L. E. Sharp

Experimental investigation of the magnetic structure in the H-1 heliac

The results of an experimental study of the magnetic structure in the H-1 heliac are presented. Electron beam magnetic mapping has confirmed the existence of closed nested flux surfaces, in good agreement with a computer model. Measurements over a wide range of helical winding currents demonstrated a variety of attainable magnetic configurations within a rotational transform range of 0.6 ≤ t0 ≤ 1.8. The observed islands can be attributed to deduced small errors in the coil alignment in H-1. A magnetic island study and a correction in the model to fit the experimental observations revealed the error sources in the magnetic field

First studies of plasma confined in a toroidal heliac

The first experimental results obtained on plasma confined in a toroidal heliac are reported. A simple method of generating highly ionized, weakly collisional plasma is described. It is found that the geometry of the plasma in general and the measured pressure profiles in particular conform closely with the calculated helical axis magnetic surfaces. Preliminary indications of plasma confinement are favourable.

Magnetic field mapping using an image-intensifying fluorescent probe

A simple, cost‐effective image‐intensifying fluorescent probe designed for mapping the magnetic surfaces in the heliac sheila is described. It consists of a phosphor‐coated metal plate which is enclosed in a grounded U‐channel that provides electrostatic shielding. An adjustable accelerating voltage is applied to the metal plate to greatly increase the cathodoluminescence produced by the directed electron beam from an electron gun, and the visible electron‐beam image is recorded by a CCD camera. The gain in the image brightness allows significant reduction of the electron‐beam energy to minimize the deviation of the measured drift surfaces from the true magnetic surfaces, and to improve resolution for detailed studies of surfaces in the newer stellarator experiments. This technique is particularly suited to electron energies below the phosphor activation threshold, when external image intensifying systems are likely to be very inefficient. Up to 36 toroidal rotations have been observed, limited mainly by ...

CO2 laser scintillation interferometer for the measurement of density fluctuations in plasma confinement devices

Density fluctuations in magnetically confined plasmas have been measured from the phase scintillations impressed onto a 10.6‐μm wavelength laser beam probe, using a Mach–Zender interferometer operating in the optical near field. By careful design, the effects of vibration and air‐borne sound waves, which limit the instrument’s sensitivity, have been reduced to a level such that the minimum detectable signal is 10−6 rad in the frequency range 10 kHz–1 MHz, corresponding to a plasma density fluctuation level (δN/N) of 10−4.

Magnetic field mapping system on the H‐1 heliaca)

The H‐1 heliac recently brought into operation is a medium‐sized 3‐field‐period heliac with major radius R0=1 m, plasma mean minor radius 〈a〉≤0.2 m and a wide range of rotational transforms 0.6≤ι–(0)≤2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150–200 toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas (p∼10−4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured ι/ profiles show very good agreement with the computer model results.

The H-1 radio frequency system and an initial study of plasma formation

Abstract Eight commercial broadcast transmitters rated at 30 kW PEP 4–26 MHz are being commissioned for plasma formation and heating in H-1. These may be either excited separately or combined into two antennas by a set of six power combiners. In this paper we describe the various components of the r.f. power transmission system. We present preliminary results using this system for plasma formation in argon and hydrogen with magnetic fields

Magnetic field mapping system on the H‐1 heliac (abstract)a)

The H‐1 heliac recently brought into operation is a medium‐sized 3‐field‐period heliac with major radius R0=1 m, plasma mean minor radius 〈a〉≤0.2 m and a wide range of rotational transforms 0.6≤■(0)≤2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150–200 of toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas (p×10−4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured ■ profiles show very good agreement with the computer model results.

Scintillation measurement of density fluctuations in a tokamak plasma (abstract)

Recent theoretical results for line integrated measurements of phase and amplitude perturbations introduced on a laser beam probing a randomly fluctuating plasma are compared with phase scintillation data obtained on the LT‐4 tokamak. It is shown that an estimate of the transverse velocity profile along the line‐of‐sight can be obtained from single shot measurements of the phase perturbations.

Optimum design of helical windings for a toroidal ℓ = 3 stellarator with large separatrix

The paper presents the results of a detailed computational study of possible l = 3 stellarator winding configurations in which the separatrix radius and position are maintained constant. It is shown that toroidal effects on both the rotational transform properties and the mechanical forces can be reduced by suitably modulating the winding pitch, which results in considerable savings in the required current and mechanical support. The existence of significant magnetic wells is demonstrated. The results generally favour the use of shallow rather than steep windings and fairly large aspect ratios.