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

Meyer, H.O.

Pancella, P.V.

Rinckel, T.

English

IUScholarWorks

COUNT-RATE FEEDBACK TO STABILIZE THE COOLER BEAM 
ON A SKIMMER TARGET 
H.O. Meyer and T. Rinckel 
Department  of Physics, Indiana University, and 
Indiana Universi ty  Cyclotron Facility, Bloomington,  Indiana 4 74 08 
P.V. Pancella 
W e s t e r n  Michigan University, Kalamazoo, Michigan 49008 
The relatively weak damping provided by electron cooling limits the thickness of 
internal targets for which a heating-cooling equilibrium can be maintained. For a 
200-MeV proton beam, for instance, internal targets should be thinner than about 1 0 ~ ~ / 2 '  
atoms/cm2, where Z is the atomic number of the target material. This rules out self- 
supporting solid targets. 
It is, however, possible to use thick, solid targets when the so-called skimmer mode is 
employed. In this mode, an unsupported edge of a solid target is placed near the beam. 
The target is then intercepted only by beam particles that have a sufficiently large betatron 
amplitude. The luminosity in this mode is a function of the distance from the target edge to 
the beam center, and can be adjusted to the requirements of the experiment by moving the 
target transversely with respect to the beam. Carbon targets of this design are routinely 
used to measure the proton beam polarization via 12C(p,p)12C scattering. 
The skimmer mode resembles slow extraction of the (hot portion of the) stored beam. 
The experiment usually requires that the target is placed such that a steady count rate 
so of a predetermined value results. To this aim, the target is mounted on a fast linear 
motion actuator (FLIM), and the count rate of a detector in the flux of scattered particles 
is processed by a ratemeter. The FLIM is controlled by a PC-based interface that also 
reads the ratemeter output and provides a feedback loop in software. 
In the ratemeter, a current that is proportional to the rate s(t) at the input is used 
to charge a capacitor. A bleeding resistor parallel to the capacitor defines the integration 
time 7. The lower the count rate, the longer the integration time has to be if one wants to 
minimize statistical fluctuations of the output r(t). The available rate information r(t) lags 
behind the actual rate, and changes in s(t) are manifest at the output only after some delay. 
Thus, the corrections applied to the FLIM position are therefore "out of date," resulting 
in large oscillations of the luminosity. This time lag of the feedback information seems 
to be a fundamental hurdle. Therefore, for many years large luminosity fluctuations were 
accepted for Cooler experiments with skimmer targets. This is illustrated in Fig. 1, which 
shows the skimmer performance during a run on March 16, 1992 of the CE20 experiment 
which used the CEO1 detector as a polarimeter. The upper trace shows the beam current 
and the lower trace the rate in the detector (ratemeter output). The latter exhibits the 
large fluctuations mentioned. 
Recently, we found that this difficulty can be overcome, because the functional depen- 
dence of the ratemeter output, r, on the input, s, i s  known. It is straightforward to show 
Figure 1. Old method: beam current I(t)  and ratemeter output r(t) for four cycles. The 
data are from a CE20 run in March 1992. The beam energy was 108 MeV. A carbon 
skimmer was mounted in the A-region, and the CEOl detector was used as a polarimeter. 
that the following relation holds: 
Here, r is the (known) time constant of the ratemeter, and a an (unimportant) calibration 
constant. The feedback code that receives the output r of the ratemeter is used to deduce 
the derivative drldt from the change of r during a short time interval. Then, the instanta- 
neous rate s at the input is calculated according to Eq. 1. Comparing s to the desired so 
yields an error signal that is used to adjust the position of the FLIM. The idea is simple 
and only a minor change to the feedback code is required. However, the improvement in 
being able to stabilize the luminosity is dramatic. This is illustrated in Fig. 2, which shows 
the skimmer performance during a CE42 run in January 1995 in which the CEOl detector 
was used as a polarimeter. The upperand lower traces show the ratemeter input s(t) and 
output r(t), respectively, for one cycle. 
loo 
cycle time 
Figure 2. New method: ratemeter input s ( t )  and output r ( t )  for a single cycle. The data 
are from a CE42 run in January 1995. The beam energy was 200 MeV. A carbon skimmer 
was mounted in the A-region, and the CEO1 detector was used as a polarimeter. 


Count Rate Feedback to Stabilize the Cooler Beam on a Skimmer Target

https://scholarworks.iu.edu/dspace/bitstream/2022/1069/1/iucf1994-1995_150-152.pdf

Count Rate Feedback to Stabilize the Cooler Beam on a Skimmer Target

Abstract

Similar works

Full text

Available Versions

IUScholarWorks