27-10-2012, 10:56 AM
Experimental Measurement of the 4-D Transverse Phase Space Map of a Heavy Ion Beam
ABSTRACT
The development and employment of a new diagnostic instrument for characterizing intense, heavy ion beams is reported
on. This instrument, the ‘Gated Beam Imager’ or ‘GBI’ was designed for use on Lawrence Livermore National Laboratory
Heavy Ion Fusion Project’s ‘Small Recirculator’, an integrated, scaled physics experiment and engineering development
project for studying the transport and control of intense heavy ion beams as inertial fusion drivers in the production of electric
power. The GBI allows rapid measurement and calculation of a heavy ion beam’s characteristics to include all the first and
second moments of the transverse phase space distribution, transverse emittance, envelope parameters and beam centroid. The
GBI, with appropriate gating produces a time history of the beam resulting in a 4-D phase-space and time ‘map’ of the beam.
A unique capability of the GBI over existing diagnostic instruments is its ability to measure the ‘cross’ moments between the
two transverse orthogonal directions. Non-zero ‘cross’ moments in the alternating gradient lattice of the Small Recirculator
are indicative of focusing element rotational misalignments contributing to beam emittance growth. This emittance growth,
while having the same effect on the ability to focus a beam as emittance growth caused by non-linear effects, is in principle
removable by an appropriate number of focusing elements. The instrument uses the pepperpot method of introducing a plate
with many pinholes into the beam and observing the images of the resulting beamlets as they interact with a detector after an
appropriate drift distance. In order to produce adequate optical signal and repeatability, the detector was chosen to be a
microchannel plate (MCP) with a phosphor readout screen. The heavy ions in the pepperpot beamlets are stopped in the MCP’s
thin front metal anode and the resulting secondary electron signal is amplified and proximity-focused onto the phosphor while
maintaining the spatial and intensity characteristics of the heavy ion beamlets. The MCP used in this manner is a sensitive,
accurate, and long-lasting detector, resistant against signal degradation experienced by previous methods of intense heavy ion
beam detection and imaging. The performance of the GBI was benchmarked against existing mechanical emittance diagnostics
and the results of sophisticated beam transport numerical simulation codes to demonstrate its usefulness as a diagnostic tool.
Amethod of beam correction to remove the effects of quadrupole focusing element rotational misalignments is proposed using
data obtainable from a GBI. An optimizing code was written to determine the parameters of the correction system elements
based on input from the GBI. The results of this code for the Small Recirculator beam are reported on.