D. W. Kerst

The Madison Symmetric Torus

AbstractThe Madison Symmetric Torus (MST) is the newest and largest reversed-field pinch (RFP) currently in operation. It incorporates a number of design features that set it apart from other pinches, including the use of the conducting shell as both a vacuum vessel and single-turn toroidal field coil. Specially insulated voltage gaps are exposed to the plasma. Magnetic field errors at these gaps as well as at the various diagnostic and pumping ports are minimized through a variety of techniques. The physics goals of MST include study of the effect of large plasma size on confinement and the detailed investigation of RFP turbulence, dynamo, and transport. Details of the design and initial operation of the device are presented.

Plasma Motion and Confinement in a Toroidal Octupole Magnetic Field

Plasma of approximately 100 eV in ion energy was injected into and trapped within an octupole magnetic field with closed lines of force contained within a toroidal region of magnetohydrodynamic stability. A zero‐field locus was near the middle of the region, and fields of 1000 to 8000 G existed along the wall and around the current‐carrying hoops forming the multipole field with 4.7 to 9.6 gyroradii across the stable plasma region. Observed electric fields caused by the self polarization charge at the edge of a moving plasma cloud enabled the injected plasma stream to penetrate the octupole field and to flow rapidly throughout the low‐field region. The high‐polarization potentials disappeared after 100 μsec as the trapped plasma evolved into a quiescent state having typically a lifetime of 0.4 msec for 100‐eV ions. The electron temperature was approximately 10 eV at all times. Density fluctations and field fluctuations during confinement were negligibly small compared to those observed in plasmas afflicted with anomalous diffusion. Electric potential fluctuations were less than 2% of the electron temperature. This quiescent condition continued while the plasma density was smoothly decreasing by loss on probes and hoop supports.

A 20‐Million Electron‐Volt Betatron or Induction Accelerator

The details of construction of an improved induction accelerator which gives electrons 20‐million electron‐volts energy are described. The accelerator has a 19‐inch diameter pole face and weighs approximately 3.5 tons. The x‐ray output as measured in a thick wall ionization chamber is 16 r.p.m. at one meter. The most important improvement incorporated in this accelerator is the electromagnetic expansion of the equilibrium orbit which can be timed to send the electrons against the target at any desired energy up to 20 million electron volts. The accelerator is now in use at the University of Illinois.

Experimental Depth Dose for 5, 10, 15 and 20-Million-Volt X-Rays

With the 20-million-volt electron beam of good intensity now produced in the University of Illinois betatron, questions about the practical use of high-energy radiations can be examined. The most promising way to use the betatron in therapy would be to send the original electrons accelerated in the vacuum tube directly into the patient. At 20 million volts these electrons will penetrate as far as 10 cm. and no farther. Thus no damage is done to the back of the patient. Furthermore, the ionization should reach a maximum 7 or 8 cm. beyond the entrance surface for the electrons, and the damage could be well localized within the body. About a 25- or 30-million-volt betatron would be ideal for this work, since it has the right energy and a reasonable size. Although a sufficiently intense beam of electrons now comes out of the betatron, it is not yet in a good enough state of collimation or control for practical use. The x-rays produced by this electron stream when it strikes a target cause an ionization intens...

An 80‐Mev Model of a 300‐Mev Betatron

The construction and operation of a flux‐biased 80‐Mev betatron is described. This betatron was built as a model to test design features later to be incorporated in a 300‐Mev betatron. Particular attention is paid to factors influencing design and to considerations of magnetic problems which are common to betatrons and synchrotrons such as the construction of field magnets which are uniform, the testing of fields at injection time, the mechanism of electron capture, and the influence of field inhomogeneities. The dimensions and operating characteristics are included together with many comparisons with the 300‐Mev betatron.

The Determination of Photon Flux for Energies between 150 and 300 Mev

A calorimetric method of determining photon flux is described. Absolute calibration was performed with an improved Pb block calorimeter using thermistors as temperature‐sensitive elements. Five‐minute irradiations were employed with a maximum of 33 milliwatts emerging from a 0.5‐in. collimator placed at 115 cm from the betatron target. A secondary standard was developed for the purpose of determining bremsstrahlung flux. This secondary standard was a 4‐in. diameter, parallel plate ionization chamber consisting of a 0.016‐in. Al center foil separated by 0.052‐in. air gaps from 0.25‐in. Cu front and back plates and adapted for use with a Victoreen M‐70 electrometer. With corrections described, the results, in ergs/esu per cm air (NTP), are 7340 at 300 Mev, 6370 at 250 Mev, 5720 at 200 Mev, and 5370 at 150 Mev. Calibration of Victoreen thimbles with various types of converter were also made.

A Van de Graaff Electrostatic Generator Operating Under High Air Pressure

An electrostatic generator of the Van de Graaff type has been developed which operates in a large steel tank under an air pressure up to 45 lbs. The maximum steady voltage obtained was about 750 kv. In applying this voltage to an evacuated tube for acceleration of protons the usable voltage is limited by the dimensions of the present tube to about 400 kv. The apparatus has given satisfactory service during a year of use in nuclear disintegration work.

TOROIDAL DISCHARGE EXPERIMENTS WITH RAPID PROGRAMMING

Five plasma configurations were studied in a fast‐programmed toroidal discharge device where magnetic field errors were minimized. The effects of the programming on the plasma were examined with magnetic probes. a Kerr cell camera, and a monochromator. Without programming, the plasma developed a well‐defined kink which corresponded to the worst mode of the instability predicted by hydromagnetic theory. By programming, the plasma lasted for 10 to 15 μsec at ion temperatures up to 130 eV and with densities up to 3 × 1016 cm−3.

Electrical circuit modeling of conductors with skin effect

The electrical impedance of a lossy conductor is a complicated function of time (or frequency) because of the skin effect. By solving the diffusion equation for magnetic fields in conductors of several prototypical shapes, the impedance can be calculated as a function of time for a step function of current. The solution suggests an electrical circuit representation that allows calculation of time‐dependent voltages and currents of arbitrary waveforms. A technique using an operational amplifier to determine the current in such a conductor by measuring some external voltage is described. Useful analytical approximations to the results are derived.

Experimental demonstration of E×B plasma divertor

The E×B drift due to an applied radial electric field in a tokamak with poloidal divertor can speed the flow of plasma out of the scrape‐off region, and provide a means of externally controlling the flow rate and thus the width of the density fall‐off. An experiment in the Wisconsin levitated toroidal octupole, using E×B drifts alone, demonstrates divertor‐like behavior, including 70% reduction of plasma density near the wall and 40% reduction of plasma flux to the wall, with no adverse effects on confinement of the main plasma.