Extraction Ion Diode Research Articles

Microsecond pulse width, intense, light-ion beam accelerator

A relatively long-pulse width (0.1–1 μs) intense ion beam accelerator has been built for materials processing applications. An applied Br, magnetically insulated extraction ion diode with dielectric flashover ion source is installed directly onto the output of a 1.2 MV, 300-kJ Marx generator. The diode is designed with the aid of multidimensional particle-in-cell simulations. Initial operation of the accelerator at 0.4 MV indicates satisfactory performance without the need for additional pulse shaping. The effect of a plasma opening switch on diode behavior is considered.

High-purity ion beam production at high current densities with a liquid-helium-cooled series-field-coil extraction ion diode

Experiments are described in which a high-purity, high-power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid-helium-cooled copper anode at 4.2 K in a series-field-coil extraction diode on the 0.7 TW HydraMITE-II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid-helium-cooled cryogenic diode experiments on small accelerators and is in the range required for high-power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen-coated cryogenic anode shows a faster initial anode turn-on than other materials. However, source-limited emission from the thin hydrogen layer results in a somewhat longer current risetime, reduced ion diode efficiency, lower proton current enhancement over the Child–Langmuir limit, and a proton spectrum of lower average energy than for the hydrocarbon anode. Techniques to overcome these limitations are discussed. Cryogenic ion sources consisting of frozen N2, CH4, and Ne have also been studied. In each case, high intensity beams consisting predominantly of components of the refrigerated gas were produced.

Ion coupling efficiency for an extraction applied-B ion diode on the HELIA linear-induction adder in positive polarity

Initial experiments to investigate coupling of the four-stage HELIA linear-induction accelerator to a uniformly insulated applied-B ion diode in planar extraction geometry are reported. Results describing the efficient operation of an applied-B extraction ion diode coupled to the HELIA linear induction accelerator operated in positive polarity are reported. Operation of a close-coupled, undermatched, applied-B diode on HELIA was consistent with magnetically insulated transmission line (MITL) electron flow intermediate between locally emitted flow and generalized flow, rather than with full-gap flow. Peak ion coupling efficiencies of 60-70% and peak ion power levels of 0.3-0.4 TW have been achieved. >

Simulation of ion beam diagnostics on extraction geometry ion diodes (abstract)

Lithium ion beams are a prime driver candidate for light‐ion inertial confinement fusion. LEVIS/LIBORS lithium ion sources are being developed for the HELIA extraction ion diode. Beam diagnostics to measure beam intensity, energy, composition, and purity are also being developed. Difficulties in diagnosing the beam are encountered because the diagnostics cannot interfere with the plasma forming and ionizing lasers of the LEVIS/LIBORS source. Efforts to model the transport of lithium ions in the self‐ and applied‐magnetic fields of the HELIA ion diode as well as the calculation of beam diagnostics using the PICDIAG computer code are described. The results directly affect the choice and design of diagnostics to be fielded. Results of the simulations are presented and comparisons with available experimental data are emphasized. This work was supported by the U.S. Department of Energy under contract No. DE‐AC04‐76DP00789.

Beam profile studies for a phase-mixed ion beam focused with a one-eighth betatron wavelength focusing cell

The focusing properties of a one-eighth betatron wavelength focusing cell are used to determine to what extent a modification of the initial phase-space distribution of an ion beam can alter the number density profile of the beam at the focal plane. It is shown that the main modification is to alter the natural 1/r profile to include an off-axis peak. The relative difficulty with which the ion beam can be concentrated into this off-axis peak is then considered. Estimates of the source brightness (extraction ion diode source current density divided by the square of the microdivergence) required to deliver a given amount of beam current into a given annular region at the focal plane are derived.

Ion source studies for particle beam accelerators

High power particle beam accelerators are being developed for use in inertial confinement fusion applications. These pulsed power accelerators require sources of low atomic number ions (e.g., protons, deuterons, carbon, or lithium). The sources must be of high purity for efficient accelerator operation and proper target coupling, must have a rapid ‘‘turn-on,’’ and must be compatible with ion diode configurations under development. A particular type of source presently being investigated is the flashover ion source which generates ions by means of the vacuum flashover of an insulating anode material when the high voltage pulse arrives at the diode. We have developed an applied-magnetic-field, extraction ion diode for the 0.03 TW Nereus accelerator specifically to investigate these sources. Extracted ion species are measured by means of a Thomson-parabola ion analyzer, dB/dt current monitors, and Faraday cups. Experiments have been performed to investigate the surface flashover mechanism and the effects of various dielectric source materials, anode preparation methods (including rf glow discharge cleaning), and vacuum conditions on ion species and diode operation.