Proceedings of IBIC2012, Tsukuba, Japan

MOPA08VARIOUS USAGES OF WALL CURRENT MONITORS FORCOMMISSIONING OF RF SYSTEMS IN J-PARC SYNCHROTRONS
Fumihiko Tamura∗ , Masanobu Yamamoto, Alexander Schnase, Masahito Yoshii, Chihiro Ohmori,
Masahiro Nomura, Makoto Toda, Taihei Shimada, Keigo Hara, Katsushi Hasegawa
J-PARC Center, KEK & JAEA, Tokai-mura, Naka-gun, Ibaraki-ken, Japan
Abstract

INTRODUCTION
The Japan Proton Accelerator Research Complex (JPARC) is a multi-purpose high intensity proton acceleratorfacility, which consists of the linac, the rapid cycling synchrotron (RCS), and the main ring synchrotron (MR). At
present, the maximum output beam power of the RCS andthe MR is 300 kW and 200 kW, respectively.
In the RCS and the MR, wall current monitors (WCM)
dedicated for rf system commissioning are installed [1, 2].
The WCMs are designed to have a high cut-off frequency
more than a few 100 MHz, which is high enough comparedto the beam frequency up to several ten MHz.
The low level rf (LLRF) control systems of the J-PARC
RCS and MR are implemented by digital circuits to realize precise and reproducible rf control. The WCM signals
are used as the input of the beam loading compensation[3], and used for diagnostics to adjust the rf parameters andpatterns.
For the high power commissioning of the RCS and theMR, the information from the WCM is important and indispensable. The analysis of the WCM waveforms taken by a∗ fumihiko.tamura@j-parc.jp

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Figure 1: Measurement setup.

Figure 2: Mountain plots of the injected bunch. (Left) incase frf = 1.671650 MHz, the momentum of the injectedbeam and the rf frequency are not matched. (Right) in casefrf = 1.671750 MHz, they are matched well.

long memory oscilloscope is performed on a PC. In thispaper, we show examples of the various analysis results ofthe WCM waveforms.

BEAM CURRENT ANALYSIS USING
WCM WAVEFORM SLICES
Since the beam frequencies are in the range up to several ten MHz, direct measurement of the WCM beam signal by using an oscilloscope is possible. As shown inFig. 1, the WCM signal is taken together with the revolution clock signal, which is generated by the digital LLRF
control system and precisely follows the accelerating frequency sweeps. The signals are recorded by a long memory oscilloscope and the waveforms are analyzed on a PC.
Slices of the WCM waveform with lengths of the revolution
period, which is calculated by using the revolution clock
signal, are generated. Since the revolution clock signal follows the frequency pattern change, no additional information is necessary to generate the slices. By stacking theslices, one can obtain a mountain plot. Mountain plots areused to analyze the motions of bunches and variations ofthe bunch shapes. Figure 2 shows an example of the inISBN 978-3-95450-119-9

Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Wall current monitors (WCM) for rf system commissioning are installed in the J-PARC synchrotrons, the RCSand the MR. The WCM signals are used as the input of thebeam loading compensation system, and also used for diagnostics to adjust the rf system parameters. Since the rf andbeam frequencies are in the range of a few MHz and several ten MHz, direct measurement of the WCM signals ispossible. For the diagnosis, the WCM signals are taken byan oscilloscope with the revolution clock signal generated
by the low level rf (LLRF) control system, and slices ofthe WCM waveform with lengths of the revolution periods
are generated. By stacking the slices, one can get a mountain plot, which shows motions of bunches and variationsof the bunch shapes. Also, time variations of the bunching
factor, which are important for acceleration of high intensity proton beams, are obtained. The harmonic analysis isperformed on the WCM signal and the cavity voltage monitor signal. By using complex amplitudes of them, one cancalculate the impedance seen by the beam. We show examples of the analyzes described above. The rf parameters
for high intensity beams have been successfully adjusted
by using these analysis methods.

MOPA08

Proceedings of IBIC2012, Tsukuba, JapanFor high current acceleration in the RCS, the bunchshape control is a key to mitigate the space-charge tune
shifts. The bunching factor (Bf ), which is defined as theratio,

Figure 3: The mountain plots up to 1000 turns of the cases(top) without and (bottom) with the longitudinal painting.

0.5

w/o longitudinal painting
w/ longitudinal painting

0.45
0.40.35
bunching factor

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Bf = (average current)/(peak current),(1)

is an important parameter. It is easy to calculate the bunching factor by using the slices of the WCM waveform.
To increase the bunching factor, the longitudinal painting scheme during the injection period is employed in theRCS. The longitudinal painting [4] is carried out by adjusting the RF system parameters and programs. We employ
wide-band (Q = 2) magnetic-alloy (MA) loaded cavities
so that the dual-harmonic operation is fully supported. In
the dual-harmonic operation, each single cavity is driven by
a superposition of the RF signals, the fundamental accelerating RF (h = 2) and the second harmonic RF (h = 4).
Also, the momentum offset injection scheme and the second harmonic phase sweep are applied.
The mountain plots up to 1000 turns of the case with
only fundamental rf and no momentum offset i.e. without the longitudinal painting, and the case with the 80 %second harmonic, the momentum offset of −0.2 % and thesecond harmonic phase sweep of 100 degrees are shown inFig. 3. Also, the bunching factor plots without and withthe longitudinal painting are shown in Fig. 4. The beam
intensity is 2.5 × 1013 ppp, which corresponds to the beam
power of 300 kW at the repetition of 25 Hz. In Fig. 3, one
can clearly see that the bunch is widened with longitudinalpainting. The bunching factor of more than 0.45 is achieved
just after injection (at 250-th turn), while it was 0.25 in thecase without longitudinal painting.
As described above, these analysis methods of the WCM
waveform slices are very effective to adjust rf parameters.

