Injector beam monitors
Fusashi Miyahara
for Injector Linac, SuperKEKB

The 23rd KEKB Accelerator Review Committee, Jun 9, 20191

Contents
1. Beam Position Monitor• BPM readout system (specification, resolution)• Function of a calibration pulse and stability of the system• Synchronized data acquisition• Quadrupole moment of the beam using 8 electrodes BPM

2. Beam Profile Monitor• Specification of a new high resolution beam profile monitor• Usability of the profile monitor

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Beam Position Monitor(BPM)

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BPM readout system in the Linac
VME based BPM readout system is used in the linac. The BPM board has two band pass filterswith bandwidth of 60 MHz (fc=180 MHz), 16bit-250Msps ADCs and a calibration pulse generators.
Position resolution of the system is smaller than requirement (σ<10 µm)

requirement

Position resolution of the BPM estimated by 3-BPM method

VME crate: 22, BPM board: 108

MVME5500 (CPU): 1
VME-EVR-230RF: 1
RAS board: 1
BPM readout board: 2-8 in a create

The oscilloscope system is used in
LTR, RTL, HER/LER BT.

The system covers the whole of the linac (A-sector – SY3) and a part of the LTR (SY2) BT.
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Calibration pulse

To calibrate gain balance between two signals from opposite electrodes and monitor the gain
drift and cable connection, the BPM board can send a calibration pulse to an electrode.

1)
The calibration pulse is sentfrom the channel Y+.

2)
The calibration pulse induces
signals on the adjacent electrodes.

ADC response to the calibration pulse.
The signal strength of ch-i is given by

U i= Gi∑ (V − Vi, ji , pedestal)

2jVi,j : ADC value

3)
The amplitude of the signals aremeasured.

Calibration of the X direction
Acknowledgment:

The gain factor is calibrated by,Gx+=1, Gx-=Ux+/UxGy+=1, Gy-=Uy+/UyUsually, the gain calibration is performed
at installation or replacing the board.

Design of our BPM system is based on LCLS one.
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We thank Dr. Steve Smith and Dr. Andrew Yang for providing many information and meaningful suggestions.

Stability of the BPM System
The calibration pulse measurement are performed twice in a hour. It also runs machine
shutdown time. Because the signal balance between electrodes affects the beam position,
monitoring the balance is important.
Variation of the ratio of the signals
from the opposite electrodes∆r = r − rr=Stable 

U x−
U x+

Position drift due to the variation of the gainbalance can be approximated by∆x ≈ a∆r≈ 0.5a∆r
1+ r𝑎𝑎 ≈8.5 mm for Sector: A − 2 ,5.2 mm (for Sector: 3 − 5)

Suddenchange Drift

Stability of BPMs (4 electrodes only)
Maximum variation of thegain balance in a month [%]<0.1

0.1-0.2

0.2-0.3>0.4

Number of BPMs (101 BPMs)

33231827

Transit of the signal ratio between opposite
electrodes to the calibration pulse in amonth.

The reason of the sudden change (without cable re-connection) and the drift is not clear.
 We have to find the reason.
The beam position shift or drift (<~20 µm) stems from the change of the signal balance
doesn’t affect the beam operation yet, but we will develop alert or feedback system. 6

Synchronized data acquisition
The event timing system allows the beam mode control (HER/LER/PF/PF-AR) andsynchronized data acquisition of the BPM, the RF system and pulsed magnet systemthrough the shot-ID.
The synchronized data are put in a PV(Waveform record including the shotID and the beam mode).

• 103 BPMs (Position, Charge)• 59 RF Monitors (Amplitude, Phase)• 98 Pulsed Magnets (Current) LIiBM:BPM_SYNC:DATA:[BeamMode] LIiRM:RFM_SYNC:DATA LIiMG:PM:sync_data:IREAD

Every shot of the those data are archived.
 We can analyze correlation of those data and check an irregular event.

