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BCM SYSTEM OPTIMIZATION FOR ESS BEAM COMMISSIONING
THROUGH THE DTL TANK 4
H. Hassanzadegan, R. A. Baron, S. Gabourin, H. Kocevar, M. Mohammednezhad, J. Murari,
S. Pavinato, K. Rosengren, T. Shea, R. Zeng, European Spallation Source ERIC, Lund, Sweden
K. Czuba, P. K. Jatczak, Warsaw University of Technology, Warsaw, Poland
Abstract
The ESS BCM system is not only used for beam measurement but it also plays an important role for machine protection particularly in the normal-conducting part of thelinac. During the previous beam commissionings to the
MEBT and DTL1 FCs and before the cavities were fully
conditioned, RF breakdowns and other types of discharges
in the cavities had a major impact on beam availability due
to the Fast machine protection functions of the BCM. Following an investigation on the root cause of the beam trips,
the configuration of the machine protection functions was
modified to improve beam availability in the more recent
beam commissioning to the DTL4 FC. In addition to this,some optimizations were made in the BCM system to improve beam measurement, and a few more functions wereadded based on new requirements. This paper reports onthese improvements and the results obtained during thebeam commissioning through the DTL4.

INTRODUCTION
The ESS Beam Current Monitor (BCM) system includes
in total nineteen AC Current Transformers (ACCTs) andone Fast Current Transformer (FCT) which are all from
Bergoz. Ten of these ACCTs are installed in the normal
conducting part of the linac that extends from the Ion
Source (ISrc) to the Drift Tube Linac (DTL) tank 5. Thesesensors overall allow beam measurement at the interface
between a section and the following one, and after the temporary beam dumps in the Low Energy Beam Transport(LEBT) and Medium Energy Beam Transport (MEBT)
sections. The other nine ACCTs will be installed in the
Linac Warm Units (LWUs) in between the cryomodules in
the Medium Beta Linac (MBL), High Beta Linac (HBL),
High Energy Beam Transport (HEBT) and in the Accelerator To Target (A2T) and Dump Line (DmpL) sections.
The analogue signal form each ACCT is buffered andamplified by a wall-mount Front End (FE) module from
Bergoz, and then filtered and further processed in a custom
designed Back End (BE) unit in the BCM rack. The signal
is then converted to digital and FPGA processed by a
Struck SIS8300-KU board in a mTCA crate. The digital
signal processing includes baseline restoration, droop compensation, digital filtering, post mortem data buffering aswell as a complete suite of machine protection functions
that are tailored to meet the ESS beam and machine requirements. Part of these functions are used to measure theamplitude, width and repetition frequency of the beampulse and send a beam abort request to the Fast Beam Interlock System (FBIS) within 1 μs if the measured values
exceed their thresholds. The BCM firmware configures themachine protection functions including the levels and activation/deactivation of the thresholds based on a beammode and a beam destination ID that are distributed overthe ESS network before starting with beam. Time windows
for the expected beam pulses from the ISrc and
LEBT/MEBT choppers are generated by the BCM firmware using some events which are distributed by the ESS
Timing System (TS) over an optical fibre network at welldefined times before the arrival of each pulse. These time
windows are then used for measuring the beam properties
including average beam current over a region of interest
and pulse length/charge. The BCM firmware also checks
the exact timing of the TS events for any inconsistency
with the selected beam mode due to possible bugs in thetiming tables, human errors etc. This feature has proven to
be very useful to avoid potential damages due to ex. awrong trigger length/frequency resulting in excessive
beam power on a temporary beam dump. Other protection
functions include differential beam interlocks with several
BCM pairs, errant beam detection and BCM internal
checks [1].
Each ACCT includes a calibration winding that is used
not only to calibrate the sensor but also to test and verify
the system with a test pulse from a waveform generator before starting with beam. A Wetest script is used to sweep
the pulse amplitude, width and frequency in multiple steps
from a minimum to a maximum, and in each step the fullsignal chain from the sensor up to the EPICS Process Variables (PVs) including all the protection functions are automatically tested and verified.

