Bunch feedback systems andluminosity
Makoto Tobiyama
KEK Accelerator Laboratory

SuperKEKB Beam Instrmentation

Outlookn

From around the end of Apr., the operation of the HER became difficult.
– Frequent beam abort due to large beam loss.
– Luminosity drop due to large vertical beam size.
nnn

Suspected the large noise near HER vertical tune affect the beam. The frequency was
not stable and was slowly changing with time.

By moving the HER vertical tune far from the noise, the operation wasresumed.
Also, by reducing the HER vertical feedback gain(2 shift gains, -12dB), theluminosity has been jumped up.
– HER beam was not stable under single beam condition due to insufficient
feedback gain, though.

nTried to find the source of the noise injected feedback system and foundthe cause (broken module). By removing the module, the noise hasdisappeared, and it was possible to increase the vertical feedback gain (1shift gain, 6dB) which enabled the stable operation under single beamconditions.

SuperKEKB Beam Instrmentation

SuperKEKB Transverse FB systems

Collaborating SLAC(US-Japan) and INFN-LNF(KEK-LNF)

2021ab Operation Summary
D01V1 headreplacement

D02V1 headreplacement

HER BxB FB noise is fixed.
NIM bin and fanout replacement• The 2021a run started on 16th February andoperated for 140 days (4 months and half).

• The first ten days were devoted to the vacuum

940 mAscrubbing.

• We operated with βy* = 2 mm to checkhardwares and to test high current operationsafely.

1000 mA• Calibrations of BPM and collimator head34

Lp 3.12x10
HER many aborts
BxB FB noise

positions, etc. were also performed by using
beams during the first two weeks.

• D01V1(HER) head was replaced. The top jaw

was short for the LER collimator head (March 23).

• We squeezed βy* down to 1 mm on 10thMarch. Beam currents increased with "bakingrun". 1000 mA / 940 mA w/o physics run• HER many aborts from April 20 to May 3.
• D02V1(LER) head was replaced due damage(June 7).

4More detail of Lsp
HERchange νy43.572 → 43.578

5HER BxB Feedback gain (vertical) decreased.
Swing shift on May 9
off-resonance (-60 MeV)

3.12x1034

Linac energy FB
troublemaintenanceBxB Feedback Noise in HER

May 6
HER BxB FB noise is fixed on June 29.
NIM bin and fanout replacement

D02V1 replacement
and maintenance

May 3

IGPF:HERDV:SRAM:PEAKFREQ2

The noise moved away from the working point on May 14.

M. Tobiyama

SuperKEKB Beam Instrmentation

Found noise in the HER BxB FB

By H.Fukuma

2nd harmonics of this signal (254kHz) affected the HER BxB FBSuperKEKB Beam InstrmentationnFound the suspicious NIM-PS for the HER timing system–––––nn

RF x4 multiplier
RF amplifiers
2GHz phase shifter
2-in 4-out Fanout (for fiducial distribution)
Huge voltage oscillation, especially -6V (Vpp~2V) with 127kHz

Prepared new NIM-ps, new RF x4 multiplier, switched the timing singal on
29/JunNo huge signal near vertical betatron tune
– Before : >20dB to baseline– After : almost no noise line (still tiny spurious remain)nReal cause:– Fanout (2-in, 4-out, Houshin elec. ~10 years old) was the real cause of the PSoscillation.
– Old technology (many large transistors, large package ECLs, etc) but notobviously damaged.

SuperKEKB Beam Instrmentation

HER vertical FB situationnMeasurement of instability growth rate and feedback
damping rate using transient domain analysis with iGp12
digital filters.
– turn off FB (10ms – 15ms) and turn-on again, record 12MB datann

by*=8mm (200mA, 370mA), by*=1mm (200mA, 400mA,600mA) with normal filling pattern of current runs.
Mode -1 is the strongest mode, which is suspected tobe caused by the resistive wall impedance.
– Unstable mode measurement at SuperKEKB Phase 1 has shown
the ion-like unstable modes (and behaviors).
– Recent measurement shows much weaker ion-like unstablemodes.

SuperKEKB Beam Instrmentation

Growth and damp of unstable modes

SuperKEKB Beam Instrmentation-1 mode dominantSuperKEKB Beam Instrmentation

Growth rate of (-1) mode
0.7
by=8mm(rate)
by=1mm(rate)

Growth Rate (1/ms)

0.6
0.5
0.4
0.3
0.2
0.1
00

100

200

300

400

Current(mA)

500

600700SuperKEKB Beam Instrmentation

During collision (by*=1mm, 670mA)

SuperKEKB Beam Instrmentation

Broadband noise in the FB system (KEKB time)nThe effect has first observed during the KEKB operation– Final amplifier in the 2GHz phase shifters (all HER and LER)
were modified during long maintenance time– the output power
had (significantly) increased. This caused the increase of thenoise level of the FB detectors (also caused the increase of thefeedback power).
– After the maintenance, the luminosity had decreased.
– Occasionally, one electrode of LER transverse feedback kicker
had failed--- I’ve turned off one final power amplifier andobserved the jumping up of the luminosity (to almost the originalluminosity level)!
– We’ve adjusted the level of the output of 2GHz phase shifterand lowered the V-FB gain.