The RF pulse shortening was found.
(T. Miura)

An irregular beam orbit caused a LER abort (Y. Seimiya).
(* Most of abort has another reason.)7

Quadrupole moment measurement of the beam
Monitoring the energy spread during the operation is very important to keep the beam quality,
but the screen monitor can’t be used for this purpose due to it causes the emittance growth.
 Non-destructive energy spread monitor is required.
Quadrupole moment of the beam depends on
the beamsize.
Jq =(1 222x − y2 + x − yR)

NJq ≈∑V cos 2θ
i =1iN∑Vi =1i

Vi: pickup voltageT. Suwada, M. Sato and K. Furukawa
Phys. Rev. Accel. Beams 6, 032801(2003)

 By monitoring the quadrupole moment at a point with a large dispersion function,
the change of the energy spread/distribution can be measured non-destructively.
(The actual beam size can’t be estimated by this way because the quadrupole moment
also depends on the beam distribution.)
The quadrupole moment was used at KEKB era, but calculation of the quadrupole
was not prepared on the new BPM readout system.
 A software IOC to calculate the quadrupole moment was developed.

88 electrodes BPM in the linac, HER and LER BT

J-ArcS8_R0_31

𝜂𝜂 ≈0.85 m

8 electrodes stripeline BPMLER BT𝜂𝜂 ≈ −1.64 m

HER BT𝜂𝜂 ≈ −0.22 m
9Correction of the beam position
The quadrupole moment depends on the beam position and the beam is jittered every shot.
A quadratic function to cancel the beam position term was applied.

Jq =Position dependency measured
by changing the strength of thesteering magnet upstream of theBPM. The red line represents thefitting function.

()

1 222x − y2 + x − yRposition term

Before correction

After correction

Beam position and quadrupole moment before (left) and after (right)
correction.

The correction works very well.
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Beam size and Quadrupole moment@J-ARC

Screen Monitor (SC_R0_31)

8 electrodes BPM (S8_R0_31)

To confirm response of the quadrupole moment of the beam, synchronous measurementsof the quadrupole moment and the beam profile had been performed.
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Beam size and Quadrupole moment@J-ARC• The quadrupole moment proportional
to square of the beam size, butsecond-order coefficient depends
on the beam distribution (klystronphase).
• Beamsize minimum corresponds tothe quadrupole moment minimum.

57 µm/pixel

The quadrupole moment and thebeamsize measured by the screenmonitor.
The klystron phase was varied
to change the distribution.

• Variance is not small.
 Statistical average should be used.
By using statistical average of the quadrupole
moment, non-destructive measurement of thechange of the beam profile or energy spread,that depend on the laser or the klystron phaseor something, becomes possible.
12Quadrupole moment and beam profile

The beam sizes (RMS)of those figures arealmost same.
But the distribution
and the quadrupole
is completely different.

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Quadrupole moment and beam profile

The quadruple moments
are almost same, but
distribution and beam
sizes are completelydifferent.

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Quadrupole moment and Horizontal beam profile

The core of the distribution
is very narrow, but thequadrupole is not small
due to a long tail.

We can only monitor
the change of thedistribution(≈energy spread)15

Quadrupole Moment and HER Injection• Clear correlation between thequadrupole moment and theinjection efficiency.
• The reason of the drift of thequad. is not clear yet.
Laser? RF? …
We have to find the sourceand develop feedback systems.

Quadrupole moment of the electron beam andInjection efficiency (average of 1 minute).

The quadrupole moment of thebeam at the J-Arc can be a goodtool for improvement of thebeam injection to the ring.

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Summary of the BPM in the Linac• The position resolution of 5-7 µm is smaller than the requirement.
• Synchronized data acquisition of BPM, RF and pulsed Magnet is very useful.
• Most of the BPM readout boards are stable (gain variation < 0.4%  ∆x, ∆y<20 µm),
but 30% of the boards have more than 0.4% gain variation.
• Change of the energy spread can be monitored by measuring the quadrupole
moment of the beam with 8 electrodes BPM at a large dispersion function point.
That affects beam injection efficiency to the ring.

TODO• Finding the reason of the unstability of BPM boards(~30%).
• Development of an alert or a feedback system to the large variation of the gain balance.
• Finding the source of the drift of the quadrupole moment (≈ energy spread) of the e- beam.
• Install new BPMs in the e+ capture section ( Enomoto’s talk).