BEAM COMMISSIONINGS THROUGH
THE LEBT, MEBT AND DTL TANK 1
With the start of the beam commissioning through the
LEBT (Sep. 2018) and MEBT (Nov. 2021) [2], a large
number of beam trips were initiated by the BCM system
and in particular the errant beam interlock. An investigation then showed that part of the trips was due to Electro
Magnetic Interference (EMI) issues caused by the safety
relay (i.e. ground relay), High Voltage Power Supply(HVPS) and the high power contactors of the ISrc. The issue was then resolved by improving the groundings of boththe ISrc cage and the electronics racks, reinforcing theshielding and grounding of the ISrc/LEBT BCM FEs, additional EMI filters, surge protectors, analogue filters on
the HVPS current/voltage readouts and digital processingin the BCM FPGA. The tests also showed that a broken
insulator around a LEBT repeller ring resulted in somesparks and these occasionally caused some large spikes on
the LEBT BCM readout thus adding to the beam trips. The
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repeller was then repaired through a LEBT intervention
and that fixed the issue with the sparks, but an uncareful
reassembly of the LEBT components then resulted in somebroken connectors as well as a short circuit through the
LEBT BCM sensor by a thermocouple cable, and these rendered this BCM temporarily non-operational. The broken
connectors were then replaced and the short-circuit was removed through another LEBT intervention. With the startof the beam commissioning through the DTL tank 1 (June2022), a large number of spikes showed up on the DTL1
BCM readout and these had a major impact on beam availability. In order to better analyse the beam trips, a Post
Mortem (PM) function was then added to the BCM firmware. This includes a configurable ring buffer that keeps
the most recent BCM data with a sample rate of 88 MSPS
from all the channels, and freezes the buffer contents assoon as the Beam Permit is changed to NOK. For the spike
analysis, the buffer size was set to 1.1 GB that is equivalent
to 1 s of BCM data (i.e. 400 ms before plus 600 ms after abeam trip). The PM data analysis then showed a correlation
between the spikes and the DTL RF/vacuum levels, but theroot causes of the spikes were understood and the issue wasresolved only after the start of the following beam commissioning through the DTL tank 4.

6.

showing up when the buncher 3 voltage goes above~130 kV.
An RF breakdown in a DTL tank induces a current
in the BCM calibration windings. In the ISrc to DTL
tank 2 sections, calibration windings of seven BCM
sensors are connected in series, and the induced current can cause beam trips by the LEBT, RFQ and
MEBT BCMs as well even if the protection functions of the DTL BCMs are disabled.

The investigation confirmed that most of the spikes
listed above are directly linked to RF and vacuum, and they
become less frequent with cavity conditioning. The spikesof type 1-3 are overall very small and do not cause an issue.
Figure 1 shows a beam pulse with an amplitude of 4.5 mA
and width of 50 μs that is measured by all the BCMs fromthe LEBT to the DTL tank 4. The differences in the pulse
amplitude (beam loss), the small spikes on the RFQ BCM
readout (multipacting) and the baseline variations due tothe secondary particles from the DTL1-3 are visible in theplot.

BEAM COMMISSIONING THROUGH
THE DTL TANK 4In April 2023 with the start of the DTL tanks 1-4 RF conditioning activities, spikes were frequently observed with
the DTL BCMs and in particular tank 2, but fewer spikes
were observed in the tank 1 compared to the previous beam
run in June 2022. A thorough investigation on the root
cause of the spikes then showed the followings:
1.

2.

3.
4.

5.

The inner surface of the DTL tanks emits electrons
when it is subject to the RF pulse. These electrons
travel a short distance after they exit from both thebeam entrance and the beam exit ports of the tanks.
Part of the electrons reach the BCM sensors which
are installed in between each two tanks and that results in a small spike of max 0.2 mA on the BCM
readout.
Two small spikes of typically -0.3 mA are observedon the Radio Frequency Quadrupole (RFQ) BCM
readout at the rising and falling edges of the RF pulse
due to multipacting.
The rising edge of the ISrc beam pulse results in asmall spike of +0.6 mA approximately on the LEBTBCM readout.
RF breakdowns and other types of discharges in thecavities result in large spikes on the DTL BCM
readouts. These spikes can easily hit the machine
protection thresholds and cause beam trips by the
BCM and the RF Local Protection System (LPS).
RF and vacuum breakdowns in the MEBT buncher
3 cavity can cause spikes with an amplitude of a few
tens of mA on the MEBT BCM 2. The spikes start

Figure 1: A low current beam pulse that is measured byeight BCMs from the LEBT to the DTL tank 4.
In order to improve the beam availability due to the spiketypes 4-5, following an assessment by a Beam Damage
Team (BDT), the interlocks on the beam amplitude and errant beams in the DTL section were replaced by Mediumspeed differential thresholds on beam losses. This however
did not compromise the errant beam interlock function because any errant beam would still be detected by the Fast
thresholds of another four BCMs located upstream of the
DTL BCMs.
Figure 2 shows the BCM PM data including a large spike(i.e. RF breakdown) on the DTL2 BCM readout causing
signal saturation with this channel as well as some ringing
on the other channels (item 6 in the spikes summary).

