OPTIONS FORTHE DESIGN OF A NEW PICK-UP

AND SCHEDULE

G. Kotzian, LIU-SPS High Bandwidth Damper Review, July 30, 2013

HBTFB - High Bandwidth Transverse Feedback

• Wideband feedback system (GHz bandwidth)• Intra-bunch GHz transverse feedback system• Help stabilize beam against Ecloud and TMCI effects• Under development with LARP

supported by: US-LARPCERN SPS LIU Project

AnalogFrontEnd

Analog BackEnd

SignalProcessing

BPM Kicker

Power AmpADC DAC

Beam Active closed loop GHz Feedback

transverse position

pre-processed sampledposition“slices”

calculatedcorrection data

correctionsignal

pre-distortion drive signal

OVERVIEW

A. Design Considerations

B. Strip-line Pick-up Options• Coupler type pick-ups• Current PU: Exponential coupler BPW(A)• Strip-line BPCL• Long Strip-line Option• Variation of strip-line for scrubbing beam optimization

B. Alternative PUs• Electromagnetic PU: Position Sensitive Wall Current Monitor• Exotic (i.e. electro-optic) PUs• Faltin-type (not treated today)

C. Schedule• On-going activities• Possible Roadmap

Design considerations

Option to split system into several bands to cover entire frequency range Centre frequencies of instabilities moving during acceleration Adequate adjustment of loop delay and for all bands (5 deg phase@1GHz approx. ~14 ps) Overlap of bands becomes delicate

Option for direct digitization with high bandwidth ADCs (GSPS) Direct representation of beam transverse motion Gain and phase equalization realizable using digitally implemented filters Loop delay adjustment follows acceleration

Analog BW: 10 MHz – 2 GHz• Lower end covered by classical damper (dipole mode, large injection oscillations require strong

damping)• Upper limit defined by Nyquist frequency for subsequent sampling (fs > 4 GSPS)

SPS 200 MHz RF System: 5ns bucket length• With 4 GSPS 20 slices/bucket• NB: observed signal is always position x intensity

holds information on longitudinal and transverse motion

Design considerations

SCOPE: PU design; requires taking into account properties of pre-processing chain (cables, filters, attenuators/amplifiers, orbit suppression signal processing)

GOAL: provide analog representation of beam transverse position for direct digitization with high bandwidth ADC (few GSPS)

Equalizer

BPM ADC

PassiveClosedOrbit

Suppression

7/8’’ transmission line

Equalizer7/8’’ transmission line

Delayadjustment

𝑉 𝑃𝑈 (𝑡 )=𝑍𝑇 𝐼𝑏 (𝑡 )∗ [𝛿 (𝑡 )−𝛿 (𝑡−𝜏 ) ]

Coupler type pick-ups

^

|ZT (w)|

w

…

ZT

Frequency DomainZT (w) = ZT j sin( /2wt ) e -jwt/2

^

f=1/(2t)

LBeam

t = 2 L/c

t

load or short

Use direct sampling and gating on single pulse: in time domain to effectively remove notch in frequency response:

notches in freq. response

PU output voltage, matched in 50 Ω direct representation

of the bunch profile

logarithmic scale: dBMax(1V/m), 70 dB range

… assuming

𝑉 𝐺 (𝑡 )=𝑉 𝑃𝑈 (𝑡 )∗𝑔 (𝑡 )=𝑍𝑇 𝐼𝑏 (𝑡 )|𝐼 𝑏=0 ,𝑡>𝜏 /2

Current PU: Exponential Stripline (BPWA)

(Courtesy W. Höfle, SPS Studies WG - August 5, 2008)

Four such couplers installed in SPS (four electrodes at 45 degrees)

… but: phase response not linear with frequency !

Developed for SPS by T. Linnecar, Reference: CERN-SPS-ARF-SPS/78/17

Special case: no notches in frequency response due to tapering of electrodes

Exponential PU – Beam response

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on

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f pic

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p [d

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vers

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gaussian bunch 4 = 1 ns, pickup l=0.375 m, a=2.48, LP 6 GHz, cos2 window

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p w

ith c

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le [d

ash

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vers

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]

gaussian bunch 4 = 1 ns, pickup l=0.375 m, a=2.48, LP 6 GHz, cos2 window

PU w/o cable (norm)PU w/o cable (norm, rev)bunch (norm)

PU with cable (norm)PU with cable (norm, rev)bunch (norm)

w/o coaxial transmission line with coaxial transmission line

Gaussian bunch

Ideal PU response

Reversed PU

PU response with dispersive and corrugated cable shows ringing in time domain

Ringing due to cable TF

Bunchlet in BPWA

2 short bunches,5ns spaced

PU responsew/o cables Reversed PU

PU response with dispersive and corrugated cable

Observation: • BWPA length of 375 mm was chosen to just separate two successive bunches• no mixing when bunches are split; however not so evident when closer• May need another iteration taking into account the implemented or an improved phase compensation

K. Pollock, Signal Equalizer for SPS ECloud/TMCI Instability Feedback Control Systemhttp://ibic12.kek.jp/prepress/papers/tupa32.pdf

(Courtesy W. Höfle)

Exponential strip-line BPW (A/B)

Advantages• mechanically short L = 375 mm• no notches in frequency response due to tapering of electrodes• 4 electrodes, can be wired horizontally or vertically

Limitations• Vacuum chamber cut-off frequency for TE11 mode at 1.64 GHz (D=107mm, BPWA/BPWB)

resp. 1.134 GHz (D=155mm, BPW)

• phase response not linear, thus group delay frequency dependent; need to compensate for PU response and cable dispersion

• impedance variation & matching was difficult to control in the design (T. Linnecar) production of matching not perfect

• 1 of the 4 existing devices may be damaged, will be inspected during LS1

For MDs and demonstrator system OK; fully functional system: what are better options?

