HOM Studies in ACC39 during the Beam Time
Period Aug. 05 – Aug. 11, 2012
Participants: N. Baboi, T. Wamsat, DESY
P. Zhang, DESY / The University of Manchester / Cockcroft Institute
Introduction
Transverse wakefields in accelerating cavities are of special concern for the beam
dynamics. They can be minimized by centering the electron beam in the accelerating modules.
Therefore monitoring the higher order modes (HOMs) can help in the operation of an
accelerator. Also, they can monitor the beam position, like a BPM. For the TESLA cavities
special electronics has been built and installed in FLASH. Similar monitors are planned to be
installed in the XFEL, and are of interest for the ILC project study.
Our project is aimed to build HOM-BPMs for the 3.9 GHz cavities at FLASH, since the
wakefields in these cavities are much stronger than in the TESLA 1.3 GHz cavities. Various
modal options for HOM-BPM have been studied in Feb 2012, and the results have been used
to finalize the design of the prototype electronics which is currently being built by Fermilab.
The focus in this series of measurements performed at FLASH in Aug. 2012 is in three folds:
first, study the position resolution systematically of HOM-BPM using the trapped cavity
modes with the analog test electronics designed by Fermilab and a μTCA-based digitizer;
second, monitor the phase of the beam by reading the phase monitor close to ACC39; third,
monitor the HOM spectrum by a real-time spectrum analyzer during the measurements.
Part of this work is in the EuCARD project (http://eucard.web.cern.ch/eucard/index.html).
Two PhD students are involved in it.
1. Measurement Setup
The schematic of the measurement setup is in shown in Fig. 1. The devices were set up at
the ACC39 board rack outside the tunnel. Three HOM couplers have been connected to a RF
multiplexer to implement a fast switch among them. The HOM signal is switchable between a
real-time spectrum analyzer (RSA) and a test electronics box. The RSA was used measure the
spectrum with 9030-9090 MHz. The local oscillator (LO) in the test electronics box was set to
down-convert the signal at 9057 MHz to an intermediate frequency (IF) of 70 MHz. The IF
signal was further processed by a μTCA digitizer at a sampling rate of 108 MHz. The signal
was under-sampled. The phase monitor signal was split into two. One went into another test
electronics box with the LO set to down-convert 9057 MHz RF. The other one was mixed
with the 1.3 GHz signal delivered from FLASH master oscillator (MO). Both IF signals from
the phase monitor were further processed with the same μTCA digitizer.
Figure 1: Schematic setup of the HOM measurements.
Fig. 2 shows schematically the measurement scheme. An electron bunch of approximately
0.5 nC is accelerated on-crest by ACC1 before entering the ACC39 module. Steering magnets
2GUN were used to produce transverse offsets of the electron bunch in ACC39. Two beam
position monitors (9ACC1 and 2UBC2) were used to record transverse beam positions before
and after ACC39. Switching off the RF in ACC39 and all quadruples close to ACC39, a
straight line trajectory of the electron bunch is produced between those two BPMs. Therefore,
the transverse offset of the electron bunch in each cavity can be determined by interpolating
the readouts of the two BPMs.
Figure 2: Schematic of measurement setup for HOM-based beam position diagnostics study
(not to scale, cavities in ACC1 are approximately three times larger than those in ACC39).
2. Resolution of HOM-Based Beam Position Diagnostics
Calibration samples were firstly taken by moving the beam in a 2D grid manner as shown
in Fig. 3(a). Then the beam was steered to several positions within the calibration range, and
validation samples were taken for each beam position with 100 beam pulses. One example is
shown in Fig. 3(b). Along with HOM signals, beam information was also recorded
synchronously by reading nearby toroids, BPMs and currents of steering magnets. The BPM9ACC1 and BPM-2UBC2 were used to interpolate the beam offset inside the cavity, which
become the measured beam position. In contrast, HOM signals are used to predict the beam
position inside the cavity for both the calibration samples. The position differences between
measurements and prediction are presented in Fig. 3(c) for x and Fig. 3(d) for y. The rms of
the residuals is the position resolution. This example suggests 28 μm for x and 32 μm for y.
Figure 3: Beam position from direct measurement (blue dots) and from prediction with HOM
signals (red dots).
Fig. 4 shows the predicted beam position versus the measured position for several sets of
validation samples. They possess good consistency.
Figure 4: Measured and predicted beam positions for various beam offsets.
3. Phase Monitor
The phase monitor signals were recorded synchronously with the HOM signals for each
measured beam pulse. An example signal for each setup is shown in Fig. 4. By looking at the
zero crossing of the waveform, a relative beam arriving time can be determined. Fig. 5 shows
12 validation samples with different beam offset and measured at different time. Preliminary
estimation suggests a ~200 ps time jitter. This corresponds to ~0.8 degree phase jitter.
(a) Mixed with 1300 MHz
(b) Mixed with 9057 MHz
Figure 5: Phase monitor signals.
Figure 6: Phase monitor signal for 12 different validation samples.
4. HOM Spectrum
In previous studies, we observed the modal frequency shift after one day. This kind of
shift occurred in this measurement as well. Fig. 7 shows the HOM spectra (after a FFT on the
waveform) for 12 validation samples. The solid lines in different colour were measured
during the dedicated beam time at the same day. The blue dashed line was measured
parasitically three days after the dedicated beam time. The frequency of the mode at ~37 MHz
clearly shifts down by ~600 kHz. The black dashed line was measured parasitically four days
after the dedicated beam time.
Figure 7: HOM spectra for 12 validation samples.
5. Summary
The data taken during the beam time constitute the first systematic study of position
resolution of the HOM-based beam position diagnostics in 3.9 GHz cavities. The signal from
the phase monitor is currently under analysis. This is the first step in the study of calibration
stability of the HOMBPM system. The frequency shift repeated in this study and requires a
long-term parasitic monitoring.
The results from this beam time will be written in two papers submitted to IBIC2012
(http://ibic12.kek.jp/).