Gas discharge ion source. I. Duoplasmatron.
The effects of the plasma expansion cup on the operation of a duoplasmatron ion source have been investigated by measuring the total ion current and the distributions of the ion energy, mass, and current density. A copper expansion cup did not affect the magnetic field near the anode of the ion source and consequently the ion current density distribution was sharply peaked near the center of the cup. Ion energy distributions were approximately symmetrical about anode potential. The dominant ionic species were D(+) (3) and D(+) at low and high arc currents, respectively. Changes in the electrical potential of the copper cup with respect to the anode produced negligible changes in the above data. A mild steel plasma expansion cup caused the magnetic field to diverge and intercept the cup walls, resulting in ion current density distributions that were flatter and more amenable to focusing than the ones with the copper cup. With the steel cup at anode potential, the ion mass distribution was similar to that from the copper cup; however, the ion energy distribution was asymmetrical about the anode potential with a peak about 10-20 V above anode potential. The total ion current from this mode of operation was about one-third the value from the copper cup. If the steel cup assumed floating potential, about 50 V below anode potential, the total current increased to the level observed from the copper cup and the ion energy distribution was similar to that observed with the copper cup but the current density distribution was much flatter than that of the copper cup. The ion mass distribution was 60%-70% atomic ions over the entire arc current range investigated. Based on these data, a modified plasma expansion cup was designed with tapered steel walls lined with a boron nitride insert. The overall performance of the duoplasmatron ion source with this cup was superior to any of the previous three modes of operation. The improved design uses a molybdenum anode insert which is far superior to copper in regard to erosion over extended operating times. A total beam current of 200 mA was produced at an arc current of 24 A.
Intense Neutron Source Development for Use in Cancer Therapy
A neutron production rate of 1.1 × 1011/s from the D(d,n)3He reaction has been measured from a 200 kV, 200 mA d. c. deuterium ion accelerator during the initial bombardment of a scandium deuteride target. Over a period of 40 h operation, the output decreased to 8 × 1010/s due to impurities in the ion beam being implanted in the target and displacing deuterium just below the target surface. Deuterium concentration depth profiles in the target have been measured which show the deuterium depletion near the surface. The initial neutron output scales to >1 × 10l3/s from the T(d,n)4He reaction for a D,T ion beam on a ScDT target, which is the output required for a clinically useful cancer therapy machine. Efforts to reduce the ion beam impurity content to acceptable levels will be evaluated in a new accelerator tube that is described.
ELECTRICAL CONDUCTIVITY OF A HIGHLY TURBULENT PLASMA.
The electrical conductivity of a highly turbulent plasma is measured in the electron density range ${10}^{10}$-${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, using hydrogen, argon, and xenon. When the applied electric field is large (\ensuremath{\gtrsim} 100 V ${\mathrm{cm}}^{\ensuremath{-}1}$), it is found that the conductivity may be described by the formula $\ensuremath{\sigma}\ensuremath{\approx}\frac{1}{2}{(\frac{M}{m})}^{\frac{1}{3}}{\ensuremath{\omega}}_{\mathrm{pe}}$, which was first suggested by Buneman.