Figure 2: A large spike on the DTL2 BCM readout causing
some ringing on the other BCM channels.

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06 Beam Charges and Current Monitors

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Given that the BCM signal drifts with temperature are
extremely small, this issue was easily resolved by disconnecting the BCM calibration cables after the system wascalibrated and the in-situ tests were completed before starting with beam.
The combination of the above-mentioned action items
fully resolved the beam availability issue with the BCM.

DIFF BCM INTERLOCKS
Differential BCM interlocks are mainly used to measure
beam losses between a BCM pair and stop the beam if thelosses exceed a threshold. The BCM firmware allows using
three types of differential interlocks, being: Fast, Mediumspeed and Slow for each pair. The Fast and Medium-speed
thresholds are set on the differential current and they can
include a configurable moving average filter as well. The
Slow threshold is however set on the cumulative charge per
cycle and it gets reset shortly before the arrival of the next
pulse.
Figure 3 shows the absolute and differential beam currents from a differential pair with the RFQ BCM and
MEBT BCM 1. The difference in the pulse lengths (20 μs
vs. 5 μs) is due to the MEBT chopper, and the 0.1 mA step
before the MEBT BCM 1 pulse is the scattered beam from
the MEBT chopper dump. For the beam commissioning to
the DTL tank 4, the Fast and Slow differential interlocks
were disabled, but the Medium-speed one was enabledwith several BCM pairs. The width of the moving averager
was set to 55 μs (LEBT-RFQ BCM pair) and 6 μs (threepairs between the consecutive DTL tanks from 1 to 4) tomake the differential interlocks less sensitive to the spikes
from the RF breakdowns, but sensitive enough to beamlosses that have the potential to cause a damage. This
proved to be an effective method to avoid unwanted beamtrips particularly at the start of the beam commissioning
and before the cavities were fully conditioned.

Figure 3a: RFQ BCM (blue) and MEBT BCM 1 (orange)
readouts. Figure 3b: Fast (blue) and Medium-speed (orange) differential currents for the above pair; the amplitudeof the Medium-speed signal is 70% lower than the Fast one
due to the moving averager.
06 Beam Charges and Current Monitors

OTHER IMPROVEMENTS
Fast BCM
The Fast BCM (i.e. FCT) is installed towards the MEBT
end and it is mainly used for verifying the 10 ns rising/falling edges of the beam pulse from the MEBT chopper. The
analogue signal from the FCT is measured by a fast oscilloscope with a sample rate of up to 20 GSPS and an overallmeasurement bandwidth of 700 MHz. Figure 4 shows ascreenshot of the FCT signal. The falling edge of the beampulse and the beam bunches and their frequency spectrum
up to the second harmonic (i.e. 2*352 MHz) can be seen in
the screenshot.

Figure 4: The falling edge of the beam pulse from the
MEBT chopper as measured by the Fast BCM.

One Hour Average Beam Current Measurementwith sub-μA Accuracy
During the beam commissioning to the DTL, a temporary
shield wall was used to isolate the DTL tank 4 from thedownstream sections. This allowed to run the beam to the
DTL4 FC and in parallel continue with the installation
work in the superconducting part of the linac. Under these
conditions, personnel safety regulations set a limit of 1 μA
average beam current over a rolling period of one hour toensure the workers would not receive a radiation dose rate
higher than 3 μS/h. The limit was imposed in the first handby only allowing certain beam modes, but the BCM system
was also used to ensure the actual beam current would notexceed the 1 μA limit. Given that the required accuracy
needed to be by ~4 orders of magnitude better than that theregular BCM accuracy (i.e. +/- 0.8 mA), this measurement
could only be successfully done after the following issues
were resolved:
 Any DC offset due to measurement errors needed to
be precisely removed from the BCM readout. Thisparticularly applies to the baseline restoration function.
 The LEBT/MEBT choppers with a reduced electrodevoltage result in some leakage beam which can potentially get accelerated by the RF in the following sections and contribute to the radiation dose. The regular
BCM channels however may not always measureleakage beams because of the chosen triggers for thebaseline restoration function. In order to fix this issue,
a copy of the MEBT BCM 2 signal was used for the
sub-μA beam measurement but its trigger was
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configured differently so that it would not be blind topotential leakage beams.
 An earlier study shows that below 85% of the nominal
RFQ voltage, the beam will be lost either in the RFQ
or in the downstream sections [3]. Then, this particular BCM channel was configured to only measure thebeam within the time window of the RF pulse, and thissignificantly improved the measurement accuracy
comparted to a continuous measurement.
Figure 5 shows a 20-hour history of the charge per pulse
as well as the 1-hour average beam current including any
leakage beams within the RF pulse window. The peakvalue is 0.27 μA which is equivalent to 27% of the“budget”. The tests showed that the BCM can indeed measure the average beam current with an accuracy significantly better than 1 μA provided that the triggers are all
correct.

