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Title: Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation

Abstract

Generation of high-intensity pulsed ion beam (HIPIB) has been studied experimentally using polyethylene as the anode polymer in magnetically insulated ion diodes (MIDs) with an external magnetic field. The HIPIB is extracted from the anode plasma produced during the surface discharging process on polyethylene under the electrical and magnetic fields in MIDs, i.e., high-voltage surface breakdown (flashover) with bombardments by electrons. The surface morphology and the microstructure of the anode polymer are characterized using scanning electron microscopy and differential scanning calorimetry, respectively. The surface roughening of the anode polymer results from the explosive release of trapped gases or newly formed gases under the high-voltage discharging, leaving fractured surfaces with bubble formation. The polyethylene in the surface layer degrades into low-molecular-weight polymers such as polyethylene wax and paraffin under the discharging process. Both the surface roughness and the fraction of low molecular polymers apparently increase as the discharging times are prolonged for multipulse HIPIB generation. The changes in the surface morphology and the composition of anode polymer lead to a noticeable decrease in the output of ion beam intensity, i.e., ion current density and diode voltage, accompanied with an increase in instability of the parameters with the prolonged discharge times. Themore » diode voltage (or surface breakdown voltage of polymer) mainly depends on the surface morphology (or roughness) of anode polymers, and the ion current density on the composition of anode polymers, which account for the two stages of anode polymer degradation observed experimentally, i.e., stage I which has a steady decrease of the two parameters and stage II which shows a slow decrease, but with an enhanced fluctuation of the two parameters with increasing pulses of HIPIB generation.« less

Authors:
; ; ; ;  [1]
  1. Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024 (China)
Publication Date:
OSTI Identifier:
20953252
Resource Type:
Journal Article
Resource Relation:
Journal Name: Review of Scientific Instruments; Journal Volume: 78; Journal Issue: 2; Other Information: DOI: 10.1063/1.2437760; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ANODES; CURRENT DENSITY; ELECTRIC POTENTIAL; FLASHOVER; GASES; ION BEAMS; IONS; MAGNETIC FIELDS; MORPHOLOGY; PARAFFIN; POLYETHYLENES; ROUGHNESS; SCANNING ELECTRON MICROSCOPY; SURFACES; THERMIONIC DIODES