SuperKEKB Beam Instrmentation

Noise effect study at KEKBnnnn

With the systematic study of the relations between the trans-verse
feedback gains and the luminosity, we have found only LER verticalfeedback gain affected the luminosity and the vertical beam size; othertransverse feedback gain, LER-H, HER-H and HER-V had no obvious
relation to the luminosity.
Though the vertical beam size slowly increased (~10%) with the feedback
gain during single-beam condition, it jumped up more than 40% with smallchange of the feedback gain during collision. The resulting luminosity
decreased around 10 to 20% with the blowup of the vertical beam size.
We have injected pure sinusoidal signals or band-limited white noises froma function generator to the V-FB system. The amplitudes of the excitedoscillation of the LER beam were detected with the bunch oscillation
recorders (BOR).
Reported at DIPAC2011 (MOPD73).

SuperKEKB Beam Instrmentation

Amplitude(% of vetical beam size)

Single frequency response (collision)250.58
0.60
0.62
0.65
0.68
0.72
0.75
0.55
0.56

2015
105000.5

11.5

2

Excite Voltatege(V )ppLarger response from tune (0.58) to 0.63 (tune + beam-beam tune shift)SuperKEKB Beam Instrmentation

Amplitude (% of beam size)

same excitation voltage
100.05V
0.1V
0.2V
0.3V
0.4V

864
2
00.5

0.55

0.6

0.65

0.7

Excited Veritical Tune

0.75

0.8

SuperKEKB Beam Instrmentation

Luminosity drop
0.55
0.56
0.58
0.60
0.62
0.65
0.68
0.72
0.75

21312.5

33

Luminosity(x10 /cm /s)

13.5

1211.51110.5
10
9.5

05

1015

20

Amplitude(% of vertical size)

25SuperKEKB Beam InstrmentationnBroadband noise in the feedback system could excite
the beam, especially near V-tune (V-tune + beam-beamtune shift)
Reducing the noise in the feedback system is essential.

nFor SuperKEKB

n– Use stripline-coupled filter to cut the lower frequency noise.
nBut cause large (>20dB) insertion loss (IL) in the system, need toamplify around 20dB to recover– main cause of the broadband noise.

– Use low noise amplifers (LNAs) to reduce noisen

Noise Figure (NF) at the head of the system is meaningful– goodLNAs after large IL might not be important, though.

SuperKEKB Beam Instrmentation

Current situationnn

Noise level from the FB detector is around 10 counts ofADC RMS (12 bit ADC)– with internal noise level of 1.1counts.
Sensitivity of the FB detector is roughly
2.0 counts/mA/um (HER upstream) to4.4 counts/mA/um (LER downstream)nn

Resulting RMS counts with zero current is around 10counts.
FB gain effect to the luminosity seems not so large now– Turning off one LER vertical loop showed small improvement ofthe luminositySuperKEKB Beam Instrmentation

SuperKEKB BxB detection system
Large insertionloss (~22dB)

SuperKEKB Beam Instrmentation

nReplacing 2GHz stripline-coupled comb filter with cabledelay type BPF (2GHz) could improve the noise figure
more than 20dB.
– Insertion loss (22dB) + Noise figure of Low noise amplifier (>1dB)

nCable-type BPF passes low frequency noise– the effectshould be checked by the beam.

SuperKEKB Beam Instrmentation

Summarynnnn

Noise injected to the HER BxB feedback system hascaused vertical beam blowup, beam loss, and luminosity
drop.
The source of the noise has been identified and removed.
Current HER vertical feedback gain seems to be enoughto suppress the CBI (mode -1).
For further reduction of the FB detector noise, we will
replace the first 2GHz comb filter with large insertion
loss to the cable-type bans pass filter, and check theresult with the beam (maybe around 2022a/b runs).

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backupSuperKEKB Beam InstrmentationMode analysis at Phase 1

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longer time after FB offSuperKEKB Beam Instrmentation

Vertical Mode100Wig OFFWig On

Growth Time Const(ms)8060402005020

5040

50605080

Mode Id

5100

5120

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Using iGp12 RMS

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ExampleniGp12 only (with offset cancel from int DAC)

1.1

nFinal DC amp only

2.5

With cable
Input terminate

14.1
14.1

n

W/O offset cancel DC
H183-4 input terminate

14.1
7.7

nW/O Final DC amp

6.0nnnn