12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3IBIC2023, Saskatoon, Canada
JACoW Publishing
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T. Lensch, D. Lipka∗ , R. Neumann, M. Werner
Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, Hamburg, Germany
Abstract
The ARES facility (Accelerator Research Experiment atSINBAD) is an accelerator to produce low charge ultrashort electron bunches within a range of currently 0.5 pC to200 pC. Especially for eFLASH experiments at ARES an absolute, non-destructive charge measurement is required. To
measure an absolute charge of individual bunches different
types of monitors are installed. A destructive Faraday Cup isused as reference charge measurement device. To measure
the charge non-destructively 2 Toroids, 1 Turbo-ICT and 2
cavity monitors are installed. The latter system consists ofthe cavity, front-end electronics with logarithmic detectors
and µTCA ADCs. The laboratory calibration of the cavity
system is performed by using an arbitrary waveform generator which generate the same waveform like the cavity
with beam. This results in a non-linear look-up table used
to calculate the ADC amplitude in charge values independent of beam-based calibration. The measured charges fromthe cavity monitors agree very well within few percent incomparison with the Faraday Cup results.

MOTIVATIONThe beam charge determination is one of the most important properties to be measured in all accelerators. For thisseveral monitor systems are developed and installed at every
accelerator, but the absolute value of charge is always indiscussion. At the Accelerator Research Experiment at SINBAD (ARES) [1–6] we installed different types of charge
monitor systems: 2 Toroids, 1 Turbo-ICT and 2 cavity monitors. The charge value results are compared with a Faraday
cup who serves as a reference system with good agreement
to the other monitors; the expected charge loss of the Faraday Cup due to lost particles is simulated and smaller than
0.6 % [7]. The cavity monitor system consists of a resonator
where the first monopole mode at 1.3 GHz carries with theamplitude the charge information. This monitor system isfirstly developed to detect the dark current from accelerators with the same accelerator frequency where the single
dark current bunches generate superimposed fields within
the resonator [8]; for this reason the system is called Dark
current Monitor (DaMon). It was found that this resonating
system is able to measure very low values of bunch charges.
Therefore, the DaMon system is installed at several accelerators at DESY [9–12] to measure the bunch charge fromseveral fC up to few nC. The calibration of the amplitude to
the charge from a resonator to get the absolute charge is theissue of this contribution.
∗ dirk.lipka@desy.deFigure 1: Picture of the resonator and the electronics box
with front panel.

SETUP
The DaMon system consists of the resonator integrated
into the beamline, a front-end electronics (see an example inFigure 1) and ADCs. One resonator has two identical outputs
to ensure symmetry in the resonator, which are connectedwith low loss coaxial cables to the front-end electronics.
Both inputs are processed differently in the electronics. Thecharge channel (Q) is in most applications attenuated to beable to provide higher charges with band pass filters, logarithmic detectors (for high dynamic range) and followed bygain and offset adjustments to adapt to the 16 bit ADC. The
dark current channel (DC) has in addition a down converterin the logarithmic detector with free oscillating reference signal to provide higher sensitivity. This channel was foreseen
to superimpose the dark current bunches with acceleration
frequency 1.3 GHz but for other accelerators this channelis used for bunch charge measurement too. Therefore, theDC channel provides higher sensitivity and can be used forlower charge values compared to the Q channel.
TUP03706 Beam Charges and Current Monitors

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CHARGE MEASUREMENT WITH RESONATORS AT ARES

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CALIBRATION PROCEDURE
The sensitivity of the resonator (or amplitude of the oscillating voltage as a function of the charge) is calculated withthe resonance frequency 𝑓, the connection impedance 𝑍, the𝑅external quality factor 𝑄𝑒𝑥𝑡 and the shunt impedance ( 𝑄) to𝑅be 𝑆 = 𝜋𝑓√ 𝑄𝑍 ( 𝑄) [13]. Here 𝑓 and 𝑄𝑒𝑥𝑡 are measured for𝑒𝑥𝑡
each resonator; the shunt impedance is used from simulation
results but should only negligible vary for each resonator.
The charge can be calculated with the measured amplitude
𝑈 to be 𝑄 = 𝑈/𝑆. Up to the electronics the amplitude is
reduced due to the attenuation of the cables, therefore eachcable attenuation is measured at the resonance frequency1.3 GHz. The electronics processes the signals differently ateach channel, described in the setup chapter. To calibrate
the electronics in the laboratory a signal at the resonancefrequency with different amplitudes are generated and theoutput are monitored. For the dark current channel a contin-Figure 2: Calibration setup in the laboratory.
uous wave (CW) was applied which was sufficient for thiskind of measurement. Since the DaMon system was developed for the dark current measurement the same CW signal
was applied for the Q channel. But this CW signal entersdifferent sections of the logarithmic detectors compared toa pulsed signal. Therefore, a pulsed signal should be moresuitable for the calibration for the charge. Especially when
the DC channel is used to measure the bunch charge since
the down conversion is different in comparison of a damped

