LBNL Office of science Department of Energy

Ion Beam Technology Group

Lawrence Berkeley National Laboratory

Accelerator Technology & Applied Physics Division

Plasma & Ion Source Technology

Many of the technologies developed by IBT are built upon a core concept that we have been developing since our days in magnetic fusion energy in the mid 1980s: a compact, efficient ion source in which a plasma is heated by radiofrequency energy and confined by a multicusp magnetic field. This highly scalable concept has been used for many purposes.

10x10 extraction array for the fast-fall-off neutron source

A beam falltime quicker than 1 microsecond has been demonstrated. This sharp end of the beam pulse is needed for differential die-away analysis.

Among our current development projects is a compact, highly portable neutron generator. To reduce its size and power consumption, advanced permanent magnets create the plasma-confining multicusp magnetic field, and the plasma is heated with microwaves (from a simple magnetron) rather than the usual medium-wave rf energy. Potential applications include neutron activation analysis and detection of explosives and nuclear threats using active interrogation. Thus far we have measured a current density of 20 mA per square inch, and are planning for sealed-tube deuterium-deuterium neutron production tests; we expect 107 to 108 neutrons per second.

Another security- and nonproliferation-related work in progress is a fast-fall-time neutron generator. Its output can be cut from full to nearly zero within ten microseconds, with a repetition rate as fast as 10 kHz. A low extraction voltage (-400 V) is the key to the fast fall time; a 10 x 10 array of extraction apertures achieves sufficient beam current in spite of this. This fast fall time lends itself to an analysis scheme called "differential die-away" for examining the contents of nuclear-waste containers.

10x10 extraction array for the fast-fall-off neutron source

The 10 x 10 emitter array allows adequate beam current to be extracted from the ion source even when the extraction voltage is kept low to ensure rapid falloff.

A new variant on our rf-driven multicusp ion source, now in development, has the potential to supply as much as 60 mA of protons continuously rather than in pulses. To meet this challenging output specification, it uses a planar, external antenna rather than our traditional spiral internal antenna to couple rf power into the plasma; and employs water-cooled permanent magnets. Thus far it has produced 130 mA/in2 with 2 kW of input power.