetized plasma

The propagation of an ion ring in a 2.5 m long, 0.30 m diam magnetized plasma column was studied using an axial array of magnetic probes and fast proton detectors, a microwave interferometer, and a grating spectrometer. The ring propagated with an initial axial velocity 3×106 m/sec, about four times the Alfvén speed in the plasma, and excited damped magnetosonic waves whose peak amplitude depended on the plasma return current. The plasma electrons were heated by about 1 eV in response to the ring, as measured by the Hγ line-to-continuum intensity ratio. This heating is primarily attributed to classical Coulomb collisions, although about 1%–10% can be attributed to the collective ring–plasma interaction. The collective interaction had no measurable effect on the dynamics of the ion ring, consistent with theoretical models, given the beam, plasma, and magnetic-field parameters in the experiment.

Sources of Fluctuating Field on Field-Reversed Configuration Plasma

Fluctuating azimuthal fields, B θ , are detected during a lifetime of a Field-Reversed Configuration plasma. The maximum strength reaches to 70 G which corresponds to about 1.5% of a confining field. Sources of the fields are discussed with relation to such plasma motions as radial shifting and tilting motions, and elliptical deformation of the plasma column triggered by a rotational instability. Analytic formulas of the B θ are derived from disturbance of the confining field due to these motions. Estimated values from the formulas agree well with field profiles obtained from a magnetic probe array.

Formation of plasma configurations with negative confined magnetic field in different gases of a theta-pinch discharge

A 105 kJ theta-pinch device with a negative bias magnetic field is used to produce plasmas in hydrogen, deuterium, helium, and neon gases. The radial distribution of electron density and temperature is determined as a function of time in the midplane by Thomson scattering. An array of magnetic probes and one flux loop make it possible to measure the plasma diamagnetism and some other important parameters during the first phase of the discharge. The flux retention and other equilibrium properties are derived by fitting a rigid-rotor profile to the experimental density data. These results are compared with theoretical and semiempirical predictions.

Measurements of rf emission and transmission in the 5–500 MHz range in TFTR (abstract)

Active and passive measurements using an array of broadband rf magnetic probes in TFTR have suggested several diagnostic applications. Swept, fast wave propagation experiments using one probe as a launching antenna have revealed absorption peaks from the dominant minority species (hydrogen and helium-3) in ohmic plasmas. A new synchronous modulation/detection system has been installed to allow operation during beam heated plasmas, where spontaneous emission is significant. Proposed experiments will investigate the localization of the absorption and evaluate the utility of this measurement as a diagnostic for tritium. The signal processing system for the probe array has also been recently modified to permit fast gating during ICRF heating pulses. This change allows measurement of the rf emission spectrum immediately following a heating pulse and will be used to look for features arising from slowing down of the high-temperature minority tail. The experiment, by analogy, may give an indication of the spectral signature that will result from alpha particles during future DT operation. This work was supported by U.S. Department of Energy Contract No. DE-AC02-76-CHO-3073.

Probe measurements in the REPUTE-1 reversed field pinch

A four-channel triple-probe and magnetic-probe array, a complex probe which consists of three types of probes and two eight-channel magnetic-probe (poloidal and toroidal) arrays, has been installed in the revised field pinch University of Tokyo experiment (REPUTE-1) reversed field pinch (RFP) device. Mean and fluctuation parts of plasma parameters, including the three components of magnetic fields, three components of electric fields, electron density, electron temperature, and space potential, are measured in 0.5a≲r≤a region. The triple-probe method and the electric field measurement are described in detail, and effects due to the fast electrons, etc., are discussed. Some experimental examples obtained in the REPUTE-1 RFP plasma are given, and the detailed results will be published elsewhere.