signal. The pulsed signal (damped signal) is generated withan arbitrary waveform generator, see Figure 2. The generated signal corresponds in frequency and decay time withthose expected from the resonator. The amplitude is varied
to get a table of the electronics response function. In Figure 2
the output of the electronics is visible with an oscilloscopefor one amplitude. The final calibration is realized with theinstalled electronics in the rack room with connected 16 bit
ADC.
The resulting electronics responds is shown in Figure 3
for the Q and DC channel. The CW signal measurements

Figure 3: Response function for Q channel on top and forthe DC channel below for electronics serial C17 installedat ARES. Each channel has three measurements: dark blue
with CW signal in the laboratory, bright blue for CW signaland 16 bit ADC and red for pulsed signal.
agree to each other but for a pulsed signal a difference isvisible for higher amplitudes. The number of measurement
points for the old CW was increased to check that charge
non-linearities are not caused due to interpolations from theelectronics responds. For the overall calibration the pulsed
signals are used to generate a look-up table for each channel
including the information about each resonator sensitivity,
cable attenuation and ADC conversion to voltage. Between
each point in the table the values are interpolated to calculate
from the measured ADC amplitude back to the charge.

TUP037274

06 Beam Charges and Current Monitors

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BEAM CHARGE
MEASUREMENT RESULTSIn Figure 4 the measurement of the first DaMon system
at ARES for the Q channel and a charge range up to 210 pC
is shown. The Q channel is attenuated with 20 dB before
the signal enters the electronics to provide a charge range
up to 1 nC. The values are compared with the Faraday Cup

Figure 4: Top: measured charge of the resonator Q channel
as a function of Faraday Cup charge; below the ratio of Q
channel to Faraday Cup charge as a function of Faraday Cup
charge. Error bars are standard deviation caused by beam
charge variation.
charges. The absolute values on top are fitted with a polynomial 1. order. The slope shows the factor between DaMon
Q channel and Faraday Cup results: with CW calibrationa disagreement of 6.7 % is shown, with the pulsed signal
the disagreement is reduced to −3.4 %. The lower diagramshows the ratio between Q channel and Faraday Cup charges
as a function of Faraday Cup charge. For charges above
100 pC the values are linear, therefore for accelerators operating above this value the DaMon Q channel shows the sameas the Faraday Cup, below this charge the CW calibrationshows non-linearities. This non-linearity is reduced to ±3 %with the pulsed signal calibration.

Figure 5 shows the charge measurement results with theDC channel in comparison with the Faraday Cup; here thecharge range is only up to 15 pC because for higher charges
the electronics is in saturation. Therefore, this channel isintended for small charges. The absolute values are fitted

Figure 5: Top: measured charge of the resonator DC channel
as a function of Faraday Cup charge; below the ratio of DC
channel to Faraday Cup charge as a function of Faraday Cupcharge. Error bars are standard deviation caused by beam
charge variation.
too and show almost the same slope; the CW values weredefined only up to 10 pC. In the lower diagram the ratios
are shown and a strong non-linearity is visible for the CW
calibration with an overestimation of the charge by a factorof almost 2. With the pulsed signal calibration the variationfrom a linearity is reduced to be only ±5 %. Still slightly
higher charge is measured with the DC channel which could
be corrected beam based.

SUMMARY AND OUTLOOKCharge measurement with resonators must be calibrated
carefully. A beam independent laboratory calibration wasdeveloped and the results were compared with a Faraday cup.
A calibration with CW pulses show non-linearities for lowcharges. A laboratory calibration with the same signal as itTUP037

06 Beam Charges and Current Monitors

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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 3 7

is expected from the resonator improves the non-linearity
and absolute charge values. The laboratory calibrations will
be improved due to automatic variation of the amplitude ofthe signal generator and readout from the ADC. In addition,a new electronics board is in development as a rear transition
module for µTCA crates with larger dynamic range due totwo additional amplifiers for different amplitudes for eachchannel.

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