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Beam Profile Monitor(Screen Monitor)

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Screen Monitors in the Linac
Screen monitor is very important to achieve production and transportation of the low
emittance electron beam.
 Downstream of the rf-gun
 • Optimization of the laser and the rf gun• intrinsic beam emittance(without the beam position jitter and the emittance growth) Key point (e.g. top of the sector) in the linac • Making a guess of a source of the emittance growth Point with a large dispersion function • Energy spread
There are about 100 screen monitors, but they have some problems.
• Conventional system is simple but it has poor resolution (~80 µm FWHM).
• Aluminum ceramic (Al2O3:Cr2O3) is used as the screen material.
It is fabricated by sintering and has a very long decay time (~ms).
That degenerate the resolution.
( The long decay time enable dynamic range control of theintensity by changing exposure time of the camera.)
Conventional (old)• Some screens have target guide lines, that disturbs beam profile
screen monitor system.
measurements.
• The CCD camera without external trigger.
 The intensity is random, because the screen shot is not synchronized with the beam.

 A new high resolution screen monitor system is required.

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High Resolution Screen Monitor (new)Layout of the optical systemcomponent of the screen monitor system

Screen SC_A4_42 (Lead shieldshield is removed in this picture)

Screen

YAG:Ce crystal (t=100 µm) or
Optical Transition Radiation (OTR) screen (70µm silicon wafer with Al coating)

Optical System

Achromatic lens x 2(f=750, f=250 mm), Bandpass Filter, ND Filter with motorized filter
wheels(variable), Iris, Mirror (depend on the direction of the view port), Lead shield

CCD Camera

GigE camera with external trigger (Sensor: 1/3-inch type, Resolution: 659x493 or 1024x768)

Some thin aluminum ceramic screens areused at some points, because it is useful tocontrol dynamic range.

Location of the new screen monitors (~June 2019)

The resolution of the optical system isabout 10 µm FWHM (estimated byspread of a sharp edge).
It is much smaller than the beam sizein the linac (σe->~100, σe+>~50 µm).

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Screen Materials

Conventional (Old) type withtarget guide lines (t= 1 mm)Thin type (t=0.1 mm)

Aluminum Ceramic (Al2O3:Cr2O3)

YAG:Ce (t=0.1 mm)

OTR Screen (t=0.07 mm)

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Positron beam profile at Linac End (SC_57_2)
Screen (single shot measurement)

Wire Scanner (multi shot measurement)

Positron beam profile measured
by the OTR screen at the linac
end.
Beam size estimated by thescreen and the wire scanner
σ x [mm]

σ y[mm]screen

0.93

0.050

wire

1.190.058

Horizontal (left) and Vertical (right) beam profile
measured by screen and wire scanner.

The beam profile measured by screen shows
good resolution and consistent with wire
scanner measurement.
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Electron beam profile at Linac End (SC_57_2)
Wire Scanner

Screen Monitor

In this case, the electron beam split into two core beams.
The horizontal profile measured by wire scanner shows fluctuation due to multi shotmeasurement with the beam jitter and the variation of the profile.
Only 2-dimentional profile helps understanding of the situation.
In case of 2 core beam at the linac end, by tuning of the beam orbit after the 2-sector,double core beams are assembled in single core.

High resolution screen monitor is important to improve the beam quality

23Summary of the Screen Monitor•New high resolution screen monitors were installed.
•The resolution of the optical system is ~10 µm FWHM which much smaller
than the beam size (σe->~100, σe+>~50 µm).
•A beam size measured by the screen monitor is consistent withthe wire scanner measurement.
•Two dimensional beam profile can be a good tool to improve the beam quality.

TODO•Beam profile measurement with the screen monitor is destructive and it disturbs
beam injection to the rings (HER, LER, PF, PF-AR) •Install a new screen to HER/LER BT.
•Replace the screen material to very thin material (e.g. aluminized Mylar).
•Development of a universal data acquisition system with auto gain control (variable
ND filter, exposure time).
•Development of a feedback system using the screen monitor.
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