Reversed Field Pinch Discharges Research Articles

Preliminary compact torus injection experiments on the Keda Torus eXperiment reversed field pinch

Abstract A new compact torus injector (KTX-CTI) has been built for injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP). The aim is to study the fundamental physics governing the compact torus (CT) central fueling processes. In experiments conducted under the sole influence of a 0.1 T toroidal magnetic field, the injected CT successfully penetrated the entire toroidal magnetic field, reaching the inner wall of the KTX vacuum vessel. Upon reaching the inner wall, the CT diffused both radially outward and toroidally within the vessel at a discernible diffusion speed. Moreover, the inherent helicity within the CT induced a modest KTX plasma current of 200A, consistent with predictions based on helicity conservation. CT injection demonstrated the capability to initiate KTX discharges at low loop voltages, suggesting its potential as a pre-ionization and current startup technique. During RFP discharges featuring CT injection, the central plasma density was found to exceed the Greenwald density limit, with more peaked density profiles, indicating the predominant confinement of CT plasma within the core region of the KTX bulk plasma.

Development of a compact torus injection system for the Keda Torus eXperiment

A compact torus injection system, KTX-CTI, has been developed for the planned injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device to investigate the physics and engineering issues associated with interaction between a compact torus (CT) and RFP. The key interests include fueling directly into the reactor center, confinement improvement, and the injection of momentum and helicity into the RFP discharges. The CT velocity and mass have been measured using a multichannel optical fiber interferometer, and for the first time the time evolution of the CT density profile during CT propagation is obtained. The effects of discharge parameters on the number of injected particles, CT velocity and CT density have been characterized: the maximum hydrogen CT plasma mass, is 50 μg, corresponding to 30% of the mass in a typical KTX plasma; the CT velocity exceeds 120 km s−1. It is observed for the first time that multiple CTs can be produced and emitted during a very short period (<100 μs) in one discharge, which is significant for the future study of repetitive CT injection, even with an ultra-high frequency.

Stepped pressure equilibrium with relaxed flow and applications in reversed-field pinch plasmas

The multi-region relaxed magnetohydrodynamics (MRxMHD) has been successful in the construction of equilibria in three-dimensional (3D) configurations. In MRxMHD, the plasma is sliced into sub-volumes separated by ideal interfaces, each undergoing relaxation, allowing the formation of islands and chaos. The resulting equilibrium has a stepped pressure profile across sub-volumes. The stepped pressure equilibrium code (SPEC) (S R Hudson et al, Phys. Plasmas 19, 112502 (2012)) was developed to calculate MRxMHD equilibria numerically. In this work, we have extended the SPEC code to compute MRxMHD equilibria with field-aligned flow and rotation, following the theoretical development to incorporate cross-helicity and angular momentum constraints. The code has been verified for convergence and compared to a Grad–Shafranov solver in 2D. We apply our new tool to study the flow profile change before and after the sawtooth crash of a reversed-field pinch discharge, in which data of the parallel flow is available. We find the promising result that under the constraints of cross-helicity and angular momentum, the parallel flow profile in post-crash SPEC equilibrium is flat in the plasma core and the amplitude of the flow matches experimental observations. Finally, we provide an example equilibrium with a 3D helical field structure as the favoured lower energy state. This will be the first 3D numerical equilibrium in which the flow effects are self-consistently calculated.

Correlation polarimetry for broadband magnetic fluctuation measurements.

Correlation techniques have been successfully utilized for plasma diagnostics like electron cyclotron emission to reduce measurement noise. We present the first application of such a technique to Faraday-effect polarimetry measurements on the Madison Symmetric Torus (MST). The MST far infrared (FIR) interferometer-polarimeter diagnostic utilizes 11 vertical chords with a chord separation of 7-8 cm and a heterodyne detection system for fluctuation measurements up to several hundred kHz. The planar-diode mixers viewing each chord represent independent noise sources; modifying the optical setup so that two different mixers view the same chord allows cross correlation between the two independent signals to reduce the noise floor in fluctuation measurements. In this manner, the noise floor in both interferometry and polarimetry measurements in reversed-field pinch discharges has been reduced by a factor of 20-30. The correlation polarimeter provides a sensitive measurement of broadband magnetic fluctuations.

