Intense Ion-Beam Treatment of Materials
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(1-10 J/cm2) or to vaporize (10-50 J/cm2) the target surface. Beam pulse durations are short ( s 1 /us) to minimize thermal conduction. Some disadvantages of IPIB processing over laser processing include
the need to form and propagate the beams in vacuum, and the need for shielding of x-rays produced by relatively low-level electron current present in IPIB accelerators. Also these beams cannot be as tightly focused onto targets as lasers, making them unsuitable for applications requiring treatment on small spatial scales. Applications investigated have included film deposition,314 alloying and mixing,15"29 glazing,29 cleaning and polishing, 30 corrosion improvement,25'29 implantation and annealing,31 polymersurface treatment,32 and nanophase powder synthesis.33 Beams with /.= 5-50 kA and £ = 1001,000 keV are produced in vacuum by magnetically insulated diodes requiring a source of ions, an accelerating voltage, and a magnetic field transverse to the acceleration gap to suppress electron flow and enhance the ion flow (Figure 1). Ion currents typically exceed the vacuum space-charge limit by 5-50 times owing
R(m)
"Cathodes
Insulating Field Coils Figure 1. A typical focused intense-ion-beam diode configuration.
MRS BULLETIN/AUGUST 1996
Intense Ion-Beam Treatment of Materials
to electrons confined in the acceleration region by the applied magnetic field. The beams are produced and transported in vacuum that can exceed 1CT4 Torr. In a conventional diode, ions are drawn from the surface of a polymer anode 34 converted to a plasma by a combination of high-voltage flashover and electron impact. Polymer anodes are unacceptable for many materials-synthesis applications because of their limited lifetime, excessive heat loading, high gas production, excessive debris, poor beam uniformity, and lack of ion-species control. Anodes that draw ions from a preformed plasma are being developed at a number of laboratories to overcome these limitations.35"37 Alternatively a negative pulse, applied to the anode prior to the main positive-polarity acceleration pulse, can be used to generate anode plasma.38 Traditional single-shot beam accelerators (using Marx generators and high-voltage pulse lines), incompatible with low-cost high-throughput commercial processing, are yielding to new high-averagepower accelerators. An ion-beam system operating at 100 Hz (in 10-shot burst mode since no active cooling was available)37 and a 300-keV intense beam source at 0.3 Hz have been demonstrated.38 Saturable magnetic switches and inductive voltage adders, 'technology borrowed from strategic-defense programs, are being used to produce a train of 80-ns pulses at up to 120 Hz.39 In another approach akin to high-power radar modulators, a system using lumped-element transmission lines and high-power thyratron switches is being developed to generate a sequence of 1-yu.s pulses. 40 Along with the development of repetitive technology, high reliability and long component and system lifetimes (>108 pulses between failure or scheduled maintenance) will
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