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GSI-ACCELERATORS-08 GSI SCIENTIFIC REPORT 2009
Beam Response on Base-Band Tune Measurement System
U. Springer 1,2 , P. Forck 1 , P. Hülsmann 1,2 , P. Kowina 1 , and P. Moritz 1
1 GSI, Darmstadt, Germany; 2 Goethe University, Frankfurt, Germany
For high current operation of SIS18 precise control of
the tune value is required. It is measured by excitation of
coherent betatron oscillations and turn-by-turn position determination
using a Beam Position Monitor (BPM). In order
to define the working area of this Tune Measurement
System using direct digitized BPM data, the influence of
beam excitation on emittance must be evaluated. The beam
width and beam loss were measured along with the frequency
spectrum obtained out of BPM data.
An example of such investigations is shown in Fig. 1 using
a beam with the following conditions: 7 · 10 9 Ar 18+
were accelerated from 11.4 to 300 MeV/u within 254 ms.
The beam was excited using band limited noise excitation
centered on expected betatron sidebands in order to actuate
a coherent betatron motion [1, 2, 3]. The bandwidth was
set broad enough to cover the expected range of tune variation
whereas the level of excitation was altered. The BPM
data are directly digitized and post-processed offline [4].
To obtain the horizontal and vertical beam envelope, and
thus beam emittance, the Ionization Profile Monitor (IPM)
installed at SIS18 was used [1, 5]. Moreover data from the
DC Current Transformer (DCCT) was analyzed to obtain
the amount of beam loss.
The measurements were performed with the given beam
parameters 250 ms after ramp start just before reaching flattop.
As the measurement system is considered to display
the tune with a ms time resolution work during complete
acceleration, all beam losses introduced by the beam excitation
during acceleration are included. The tune value is
obtained from SIS18 section 5 turn-by-turn position data by
Fourier Transformation. The Signal/Noise ratio presented
in Fig. 1 is calculated by integrating the tune peak in the
Fourier amplitude spectrum divided by the corresponding
part of spectrum of equal width outside the resonance. For
each data point in Fig. 1(top) 26 spectra were averaged and
their fluctuations are represented in the error bars. For the
DCCT data an average of 100 measurements is taken. The
beam width is determined from transverse profiles recorded
by the IPM using an average over 60 measurements.
It was observed that a S/N ratio of about 3 is enough to
achieve stable measurements, meaning that the evolution of
tune can be reconstructed each 512 turns all along the ramp.
For this S/N an Exciter Power of about 2 W is needed. On
the other hand no significant beam loss exceeding 2 % was
observed for excitation levels up to 10 W. An alteration in
beam profile was measured exceeding 8.5 W. A working
area providing sufficient signal strength for tune determination
and low transverse emittance enlargement thus could
be defined between 2-8.5 W of Exciter Power. The results
may be scaled (dep. on Z, A, E) for other ion species and
energy ranges.
136
Figure 1: The influence of beam excitation using band limited
noise is shown using BPM (upper part), DCCT (middle) and IPM
data (lower part). For measurement parameters see text.
References
[1] U. Rauch et al, “Baseband Tune Measurements at GSI SIS-18
using Direct Digitized BPM signals”, Proc. DIPAC’09, Basel,
Switzerland
[2] K. Blasche et al,“SIS Status Report”, GSI Scientific Report
2000, p.184
[3] U. Rauch, et al, “Base-Band Tune Measurements at SIS-18
using Direct Digitized BPM signals”, GSI Scientific Report
2008, p.122
[4] U. Rauch et al, “Investigations on BaseBand Tune Measurements
using Direct Digitized BPM Signals”, Proc. of 5th
CARE-HHH-ABI Workshop, Chamonix, Dec. 2007, p.58
[5] T. Giacomini et al,“Development of Residual Gas Profile
Monitors at GSI”, Proc. of 11th Beam Instrumentation Workshop
BIW’04, Knoxville, USA