Overview of Keda Torus eXperiment initial results

The Keda Torus eXperiment (KTX) is a new reversed field pinch (RFP) device at the University of Science and Technology of China. The construction and assembly of KTX, including the vacuum chamber, conducting shell, magnetic field windings, power supply system, active control coils, vacuum pump and data acquisition system, have been completed on August 1, 2015. Immediately following that, the first plasma was obtained on August 15, 2015. Intensive conditioning of the machine is underway to ramp up the plasma current toward its full operation. An active feedback mode control system has been built and has been implemented to control the error field around the vertical gaps of the conducting shell. The pulsed power supply systems of ohmic heating field and toroidal field (TF), using thyristor and energy storage capacitors, have been tested and commissioned. The TF power supply has flexibility in being able to operate with a reversed TF configuration and stable TF configuration. The fundamental diagnostic tools are developed for early KTX operation. Currently, the plasma current is up to 205 kA and the maximum discharge length is 21 ms, approaching to the conducting shell penetration time. Furthermore, typical RFP discharges are being produced with RFP state lasting 2 ms. These initial operation results for KTX are described in detail.

X-ray analysis of electron Bernstein wave heating in MST.

A pulse height analyzing x-ray tomography system has been developed to detect x-rays from electron Bernstein wave heated electrons in the Madison symmetric torus reversed field pinch (RFP). Cadmium zinc telluride detectors are arranged in a parallel beam array with two orthogonal multi-chord detectors that may be used for tomography. In addition a repositionable 16 channel fan beam camera with a 55° field of view is used to augment data collected with the Hard X-ray array. The chord integrated signals identify target emission from RF heated electrons striking a limiter located 12° toroidally away from the RF injection port. This provides information on heated electron spectrum, transport, and diffusion. RF induced x-ray emission from absorption on harmonic electron cyclotron resonances in low current (<250 kA) RFP discharges has been observed.

Dynamics of a reconnection-driven runaway ion tail in a reversed field pinch plasma

While reconnection-driven ion heating is common in laboratory and astrophysical plasmas, the underlying mechanisms for converting magnetic to kinetic energy remain not fully understood. Reversed field pinch discharges are often characterized by rapid ion heating during impulsive reconnection, generating an ion distribution with an enhanced bulk temperature, mainly perpendicular to magnetic field. In the Madison Symmetric Torus, a subset of discharges with the strongest reconnection events develop a very anisotropic, high energy tail parallel to magnetic field in addition to bulk perpendicular heating, which produces a fusion neutron flux orders of magnitude higher than that expected from a Maxwellian distribution. Here, we demonstrate that two factors in addition to a perpendicular bulk heating mechanism must be considered to explain this distribution. First, ion runaway can occur in the strong parallel-to-B electric field induced by a rapid equilibrium change triggered by reconnection-based relaxation; this effect is particularly strong on perpendicularly heated ions which experience a reduced frictional drag relative to bulk ions. Second, the confinement of ions varies dramatically as a function of velocity. Whereas thermal ions are governed by stochastic diffusion along tearing-altered field lines (and radial diffusion increases with parallel speed), sufficiently energetic ions are well confined, only weakly affected by a stochastic magnetic field. High energy ions traveling mainly in the direction of toroidal plasma current are nearly classically confined, while counter-propagating ions experience an intermediate confinement, greater than that of thermal ions but significantly less than classical expectations. The details of ion confinement tend to reinforce the asymmetric drive of the parallel electric field, resulting in a very asymmetric, anisotropic distribution.

Overview of gyrokinetic studies of finite-β microturbulence

Recent results on electromagnetic turbulence from gyrokinetic studies in different magnetic configurations are overviewed, detailing the physics of electromagnetic turbulence and transport, and the effect of equilibrium magnetic field scale lengths. Ion temperature gradient (ITG) turbulence is shown to produce magnetic stochasticity through nonlinear excitation of linearly stable tearing-parity modes. The excitation, which is catalyzed by the zonal flow, produces an electron heat flux proportional to β2 that deviates markedly from quasilinear theory. Above a critical beta known as the non-zonal transition (NZT), the magnetic fluctuations disable zonal flows by allowing electron streaming that shorts zonal potential between flux surfaces. This leads to a regime of very high transport levels. Kinetic ballooning mode (KBM) saturation is described. For tokamaks saturation involves twisted structures arising from magnetic shear; for helical plasmas oppositely inclined convection cells interact by mutual shearing. Microtearing modes are unstable in the magnetic geometry of tokamaks and the reversed field pinch (RFP). In NSTX instability requires finite collisionality, large beta, and is favored by increasing magnetic shear and decreasing safety factor. In the RFP, a new branch of microtearing with finite growth rate at vanishing collisionality is shown from analytic theory to require the electron grad-B/curvature drift resonance. However, gyrokinetic modeling of experimental MST RFP discharges at finite beta reveals turbulence that is electrostatic, has large zonal flows, and a large Dimits shift. Analysis shows that the shorter equilibrium magnetic field scale lengths increase the critical gradients associated with the instability of trapped electron modes, ITG and microtearing, while increasing beta thresholds for KBM instability and the NZT.

