Bunch by bunch feedback systems for KEKB

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Bunch by bunch feedback syste ms for KEKB Makoto Tobiyama KEK Accelerator Labo ratory

description

Bunch by bunch feedback systems for KEKB. Makoto Tobiyama KEK Accelerator Laboratory. KEKB Two rings with L=3km, f RF =508.8MHz, h=5120 HER:electron 8GeV 1.3A LER:positron 3.5GeV 1.7A. Requirements for bunch by bunch feedback systems. Feedback system should work - PowerPoint PPT Presentation

Transcript of Bunch by bunch feedback systems for KEKB

Page 1: Bunch by bunch feedback systems for KEKB

Bunch by bunch feedback systems for KEKB

Makoto Tobiyama

KEK Accelerator Laboratory

Page 2: Bunch by bunch feedback systems for KEKB

KEKB

Two rings with L=3km, fRF=508.8MHz, h=5120

HER:electron 8GeV 1.3A

LER:positron 3.5GeV 1.7A

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Requirements for bunch by bunch feedback systems

Feedback system should work • both horizontal and vertical

• Longitudinal– LER(optional)

• minimum bunch spacing of 2ns

• various modes coming from unknown sources• Resistive wall (HER, LER)

• Photo electron instability (LER)

• Fast ion instability (HER)

Feedback damping time required• 1ms (100 turns) for transverse

• Transverse radiation damping time:41ms

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Page 5: Bunch by bunch feedback systems for KEKB

0 0.5 1 1.5 2 2.5 3 3.5 4-80

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T ime(ns)

0 0.5 1 1.5 2 2.5 3 3.5 4-4

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T ime(ns)

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180 180

AR 250A250 250W amp.

40cm wideband kicker

sumsum

0iso

0

180

0

180

0

sumsumTune X excite

509MHz

DC OFFSET

Vector1

4xRF750MHz

750MHz

BPM(Downstream)

iso

2 Tap FIR Filter

4xRF

BPM(Upstream)

From verticaldetector

Beam

Vector2

KEKB Transverse Bunch Feedback System

DC OFFSET

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Page 8: Bunch by bunch feedback systems for KEKB

Signal processing

Hardware 2-tap FIR filter :simplest digital filter • DC rejection

• 90-degree phase shift (for longitudinal plane)

• One-turn delay adjustment

Kick=(A-B)

B

B A

Kick

A

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PECL-ECL4bit

4bit

8bit/254MSPS

ECL-PECL

4bit

4bit

SubtractMemory(SRAM)

16bit

16bit 8bit8bit

Data ShiftLogic

LvTTL

A-F(8bit)

A-0(8bit)

SYNC

FMUX

FMUXFDMUX

FDMUX

A-F

A-0

B-F

A-0

A-F

FDMUX

4bit

4bit

SYNC

Precise Timing

Generator

SLCK/SYNCRead/Write

Address/Interface FPGA

VME Interface

SubtractData ShiftLogic

Memory(SRAM)

B-F(8bit)B-0(8bit) FMUX

FMUXFDMUX

B-F

EX data ADR CONT

B-0

B-0

DACTQ6122-MSYNC

MAX101

FADC

254MHz

8bit/254MSPS

8bit/254MSPS

508.9MHz RF

254MHz

Feedback Signal

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Digital filter system A/D(MAX101, 8bit) works with ring RF clock=509MHz. Demultiplex the output of A/D down to manageable frequ

ency: 509MHz/32=15MHz. Write the data to two ring memories (M1 and M2) simulta

neously, and read two different data (M1 and M2), make subtract (M1-M2) within 1/15MHz(67ns): two actions /cycle.• Address counter: one for data-write address and two full-ad

der (subtracter) to get two data-read address Multiplex the calculated result up to 509MHz/2 (D/A:TQ61

22-M has internal-multiplexer).

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Page 12: Bunch by bunch feedback systems for KEKB

Custom LSIs

0.5 gated GaAs DC FET logic, 136pins QFP Synchronizing circuit between chips FDMUX

• Demultiplex 4bit signal(PECL) into 16ch x 4bit(LvTTL) signal

• 600MHz(max),1.5k gates

FMUX• Multiplex 16ch x 4bit(LvTTL) signal to 4bit PECL signal

• 600MHz(max),1.7k gates

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Application to related systems

Memory board (20MB)

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Bunch current monitor

•Injection trigger stops the data-taking of the BCM

•BCM makes interrupts to VME-bus

•Data transfer from BCM to VME CPU

•Convert the data to bunch current, write the information to reflective memory and EPICS record

•Reflective memory makes interrupts to bucket selection code

Trigger to Ref. memory ~1.4ms

Maximum inj: 50Hz(20ms)

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Bunch oscillation recorder

Post-mortem analysis of beam abort• Beam-loss trigger from

DCCT

• Automatic data transfer Transient-domain analysis of

instabilities• Dominant mode of the

instability

• Clear (linear) behavior with small amplitude of oscillation

Precise oscillation measurement with long-time data accumulation• Measurement of sideband

of PEI during collision

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Performance of the system

Transverse feedback damping time at high current:~0.2ms (20 turns)• Without feedback systems, we observe strong

transverse instabilities starting from 20~40mA

• With feedback system on (both horizontal and vertical planes), we can inject single beam up to maximum current.

Very flat filling pattern for good luminosity Understanding the new instabilities

• Photo Electron Instability, Fast Ion Instability

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Lessons learned with the operation of transverse feedback systems

Problem: A/D easily saturates with the change of residual offset of the detection system• Continuous Closed Orbit Correction• Feedback of reference RF phase• Gain optimization before/after the digital filter• Need to monitor the “real” offset observed by the digital filter

Problem: Difficulty to find the best analog vector sum of the detection system• Fine tuning with minimum feedback gain• The best position are affected by the dynamic beta effect• Desirable to use only one pickup and tune the phase in the digital filter

Problem: Not easy to find the trouble in the filter• Need self checking system

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Specific Luminosity vs FB Gain

+3 -30 +1.5

+3+3

+3

-2 -1

FB gain of the LER vertical affects the specific luminosity. The other gains (LER H, HER H/V) bring no effects.

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Luminosity depends on LER-V feedback gain

Speculated sources• Feedback gain is too high??

• Maximum stable damping rate of the system from simple simulation shows less than 10 turns

• Reactive component of the feedback kick affect the luminosity??• No obvious tune-shift due to feedback gain is observed

• Residual noise blowups the vertical size??• Beam-beam simulation shows only a few percent of blowup

of the vertical beam size reduces the luminosity greatly.

Might be showing the limitations of two-tap FIR filter system

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Summary

Bunch by bunch feedback systems for KEKB are working very well and contributing to improving in ring operation and physics runs.

Effect of the feedback systems to luminosity might be coming from the limitations of hardware two-tap FIR filter system.• Need new generation digital filter system:

• Multi-tap, non-downsamping digital filter

• Gboard is under development with the collaboration of KEK, SLAC and INFN