0.3

HARMONIC ANALYSIS

0.25
0.2
0.15
0.1
0.05
00100

200

300

400 500 600
slice number

700

800

900 1000

Figure 4: Comparison of the bunching factor up to1000 turns of the cases without and with the longitudinalpainting.

jection frequency correction in the MR using the mountainplots. If the injection rf frequency is not matched to themomentum of the injected bunch, dipole oscillations occur. In case of an rf frequency of frf = 1.671650 MHz(Fig. 2 left), a dipole oscillation is observed and the amplitude corresponds to momentum mismatch of 0.1%. By
using the slippage η = −0.058, the frequency is corrected,
frf = 1.671750 MHz (Fig. 2 right). The dipole oscillation
disappears.

The harmonic analysis of the beam current and the cavity voltage is important to investigate the beam loading andthe impedance seen by the beam, because the wide-band
MA cavity covers not only accelerating harmonic but also
higher harmonics and the wake voltage in the cavity consists of these harmonics.
The harmonic analysis of the waveform is performed asfollows. The full waveforms of the WCM and the cavity voltage monitor from the injection to the extraction arerecorded together with the revolution clock signal. Fromrevolution clock waveform, one can generate sine and cosine waveforms of the selected harmonics (h = 2, 4, 6 incase of the RCS) with unity amplitude on the PC. Then, theI/Q demodulation for the selected harmonic is performedby multiplying the sine and cosine waveforms with themonitor signal waveforms and applying a low-pass filter.
The sine and cosine waveforms follow the accelerating frequency sweep, since they are generated from the revolution
clock signal. By the I/Q demodulation, the complex amplitude of the selected harmonic is obtained.

ISBN 978-3-95450-119-9
2Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, Japan

MOPA08Figure 5: Typical harmonic components of (top) the WCM
signal and (bottom) the cavity gap monitor.

By turning off the driving rf voltage of one of the cavities, it is possible to measure the wake voltage directly. Thetypical harmonic components of the WCM signal and thecavity gap monitor are plotted in Fig. 5. The beam intensity is 2.5 × 1013 ppp. Since the frequency sweep is slow
compared to the wide frequency response of the MA cavity(Q = 2), a steady-state equation is a good approximation
to calculate the time variation of the cavity gap impedance
seen by the beam. Assuming steady state, the wake voltage
in frequency domain is expressed as
Vcav (ω) = Zcav (ω) · Ibeam (ω),

beam commissioning. The analysis is applied to commissioning of the multiharmonic feedforward system for thebeam loading compensation [3]. In the commissioning
methodology described in the reference, the harmonic analysis is also used for the LLRF driving signal, so that theimpedance seen by the beam with generating accelerating
rf voltages and feedforward signals is analyzed. The feedforward system has been successfully commissioned andthe impedance seen by the high power beam was reduced
down to 1/30.

SUMMARY
In the RCS and the MR, WCMs dedicated for rf system commissioning are installed. As described above, various analysis methods on waveforms of the WCM are performed. The key is that the WCM waveform is captured
with the revolution clock signal generated by the digital
LLRF control system, which follows precisely the accelerating frequency sweeps. The analysis is performed on a PC.
Commissioning of the rf system parameters using the analysis has been successfully performed for the high power
operation in the RCS and the MR.

(2)

where ω is the angular frequency, Vcav (ω) the gap voltage,
Zcav (ω) the cavity gap impedance, and Ibeam (ω) the beam
current. By using the equation and the conversions of theWCM and the cavity gap voltage monitor (beam current[A] = WCM voltage [V] / 0.05, gap voltage [V] = monitor
voltage [V] ·3.35×104 ), the cavity gap impedance Zcav (ω)
is obtained as a function of time. By using the frequency
program, one can reconstruct the impedance curve in frequency domain. In Fig. 6, the reconstructed frequency response of the impedance seen by the beam is plotted. In theplot, the frequency response of the parallel circuit model(R = 850 Ω, L = 50 µH, C = 175 pF) is also plotted. The
measured impedance and the circuit model agree well.
Thus, the harmonic analysis of the WCM and cavity
voltage monitor signal is very important for high power

REFERENCES[1] N. Hayashi, et al.: “Beam instrumentations for the J-PARC
RCS commissioning”, in Proceedings of EPAC08, Genoa,
pp. 1125–1127, 2008.
[2] T. Toyama, et al.: “Beam diagnostics for the J-PARC main
ring synchrotron”, in Proceedings of Particle Accelerator
Conference, Knoxville, Tennessee, pp. 958–960, 2005
[3] F. Tamura, et al.: “Multiharmonic rf feedforward systemfor beam loading compensation in wide-band cavities of a
rapid cycling synchrotron”, Phys. Rev. ST Accel. Beams, 14,051004 (2011).
[4] F. Tamura, et al.: “Longitudinal painting with large amplitudesecond harmonic rf voltages in the rapid cycling synchrotronof the Japan Proton Accelerator Research Complex”, Phys.
Rev. ST Accel. Beams, 12, 041001 (2009).

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 6: Translation to impedance. Z(ω) = V (ω)/I(ω).