Production of 0.5-TW Proton Pulses with a Spherical Focusing, Magnetically Insulated Diode

Abstract

The production, focusing, and numerical simulation of a 0.5-TW proton beam is reported. This beam is produced with a spherical, magnetically insulated, ion diode fed symmetrically by the dual-pulse-line Proto I generator. The ions are accelerated with electric fields due to a virtual cathode supported by magnetic field surfaces. Approximately 75% of the diode electrical power is delivered to ions and 25% of the ion beam is focused upon thin, 1-cmdiam, 1-cm-long conical targets to produce the first experimental ion-driven implosions.Recent advances have been made in sources of intense pulsed ion beams for magnetic and inertial confinement fusion. 1 " 3 The reduction of electron loss is accomplished via the self magnetic field in a pinched electron-beam diode, 1 the reflexing of electrons in a reflex triode, 2 and an applied magnetic field in a magnetically insulated (MI) diode. 3 We report the production of a converging proton beam with a MI diode driven by the 10 12 -W Proto I dual-puljse-line generator. 4 A proton beam with peak current of 360 kA at voltages between 0.8 and 1.4 MV has been diagnosed and focused with 25% efficiency onto thin conical targets with nominal lengths and diameters of 1 cm.The apparatus is shown in Fig. 1. The applied magnetic field rises in 70 jzsec and is spherically contoured in the diode gap by the 15-cm radius aluminum anode. The anode then is pulsed positive via the center electrode of the disk triplate transmission line of the Proto I generator. A virtual cathode then is produced by electrons which are emitted from the cathode disks and spiral axially along magnetic field lines to establish a uniform equipotential surface near the anode. Anode plasma from a nylon mesh mounted upon the anode (thought to be formed by surface flashover or leakage electrons) is the source of protons or higher-Z ions which are accelerated. Since all magnetic field lines are contained between the anode surface (neglecting a 2-mm magnetic field soak in) and diode center line, the ions are emitted from the anode with zero canonical angular momentum and ideally can be focused to the center line. When drifting through the magnetic field, the ions are space-charge neutralized by electrons emitted from the walls of the drift region.The diode was numerically simulated with a two-dimensional (2D) steady-state, particle-incell code 5 with electron and ion motion treated in self-consistent electric and magnetic fields. The applied and self magnetic fields, which are excluded from the anode but fully penetrate the thin disk cathodes, are computed by solving the one-component vector-potential equation Figure 2 contains a typical instantaneous ion simulationparticle map together with lines of equal intensity I? app assuming cylindrical symmetry for the diode. The assumed diode voltage is 1.2 MV and £ app in the diode gap is 13 kG. Atypical electron trajectory is shown at the position of the virtual cathode near one Z? app field line. Complete spacecharge neutralization was assumed between the disks in this simulat...

High-gain, low-intensity ICF targets for a charged-particle beam fusion driver

“…Experiment and theory concerning the transport and focus of multiple electron beams [21 -23] and the focus of a light ion beam [24] suggest that the diameter of a reactor-size target should be of the order of 1 cm. Such large diameters can have a drastic effect on the energy and power required for a given ignition condition with conventional ablative drive.…”

Field-reversed configurations with a component of energetic particles

“…This puts an extraordinary demand on the ion source and although present-day ion diodes and pulsed-power machines (see, e.g. Refs [18][19][20]) are capable of providing this amount of ion charge per pulse, particular care must be exercised to optimize the injection ' efficiency. Employing a 1-TW pulsed-power machine, GAMBLE II, at the Naval Research Laboratory, Kapetanakos et al [21 ] were successful in creating a transient (~ 30 ns) field reversal by injecting a pulse of ions through a magnetic cusp.…”

Neutralized, intense-ion beams for fusion

“…An intense-ion beam (IIB) is a plasma beam with far greater particle energy and instantaneous intensity, with typical output parameters, W i ≃100-1000's of keV ion energy, n i ≲ 10 17 − 10 19 m −3 ion density, τ b ≃ 0.1 − 1 µs pulse-duration, and Pb ≃ 10 9 − 10 12 W instantaneous-peak power. IIBs were originally developed in the late 70's for light-ion, inertialconfinement fusion [9,10] and later adapted for use in fieldreversed fusion plasmas [11,12] and materials processing [13,14]. Early designs used flashover discharges to produce a high-density plasma from which the ions were extracted and then accelerated.…”