High precision superconducting cavity diagnostics with higher order mode measurements Page: 4 of 13
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cavity at an angle will enter and leave each of the
cells with a different offset.
Figure 5: Beam passing through a two-cell
cavity with an angle offset, but zero position
offset at the cavity centre.
Since the length of each of the cells, and the
phase change per cell of the mode are known, it
is possible to calculate an approximate value for
the amplitude scaling between (for example) the
mode induced by a 1 mm offset, and the mode
induced by a 1 mrad angle.
Dipole Mode Measurement
The linear relationship between the dipole
mode voltage induced, and the offset of the beam
with respect to the axis of that mode, implies that
the voltage waveform can be examined to find
the position of the beam as it traverses each
cavity. Promising preliminary
measurements[ 15] made on the first FLASH
accelerating module showed that a beam position
resolution of 50 pm could be easily achieved by
measuring the time-domain intensity of the
dipolar modes excitation with a variable
bandwidth spectrum analyser. The following will
explain how the TESLA cavities at the FLASH
facility were instrumented in order to examine a
particular dipole line, and to explore the
possibility of determining the beam position
from an analysis of its properties.
Electronics
Simulations of the TESLA cavities[11] show
that the sixth mode in the first dipole passband,
TE111-6, has a strong coupling to the beam, with
an R/Q of ~5.5 Ohms/cm2. Its frequency is ~1.7
GHz, with small variations from cavity to cavity.
A circuit was developed to filter the HOM output
around this frequency, and downmix to a lower
frequency for digitisation. Its block diagram is
shown in figure 6. After filtering (20 MHz
bandwidth), the signal is mixed with a 1.679
GHz (186 times the 9.0275 MHz accelerator
reference frequency) local oscillator (LO). The
resulting 20 MHz signal is then digitized at 108
MHz (12 times 9.0275 MHz), with 14 bit
resolution. The purpose of locking the LO andthe digitiser clock with the accelerator reference
frequency is to allow meaningful phase
measurements of the signal.
Figure 7 shows a more detailed schematic of
the electronics[16]. An input coupler sends a
small part of the input signal to a test port. A
second coupler introduces a 1.697 GHz (188
times 9.0275 MHz) calibration signal to measure
the signal amplitude and phase. A two section
ceramic filter is used to attenuate signals outside
of a 20 MHz bandwidth of 1.7 GHz mode
frequency to prevent amplifier saturation.
Frcrni HC)L E -lrFpas
H Filter +.#tt er - Digizer I
GHz 2OMHZ IF
Figure 6: Mix-down electronics.
A RF limiter, rated at 100 W peak power is
used to protect the downstream active electronics
from possible high output power from the HOM
coupler. Note that leakage of the cavity
fundamental power at 1.3 GHz is blocked by the
passive band pass filter.
A low noise (1.1 dB NF), high linearity (27
dBm OIP3) preamplifier is used, followed by a
four section, 20 MHz bandwidth ceramic filter.
This second filter blocks all frequencies which
might alias into the signal band. A high linearity
mixer (30 dBm IIP3) is used to mix the signal
down to the approximately 20 MHz IF.
The mixer is followed by two stages of IF
amplifier to drive the required +/-1 V, 50 Ohm
input of the digitizer. A low pass filter at 36
MHz is used to eliminate amplifier noise.
The digitizer is a Struck Innovative Systems
SIS3301, eight channel, 14 bit, VME digitizer,
operating at 108 Ms/s. Data is collected from the
digitizer with the DESY DOOCS[17] control
system, and processed offline using Matlab[ 18].
Due to the risk of damage to the electronics
from high power HOM signals produced by
large beam offsets during the experiment, 10dB
attenuators were added to the input of the
electronics. This provides increased
measurement range, but is expected to degrade
the system resolution by a factor of ~3, assuming
the electronic noise is the limiting factor.
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Molloy, S.; Frisch, J.; McCormick, D.; May, J.; Ross, M.; Smith, T. et al. High precision superconducting cavity diagnostics with higher order mode measurements, article, January 1, 2006; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc886549/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.