Citation Formats

Zhu, X. P., Dong, Z. H., Han, X. G., Xin, J. P., and Lei, M. K. Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation. United States: N. p., 2007. Web. doi:10.1063/1.2437760.
Zhu, X. P., Dong, Z. H., Han, X. G., Xin, J. P., & Lei, M. K. Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation. United States. doi:10.1063/1.2437760.
Zhu, X. P., Dong, Z. H., Han, X. G., Xin, J. P., and Lei, M. K. Thu . "Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation". United States. doi:10.1063/1.2437760.
@article{osti_20953252,
title = {Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation},
author = {Zhu, X. P. and Dong, Z. H. and Han, X. G. and Xin, J. P. and Lei, M. K.},
abstractNote = {Generation of high-intensity pulsed ion beam (HIPIB) has been studied experimentally using polyethylene as the anode polymer in magnetically insulated ion diodes (MIDs) with an external magnetic field. The HIPIB is extracted from the anode plasma produced during the surface discharging process on polyethylene under the electrical and magnetic fields in MIDs, i.e., high-voltage surface breakdown (flashover) with bombardments by electrons. The surface morphology and the microstructure of the anode polymer are characterized using scanning electron microscopy and differential scanning calorimetry, respectively. The surface roughening of the anode polymer results from the explosive release of trapped gases or newly formed gases under the high-voltage discharging, leaving fractured surfaces with bubble formation. The polyethylene in the surface layer degrades into low-molecular-weight polymers such as polyethylene wax and paraffin under the discharging process. Both the surface roughness and the fraction of low molecular polymers apparently increase as the discharging times are prolonged for multipulse HIPIB generation. The changes in the surface morphology and the composition of anode polymer lead to a noticeable decrease in the output of ion beam intensity, i.e., ion current density and diode voltage, accompanied with an increase in instability of the parameters with the prolonged discharge times. The diode voltage (or surface breakdown voltage of polymer) mainly depends on the surface morphology (or roughness) of anode polymers, and the ion current density on the composition of anode polymers, which account for the two stages of anode polymer degradation observed experimentally, i.e., stage I which has a steady decrease of the two parameters and stage II which shows a slow decrease, but with an enhanced fluctuation of the two parameters with increasing pulses of HIPIB generation.},
doi = {10.1063/1.2437760},
journal = {Review of Scientific Instruments},
number = 2,
volume = 78,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • High-intensity pulsed ion beam (HIPIB) with ion current density above Child-Langmuir limit is achieved by extracting ion beam from anode plasma of ion diodes with suppressing electron flow under magnetic field insulation. It was theoretically estimated that with increasing the magnetic field, a maximal value of ion current density may reach nearly 3 times that of Child-Langmuir limit in a non-relativistic mode and close to 6 times in a highly relativistic mode. In this study, the behavior of ion beam enhancement by magnetic insulation is systematically investigated in three types of magnetically insulated ion diodes (MIDs) with passive anode, takingmore » into account the anode plasma generation process on the anode surface. A maximal enhancement factor higher than 6 over the Child-Langmuir limit can be obtained in the non-relativistic mode with accelerating voltage of 200–300 kV. The MIDs differ in two anode plasma formation mechanisms, i.e., surface flashover of a dielectric coating on the anode and explosive emission of electrons from the anode, as well as in two insulation modes of external-magnetic field and self-magnetic field with either non-closed or closed drift of electrons in the anode-cathode (A-K) gap, respectively. Combined with ion current density measurement, energy density characterization is employed to resolve the spatial distribution of energy density before focusing for exploring the ion beam generation process. Consistent results are obtained on three types of MIDs concerning control of neutralizing electron flows for the space charge of ions where the high ion beam enhancement is determined by effective electron neutralization in the A-K gap, while the HIPIB composition of different ion species downstream from the diode may be considerably affected by the ion beam neutralization during propagation.« less
  • The temperature effect of anode dielectric materials on ion beam turn-on time is examined in a magnetically insulated pulsed ion diode. It is observed that the turn-on time decreased with temperature increase. This effect may be explained by a simple model in which electrons bombard the anode surface to generate desorbed and evaporated gas and trigger surface flashover to produce an anode plasma.
  • Intense pulsed heavy ion beam is expected to be applied to materials processing including surface modification and ion implantation. For those applications, it is very important to generate high-purity ion beams with various ion species. For this purpose, we have developed a new type of a magnetically insulated ion diode with an active ion source of a gas puff plasma gun. When the ion diode was operated at a diode voltage of about 190 kV, a diode current of about 15 kA, and a pulse duration of about 100 ns, the ion beam with an ion current density of 54more » A/cm{sup 2} was obtained at 50 mm downstream from the anode. By evaluating the ion species and the energy spectrum of the ion beam via a Thomson parabola spectrometer, it was confirmed that the ion beam consists of nitrogen ions (N{sup +} and N{sup 2+}) of energy of 100-400 keV and the proton impurities of energy of 90-200 keV. The purity of the beam was evaluated to be 94%. The high-purity pulsed nitrogen ion beam was successfully obtained by the developed ion diode system.« less
  • We show that anode plasma sheath heating can cause undesirable in situ ionization of the anode plasma in magnetically insulated ion diodes. We find that extremely high electron energies will occur in the anode plasma sheath due to an increasing magnetic field at the anode surface during the diode pulse. These high-energy electrons can collisionally ionize the plasma ions to an unwanted ionization state. Multiply ionized anode plasma ions represent a beam contaminant that may cause undesired preheat in inertial confinement fusion (ICF) capsules. The fraction of unwanted higher ionization states increases with anode plasma density. Therefore, this effect placesmore » an upper limit on the anode plasma density that can be used in an ICF application. As an example, we estimate the fraction of Li/sup + +/ generated by sheath heating in the PBFA-II Applied-B ion diode as a function of the initial Li/sup +/ anode plasma density and obtain an upper limit for a lithium anode plasma density of approximately 1 x 10/sup 16/ cm/sup -3/. At anode plasma densities of 1 x 10/sup 16/ cm/sup -3/ or less there will be substantial motion of the ion emitting surface during the diode pulse. We discuss the advantages and disadvantages of this plasma motion.« less
  • Research in light-ion-driven inertial-confinement fusion is emphasizing high-voltage, nonprotonic ions to obtain improved magnetic stiffness. Target preheat by coaccelerated proton impurities is a serious concern. A method is described which should essentially eliminate protonic preheat for applied magnetic field, magnetically insulated (applied-B) ion diodes. Briefly, the charge state of the principal ion species is changed by passing through a thin foil in the applied magnetic field. Due to the change in canonical angular momentum, the ions' closest approach to the axis of an axisymmetric diode is different from that of the proton contaminant. This difference provides effective separation of protonsmore » from the principal species.« less