Magnetohydrodynamic Activity Research Articles

Gridded ionization chamber for detection of x-ray wave activity in tokamak plasmas

In order to carry out x-ray observations of magnetohydrodynamic wave activity of the plasma during DD and DT (deuterium-beam-heated deuterium and tritium plasmas, respectively) operation of the Tokamak Fusion Test Reactor (TFTR), we will need detectors not susceptible to nuclear radiation damage. We have investigated the use of gridded ionization chambers as fast nondamageable x-ray detectors. A prototype chamber is described which was tested on the PDX tokamak. These tests and laboratory tests with a pulsed x-ray source suggest that the detector has sufficient sensitivity and speed for the required measurements.

Special Section Authors

Special Section Authors

Magnetohydrodynamic Activity in High-β, Currentless Plasmas in Heliotron-E

Electron-cyclotron and neutral-beam heating have been used to produce currentless plasma with $〈\ensuremath{\beta}〉\ensuremath{\approx}(1\ensuremath{-}2)%$ in the Heliotron-$E$ device. For moderately peaked profiles, magnetohydrodynamic activity that peaks at the $t=1$ surface has been observed. The characteristics of the fluctuations are consistent with theoretical expectations for an $m=1$ pressure-driven instability. The magnetohydrodynamic activity is associated with particle and energy losses but has not yet been directly linked to global confinement.

The LT-4 Tokamak. I. Description of the Apparatus and Its Operation

The LT-4 is a relatively small (major radius 0�5 m, minor radius 1'10� 1 m) experimental tokamak using a homopolar generator as its main power source to produce a toroidal field of 3 T. A fast currentforcing feedback control system maintains the plasma position constant to 1'1 1 mm. Fairly standard diagnostics are supported by a data acquisition system which handles 200 kbytes of data through 70 CAMAC controlled ADCs per discharge. Illustrative examples of data obtained during .typical operation show magnetohydrodynamic (MHD) activity (magnetic pick-up coils and soft X-ray detectors) as well as relaxation oscillations within the plasma.

D-3 He reaction measurements during fast wave minority heating in the PLT tokamak experiment

Time- and energy-resolved d-3He fusion reactions have been measured to infer the energy of the d+ or He++ minority ions heated near their cyclotron frequency by the magnetosonic fast wave. The average energy of the reacting 3He ions during 3He minority heating is in the range of 100–400 keV, as deduced from the magnitude of the reaction rate, its decay time, and the energy spread of the proton reaction products. The observed reaction rate and its scaling with wave power and electron density and temperature are in qualitative agreement with a radial reaction rate model using the minority distribution predicted from quasilinear velocity space diffusion. Oscillations in the reaction rate are observed concurrent with sawtooth and m=2 magnetohydrodynamic activity in the plasma.

Study of High-Beta Magnetohydrodynamic Modes and Fast-Ion Losses in PDX

Strong magnetohydrodynamic activity has been observed in PDX neutral-beam-heated discharges. It occurs for βTq>~0.045 and is associated with a significant loss of fast ions and a drop in neutron emission. As much as 20%-40% of the beam heating power may be lost. The instability occurs in repetitive bursts of oscillations of ≤ 1 msec duration at 1-6-msec intervals. The magnetohydrodynamic activity has been dubbed the "fishbone instability" from its characteristic signature on the Mirnov coils.Received 29 September 1982DOI:https://doi.org/10.1103/PhysRevLett.50.891©1983 American Physical Society

Density Threshold for Magnetohydrodynamic Activity in AlcatorC

Experimental measurements of magnetic fluctuations arising from resistive magnetohydrodynamic tearing modes reveal unexpected behavior in Alcator $C$. After the plasma current has risen to its steady-state level, no significant magnetohydrodynamic activity is observed unless the electron density is above a threshold value, ${\overline{n}}_{c}$. For a given toroidal field, ${\overline{n}}_{c}$ is remarkably repeatable. We find ${\overline{n}}_{c}$ to be proportional to ${B}^{2}$ and independent of ${I}_{p}$.

Finite β effects on the nonlinear evolution of the (m = 1; n = 1) mode in tokamaks

The stability and evolution of ISX-B-like plasmas are numerically studied using a reduced set of resistive magnetohydrodynamic equations. For a sequence of equilibria stable to ideal modes, the n = 1 mode changes from a tearing branch to a pressure-driven branch as βp is increased. When this mode is unstable at low β, it is just the (m = 1; n = 1) tearing mode. Higher n modes also become linearly unstable with increasing βp; they are essentially pressure-driven and have ballooning character. For low values of β, the instability is best described as a βp distortion of the (m = 1; n = 1) tearing mode. This mode drives many other helicities through toroidal and nonlinear couplings. As βp is increased, the growth of the m = 1 island slows down in time, going from exponential to linear before reconnection occurs. If βp is large enough, the island saturates without reconnection. A broad spectrum of other modes, driven by the (m = 1; n = 1) instability, is produced. These results agree with some observed features of magnetohydrodynamic activity in ISX-B.

Magnetohydrodynamic Instability with Neutral-Beam Heating in the ISX-BTokamak

This paper describes observations of magnetohydrodynamic instability with neutral-beam heating in the ISX-B tokamak and the theory specifically developed to support these experiments. The observed magnetohydrodynamic activity is explained by the resistive model presented but is not responsible for the observed degradation of confinement. Increasingly important n>1 pressure-driven modes are predicted by the theory for the higher experimental ..beta../sub p/ values, but there is no experimental verification of their presence.

Magnetic energy storage and conversion in the solar atmosphere

Until man broke gravity’s shackles, now more than 20 years ago, our knowledge of the multifaceted role played by magnetic fields on the Sun was meager. The past few decades of steadily improving X-ray and ultraviolet observations above the atmosphere have fostered a corresponding growth in our understanding of the importance of magnetic energy conversion and release and magnetohydrodynamic activity to virtually all components of solar variability which affect our environment here on Earth. Every experimental improvement has brought us a closer and better view of the dynamic, sometimes violent, MHD and plasma phenomena which are now generally agreed to control the solar transition region, corona, solar wind, flares, spots, bright points, and spicules.

Magnetohydrodynamic activity in the JFT-2 tokamak with high-power neutral-beam-injection heating

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged ⟨β⟩ of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.