Effect on Plasma Performance of a Single MHD Mode Feedback Control in Low-Aspect-Ratio RFP RELAX

A feedback control system for the stabilization of resistive wall mode (RWM) was applied to a low-aspect-ratio reversed field pinch (RFP) with minimum power supply capabilities to control the single mode. The system consists of 64 saddle coils (4 and 16 in poloidal and toroidal direction, respectively) in the actuator covering the whole torus on the outer surface of the vacuum vessel. The sensor coils also have the same structure. The saddle coils are connected in series to control the single m/n = 1/2 mode, which has the largest growth rate in RELAX. The radial component of the magnetic field from the sensor coils was suppressed to the preset level and the m/n = 1/2 magnetic mode, which otherwise grows with field penetration time of the vessel, was reduced to 0.1% of the edge poloidal field throughout the discharge. The RFP discharge duration has been extended to ∼3.5 ms, the upper bound determined by the saturation of the iron core. Finally, the MHD control issues in a low-A machine are discussed.

Heavy ion beam probe operation in time varying equilibria of improved confinement reversed field pinch discharges

Operation of a heavy ion beam probe (HIBP) on a reversed field pinch is unique from other toroidal applications because the magnetic field is more temporal and largely produced by plasma current. Improved confinement, produced through the transient application of a poloidal electric field which leads to a reduction of dynamo activity, exhibits gradual changes in equilibrium plasma quantities. A consequence of this is sweeping of the HIBP trajectories by the dynamic magnetic field, resulting in motion of the sample volume. In addition, the plasma potential evolves with the magnetic equilibrium. Measurement of the potential as a function of time is thus a combination of temporal changes of the equilibrium and motion of the sample volume. A frequent additional complication is a nonideal balance of ion current on the detectors resulting from changes in the beam trajectory (magnetic field) and energy (plasma potential). This necessitates use of data selection criteria. Nevertheless, the HIBP on the Madison Symmetric Torus has acquired measurements as a function of time throughout improved confinement. A technique developed to infer the potential in the improved confinement reversed field pinch from HIBP data in light of the time varying plasma equilibrium will be discussed.

Plasma behaviour at high β and high density in the Madison Symmetric Torus RFP

Pellet fuelling of improved confinement Madison Symmetric Torus (MST) plasmas has resulted in high density and high plasma beta. The density in improved confinement discharges has been increased fourfold, and a record plasma beta (βtot = 26%) for the improved confinement reversed-field pinch (RFP) has been achieved. At higher β, a new regime for instabilities is accessed in which local interchange and global tearing instabilities are calculated to be linearly unstable, but experimentally, no severe effect, e.g., a disruption, is observed. The tearing instability, normally driven by the current gradient, is driven by the pressure gradient in this case, and there are indications of increased energy transport (as compared with low-density improved confinement). Pellet fuelling is also compared with enhanced edge fuelling of standard confinement RFP discharges for the purpose of searching for a density limit in MST. In standard-confinement discharges, pellet fuelling peaks the density profile where edge fuelling cannot, but transport appears unchanged. For a limited range of plasma current, MST discharges with edge fuelling are constrained to a maximum density corresponding to the Greenwald limit. This limit is surpassed in pellet-fuelled improved confinement discharges.

One-dimensional simulations of reversed field pinch discharges

A simple one-dimensional time-dependent model for simulating the magnetic field profiles in a reversed field pinch (RFP) plasma is presented. The model, which is derived from basic MHD equations with the addition of a phenomenological dynamo electric field, is able, through the tuning of a few free parameters, to simulate the time evolution of RFP discharges in a large range of operating conditions, successfully reproducing quantities such as the average toroidal field, the loop voltage and the on-axis safety factor. The application to several experimental conditions obtained on the RFX-mod machine [P. Sonato et al., Fusion Eng. Des. 66, 161 (2003)], including oscillating poloidal current drive (OPCD) is shown and the simulation outcome is discussed.

MHD Properties of Low-aspect Ratio RFP in RELAX

The low-aspect-ratio (A) reversed field pinch (RFP) offers attractive properties such as enhanced bootstrap current and simpler MHD mode dynamics. The RELAX (REversed field pinch of Low-Aspect ratio eXperiment) machine with the world’s lowest A of 2 (R/a = 0.5 m/0.25 m) has been constructed to explore the RFP properties in low-A regime. In flat-topped low-A RFP discharges in RELAX, plasma current of ~50 kA has been attained with discharge duration of ~2 ms. In round-topped discharges with plasma current of ~70 kA, quasi-periodic growth of a single helical mode has been observed. When the dominant m = 1/n = 4 mode grows, the toroidal mode spectrum looks like that of the quasi-single helicity (QSH) RFP state with higher amplitude. MHD equilibrium analyses using a reconstruction code have shown that the bootstrap current fraction is lower than ~5% in the present RELAX plasmas, and it will be ~25% if we could achieve the plasma density of 4 × 1019 m−3 and electron temperature of 300 eV at plasma current of ~100 kA.