Figure 5: 20-hour history of the charge per pulse as measured by the MEBT BCM 2 (green) and the 1-hour average
beam current with sub-μA measurement accuracy (red).

LVDS to RS-485 / Optical Converter
The existing BCM-FBIS interface uses four Low Voltage Differential Signalling (LVDS) channels, being three
discrete and one customized serial data link. The discrete
channels are used for the Beam Permit, BCM Ready andBeam Presence signals. The serial data is Manchester encoded and used for sending the same information as thediscrete channels (for redundancy purposes) as well assome more information including beam mode/destination
IDs, life-sign counter, beam exist flags etc. with an update
rate of 100 kHz. Although this interface has so far worked
without a major issue, concerns over its reliability withlong cables in noisy environments triggered the development of some new electronics in collaboration with the
Warsaw University of Technology (WUT) with the aim ofmaking the interface more robust. The new electronics consist of a transmitter Printed Circuit Board (PCB) for data
aggregation and LVDS conversion to RS-485/optical aswell as a receiver PCB for optical signal conversion to RS485. The transmitter comes in two form factors being: 1)
mTCA Advanced Mezzanine Card (AMC) with two channels, and 2) Rack-mount unit with up to four channels. The
two form factors are planned respectively for the BCM and
the Beam Position Monitor (BPM) systems. Figure 6
shows the first prototype of the transmitter AMC that was
successfully tested at ESS in April 2023 together with a receiver unit and the BCM/FBIS electronics.

Figure 6: Photo of the new LVDS to RS-485/optical converter AMC (Courtesy of Pawel. K. Jatczak - WUT).

SUMMARY AND OUTLOOKDuring the ESS beam commissioning through the DTL
tank 4, nine BCM channels were successfully used forbeam measurement, machine protection and timing synchronizations. At the start of the commissioning, the beam
availability was impacted by a large number of beam trips
due to spikes on the BCM readouts. An investigation thenshowed that the spikes were partly due to EMI and othertypes of external disturbances, and partly due to RF / vacuum breakdowns particularly in the DTL section. The issue
was then resolved by addressing the EMI issues and reconfiguring part of the BCM protection functions to makethem less sensitive to the spikes but sensitive enough tobeam losses that pose a damage potential. During the samecommissioning run, a Fast BCM, a Medium-speed differential interlock and a special BCM channel with sub-μA
measurement accuracy were successfully tested and verified with beam as well. It is planned to continue with the
BCM activities in 2023-2024 by installing in total ten new
BCMs in the DTL tank 5 and the superconducting part ofthe linac as well as three BCMs in a new ISrc-LEBT Test
Stand that is under construction. It is also planned to makethe BCM-FBIS interface more robust by deploying a newly
developed RS-485/optical interface module. The otherparts of the system including the electronics, FPGA firmware, software and the operator interfaces have already become mature following a few rounds of improvements,
hence no major changes are planned in the near future.

REFERENCES[1] H. Hassanzadegan et al., “Machine Protection Features of the
ESS Beam Current Monitor System”, in Proc. IPAC'18, Vancouver, Canada, Apr.-May 2018, pp. 2058-2060.
doi:10.18429/JACoW-IPAC2018-WEPAF088[2] R. Miyamoto et al., “Beam Commissioning of Normal Conducting Part and Status of ESS Project”, in Proc. LINAC'22,
Liverpool,UK,
Aug.-Sep.
2022,
pp.
18-24.
doi:10.18429/JACoW-LINAC2022-MO1PA02[3] E. Sargsyan, “ESS RFQ Error Study”, May 2014, ESS Internal Report.

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