This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Klasen, M.

Lash, J.

Pelath, M.

Pollock, R.

Sarrazine, A.

Sloan, T.

English

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CE-30 - TOWARD A TRAPPED PARTICLE TARGET 
M. Klasen, J. Lash, M. Pelath, R. Pollock, A. Sarrazine, and T. Sloan 
Indiana Universi ty  Cyclotron Facility, Bloomington,  Indiana 47408 
A magnetic solenoid with a vacuum chamber designed for storage ring use was con- 
- 
strutted in 1991/92 and first used to develop a stored beam diagnostic based on measured 
deflection of a parallel low energy electron beam. This work has been published.1 
During the summer of 1992, the interior was refitted to serve as a large volume Penning 
trap for the study of problems in filling, observing and establishing stability limits for a 
trapped electron cloud. The geometry is shown in Fig. 1. The gradient columns at the 
trap ends established a "bathtub"-shaped potential which is isochronous for axial motion 
of single particles near the maximum contained energy. The electron gun is off-axis to keep 
the trap centerline clear for future passage of a stored ion beam. Initial trap tests were 
made at a pressure of 1 pPa. The pumping system was augmented in the summer of 1992 
so that 0.1 pPa has been obtained after a 180 " C bakeout. 
The trapped cloud has been observed by three methods. A collector on the end op- 
posite the gun shows the expected 1.4 MHz axial bounce frequency when 0.1 ps pulses are 
injected with kinetic energy near the top of the containment potential. An azimuthally- 
segmented "cage" surrounding the central region of the trap can be connected to a pream- 
plifier to sense a time-dependent line dipole moment of the trapped ion cloud. Under 
suitable conditions with continuous injection, a strong sustained oscillation of a few kHz is 
observed, indicating coherent magnetron (diocotron) motion. The minimum period is ob- 
served to vary nearly linearly with magnetic field and inversely with trap endcap potential 
as shown in Fig. 2. This behavior is consistent with the rotation of an ion column driven 
by an image charge and with the number of trapped electrons limited by axial confinement 
at 13 V to about 7x lo8  particles. 
After filling the trap, one endcap can be ramped to zero potential and the time- 
dependent current collected on the endcap may be used to measure the number of trapped 
particles and the energy distribution. The number observed by this method grows to a few 
times lo8 over a time of order 0.1 s in a continuous-fill periodic-dump cycle. The filling 
time indicates an inefficient injection mechanism of unexplained origin. The observed 
distributions show that the trap may be tuned to contain electrons near the injection 
energy and/or a component near zero energy as shown in Fig. 3. 
Phenomena are observed with a time scale of a few seconds that may be associated 
with partial neutralization by positive ions created by ionization in the imperfect vacuum. 
The trap is being modified to allow controlled retention or ejection of positives. 
Studies are in progress to elucidate the nature of the filling mechanism, the physics 
responsible for the redistribution of longitudinal momentum, and to determine the radial 
and azimuthal distributions of density in the trapped cloud. For use as a target, the cloud 
must be maneuvered to the trap centerline with the aid of external steering by feedback 
of the magnetron signal. The number of particles can be increased by operation at higher 
confinement potential. 
The methods developed in this work may be extended to a trap for bare nuclear ions. 
I 
I 
I 
1 1 1 1 1 1 1 1 1 1 1 1  I I 1 1 1 1 1 1 1 1 1 1 1 1  
COLUMN I I 
I 
I 
I 
I 
- 
--------------------------------------*.------------------------------------------ 
I 
I 7 
I 
I 
I 
I 1 1 1 1 1 1 1 1 1 1 1 1  
STEERER : CAGE 
I 
1 COLLECTOR 
t 0.4 m 
Figure 1. Plan view of bathtub trap. The gradient columns, steerer cage and endcaps 
have cylindrical symmetry. 
- 
Minimum Period (ms) 
vs Solenoid Field ( T) 
- forEndcapVoltages: 
- 1 3 V (solld square) 
- 50 V (open square) 
- 
m 
O O U  
Trap B Field ( Tesla) + 
I I I I I 1 
Figure 2. Dependence of minimum period of observed magnetron motion on the applied 
magnetic field for two values of confinement implies that the number of trapped electrons 
N is independent of the magnetic field. The data at 50 eV appear to indicate an increase of 
N with magnetic field below 0.08 T and a linear increase of N with confinement potential . 
above this threshold. 
Figure 3. Representative oscilloscope photos of the time distribution of electron current 
collected on the gun endcap when the potential on this end of the bathtub is lowered 
linearly past zero. The lower trace in each case is the collector response with the trap 
empty. The amplifier inverts the negative current signal. Current collected early (as at 
left) is indicative of higher longitudinal energy in the trapped distribution. The peak near 
80 ps (as at right) is a low energy component. The central picture is an example of a 
nearly uniform distribution. 
A higher field solenoid must be employed for construction of a large-capacity heavy-particle 
trap. 
1. R.E. Pollock, e t  al., Nucl. Instrum. Methods A330, 27 (1993). 


CE-30 - Toward a Trapped Particle Target

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CE-30 - Toward a Trapped Particle Target

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