Observation of Large-Scale Profile Change of Magnetic Field in a Low-Aspect Ratio Reversed Field Pinch

Observation of Large-Scale Profile Change of Magnetic Field in a Low-Aspect Ratio Reversed Field Pinch

Reduction of Thermal Wall Load by Gas Puffing in Reversed-Field Pinch Device, TPE-RX

We report that the thermal wall load due to locked modes can be decreased by means of gas puffing in the reversed-field pinch (RFP) device, TPE-RX [Y. Yagi et al.: Fus. Eng. Design 45 (1999) 409]. Under standard RFP operation in TPE-RX, the phase- and wall-locked modes, i.e., spatially localized and stationary magnetic perturbations, respectively, appear after the current-rising phase. The locked modes cause a severe thermal wall load of the order of 100 MW/m2 on the first wall. It is estimated that the temperature on the plasma-facing wall at the mode-locking position sometimes exceeds the melting temperature of stainless steel. The thermal wall load increases with plasma current, Ip. We have succeeded in mitigating the high thermal wall load by gas puffing during discharges. It is shown that the maximum temperature increment of the vacuum vessel wall is halved when a gas-puffing rate of 200 Pa m3/s is applied at Ip=350 kA. The characteristics of the temperature increase of the vacuum vessel are presented, and they indicate the nature of the locked mode in RFP discharges in TPE-RX.

Characteristics of global confinement properties in TPE-series reversed-field pinch devices

A confinement database of the reversed-field pinch (RFP) devices of the toroidal pinch experiment (TPE) series has been established. The TPE database contains global confinement properties under various experimental conditions for four different devices and seven different cases, which cover two decades of experiments. Characteristics of the TPE database have been extracted under consistent experimental conditions of conventional RFP discharges, and scaling laws of the poloidal beta, βp and energy confinement time, τ E, are given against independent parameters that include the size of the plasma and the pinch parameter Θ. Scaling laws of and are obtained for a set of scans, where a is the plasma minor radius, Ip the plasma current and N the column density. The scaling of τ E on a, Ip, and I/N is similar to that dominated by the transport under tearing instability. Another scaling shows that τ E is inversely proportional to the Joule input power. The scaling for the apparent effective charge shows a positive correlation both with Ip and I/N.

High-Density Current Injection to Magnetically Confined Plasmas with a Miniature Plasma Source

Influences of high-density current injection with a miniature plasma source on a reversed-field pinch (RFP) have been studied in the Separatrix Test Experiment-2 (STE-2) device. The current injector has successfully produced a unidirectional field-aligned electron beam. No deterioration or slight improvement of the RFP discharge was observed with injection of poloidal current of up to 1 kA. The deepening of the local edge toroidal magnetic field reversal has been observed in the vicinity of the injection port. The global change in the current profile and the stabilization of m=1 tearing modes will require current injection at multiple toroidal locations with a higher bias voltage. However, the gas throughput from the injector is too high, hindering the addition of more injectors in the STE-2 device. Global current profile control experiments with multiple miniature plasma sources are expected in larger devices.

RFP plasma response to external helical perturbations

Plasma response to four kinds of static helical perturbations with different helicities has been studied in the STE-2 reversed field pinch (RFP). With application of externally non-resonant helical fields which have no resonance between the field reversal surface and shell, RFP discharges have been slightly improved with characteristics of higher pinch parameter and suppressed coupling of m = 0 modes for perturbation level of up to . On the other hand, degradation of RFP discharges have been caused by low level of helical fields resonant inside the reversal surface with indications of enhanced m = 1 mode coupling. The degradation is shown to be caused by sufficient overlapping of the static and inherent magnetic islands, the latter being associated with m = 1 tearing modes.

Superthermal electron generation and transport in the reversed-field pinch

A new technique for the analysis and interpretation of reversed-field pinch discharges is presented, using the fast electron population in the plasma core as an indicator of the level of magnetic turbulence. The method combines x-ray measurements of the superthermal electron distribution in the plasma core with a self-consistent three-dimensional Fokker - Planck code. This technique is illustrated by an application to low-density discharges of the MST device, using measured soft x-ray spectra to constrain the simultaneous computation of the electron distribution function and of the magnetic fields. The consistency of the experimental results, analysed in this way with both kinetic and MHD dynamo theories is discussed. The best agreement between the code and the measured x-ray spectrum occurs when an MHD dynamo field of the appropriate level and shape is included with radial diffusion due to fluctuating fields %.

Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

Most Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2–5 ms, corresponding to magnetohydrodynamic (MHD) instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent.