Scaled beam merging experiment for heavy ion inertial fusion

Abstract

Transverse beam combining is a cost-saving option employed in many designs for heavy ion fusion drivers. However, the resultant transverse phase space dilution must be minimized so as not to sacrifice focusability at the target. A prototype combining experiment has been completed employing four 3-mA Cs beams injected at 160 keV. The focusing elements upstream of the merge consist of four quadrupoles and a final combined-function element (quadrupole and dipole). Following the merge, the resultant single beam is transported in a single alternating gradient channel where the subsequent evolution of the distribution function is diagnosed. The results are in fair agreement with particle-incell simulations. They indicate that for some heavy ion fusion driver designs, the phase space dilution from merging is acceptable.

High current transport experiment for heavy ion inertial fusion

“…The drift distance between the slit and the slit-cup is chosen such that <1% of the measured transverse envelope expansion of the transmitted slice is due to the remaining spacecharge forces. This procedure is repeated to map phasespace density projections [13,15,23]. The signal-to-noise ratio varies from 10:1 to 300:1 depending on the diagnostic station and the current density at the collector.…”

“…Previous scaled experiments at LBNL [11][12][13][14] were designed with the appropriate perveance for studying driv-erlike phenomena, principally transport with sufficient current to highly depress the single-particle betatron tunes, but the beam current was kept relatively low. For example, in the Single Beam Transport Experiment [15], the maximum generalized perveance was 2:2 10 ÿ3 -higher than generally envisioned for a fusion driver-but the linecharge density remained about 1 order of magnitude lower than for the HCX or a fusion driver front end.…”

Research and development toward heavy ion driven inertial fusion energy

“…(c) The transverse merging of four beams into a single quadrupole channel may be advantageous at the transition from electric quadrupole focusing to magnetic quadrupole focusing and the question of emittance growth due to beam-beam space-charge forces was important to address via experiment. This was demonstrated while retaining good beam quality in a scaled experiment [13][14][15]. A related application of transverse merging of beams is for the initial formation and injection of high-current beams.…”

“…Particle-in-cell (PIC) codes for beam and beam-plasma interaction have both guided and been validated by comparison to the above experiments [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Computational tools were developed for the self-consistent modeling of the beams (and plasma if present) subject to the externally applied focusing field, acceleration field, self-field of the beam, as well as the fields from induced image-charge and currents.…”