Results from a scaled final focus experiment for heavy ion fusion

Abstract

Negative chlorine ions from multicusp radio frequency ion source for heavy ion fusion applications Rev.A one-tenth-scale version of a final focus subsystem for a heavy-ion-fusion driver has been built and used for experimental tests of concept. By properly scaling the parameters that relate ion energy and mass, current, emittance, and focusing fields, the transverse dynamics of a representative driver final focus have been replicated in a small laboratory beam. Whereas the driver beam parameters considered are 10 GeV Bi ϩ at 1.25 kA, the scaled experiment used a 95 A beam of 160 keV Cs ϩ ions brought to a ballistic focus through a series of six quadrupole magnets. The measured focal spot size was consistent with calculations in the report of the design on which the experiment is based. In a second experimental program, a 400 A beam was propagated through the focal system and partially neutralized after the last magnet using electrons released from a hot tungsten filament to test the predictions of the benefits of neutralization. The increase in beam current resulted in a corresponding increase in spot radius in the absence of electrons. A reduction of the spot radius and modification of its shape were observed in the presence of neutralizing electrons.

HEAVY ION FUSION SCIENCE VIRTUAL NATIONAL LABORATORY 3nd QUARTER 2009 MILESTONE REPORT: Upgrade plasma source configuration and carry out initial experiments. Characterize improvements in focal spot beam intensity

“…Metal grids are a known method of generating electrons due to beam ion impact [1,2]. Depending on the angle of incidence, the yield of electrons ranges from ~10 to ~100 per primary ion, with the highest values at glancing angles [3].…”

“…The peak power in the compressed pulse delivered to the target plane was measured at ~470 kW, with ~1.5 mJ in the ~5ns (FWHM) compressed pulse. 1 The {} braces denote that the magnetic field is scaled proportional to E b 1/2 to keep the rms beam envelope in the matching solenoids and un-neutralized drift sections unchanged for a space charge dominated beam. …”

Heavy Ion Fusion — Using Heavy Ions to Make Electricity

“…The Multiple Beam Experiment [6] accelerated four beams simultaneously from 200 keV to 0.9 MeV, and compressed them by a factor of 7. Experiments were also done to combine beams transversely [7], and focus them to a spot (in a 1/10 scale model in linear dimension of a driver focusing system) [8]. Finally, a target tracking experiment showed that targets could be injected and tracked into the target chamber accurately enough for the beams to properly hit the target [9].…”

Results on intense beam focusing and neutralization from the neutralized beam experiment

“…The key scaling parameter for beam transport is the dimensionless perveance defined as the ratio of the beam space charge to kinetic energy (K = 2I [27]. Previous neutralization experiments [26,28] have provided, to some degree, confirmation of this limit.…”

“…An earlier experiment [26] examined the charge neutralization of a heavy ion beam by electrons drawn from a localized source as the beam was focused. The electron source was a glowing tungsten filament placed in the beam path, enabling the supply of thermionicallyemitted electrons inside of the beam.…”