Name Position Description Diameter/TE11 cut-off

BPW 317.98 horizontal PU, 3x H-183; for beam observation in CCR 155 mm / 1.134 GHz

BPWA 319.01 vertical PU (reversed), used as vertical kicker (RF injected in downstream end) 107 mm / 1.64 GHz

BPWA 319.31 vertical PU, 3x H-183, 2x 6dB attn. at each coupling port; for beam observation in CCR, may be damaged inspection during LS1

107 mm / 1.64 GHz

BPWB 321.01 vertical PU, 6 dB/12W attn. at each coupling port; H-183 hybrids replaced by two resistive combiners (2008-10-12)

107 mm / 1.64 GHz

Strip-line BPCL

Existing device: SPS BPCL• L = 600 mm, two planes• 50 Ω downstream termination• Flat response up to ~3 GHz

SPS head-tail application: • Performs a 2-dim Wiener de-convolution• re-aligns the initial signal and successive reflection

Advantages:• Existing devices• possible for bunch lengths up to max. 4ns

Limitations:• 600 mm electrode length not sufficient for max. bunch

length of 5ns (• higher order cut-off frequency in PU body fc=1.32 GHz

(problematic? possible damping with ferrites?)

R. Steinhagen

Courtesy: R. Steinhagen

Courtesy: R. Steinhagen

Long strip-line Option

Optimal solution: separate positive and negative output pulses • Required minimum length • Reserved location: 319.31 (existing BPWA to 311.01 during LS1)• Available length: 1426 mm (see H. Bartosik)

Assuming a beam tube inner radius of (:• TE11 cut-off frequency

The stay-clear half-aperture for the pick-up is .

For an electrode thickness of • height of strip-line gap

For a strip-line design • strip-line width

Find trade-off between TE11 cut-off (i.e. ) and expected signal level ()

Strip-line with this length probably needs intermediate supports. Details are subject to future studies.

calculations based on: J-P. Papis, L. Vos, CERN SL/91-g (BI)

80 90 100 110 120 130 140 1501

1.2

1.4

1.6

1.8

2

2.2X: 83Y: 2.119

Beamtube inner diameter / mm

TE

11

cu

t-o

ff / G

Hz

Geometry dependent TE11 cut-off in circular waveguide

X: 93Y: 1.891

X: 107Y: 1.643

X: 133Y: 1.322

BPWA

BPCL

stay-clear aperture limit

Long strip-line – Option II

Due to pattern of the bunchlet beam (bunch spacing is here 5ns!):

signal of the second bunch will overlap with residual pulse (if )• In order to resolve doublet scrubbing beam requires strip-line with delay• Required minimum length • Too long for 319.31 (1426 mm), needs also 319.32 to be freed up

(current plan for LS2: BPWA 319.01 319.32)• If full length available, i.e. 2286 mm leaves comfortable margin for PU, flanges, bellows, etc.

Constraints for coupler-type PUs: • The strip-line length must be longer than the bunch length

in order to have no overlap between the bipolar pulses for 5ns bunches for 10ns doublets

• Gap between successive bunches (or bunchlets) necessary, otherwise cancelation of beam signal with residual reflection

gap length ≥ strip-line delay

Remark: if PU length not sufficient for bunchlet beam then one could think of using the negative pulse of the second bunch which – in the ideal case – is unperturbed and with a negative gate (time alignment of pulse w.r.t. to first bunch required).

Variation of strip-line for scrubbing beam optimization

Challenge in direct representation of bunch profiles using coupler-type PUs: the residual reflection spoils the response. exponential coupler uses tapered electrodes long strip-line with delayed reflection

Quest: Absorb the residual reflection in bulk material

Studies on-going …

5.002.001.000.500.200.00

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8090100110

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Strip2LoadSmith Chart 1Curve Info

St(strip_T1,strip_T1)Setup1 : DC_5GHz

0.00 1.00 2.00 3.00 4.00 5.00Freq [GHz]

-55.00

-45.00

-35.00

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dB

(St(

stri

p_

T1

,str

ip_

T1

))

Strip2LoadTerminal S Parameter

Curve Info

dB(St(strip_T1,strip_T1))Setup1 : DC_5GHz

localized load materialwith adjusted bulk conductivity

Position Sensitive Wall-Current Monitor Option

BPM based on M. Gasior designAB-Note-2003-082-BDI: A proposal for an Inductive Pick-Up for Measuring the Position and Current of Proton Beams in the Transfer Lines between the Linac 2 and the PSB

• adapted from and based on an inductive pick-up (IPU), developed for position and current measurement in CTF3

• Bandwidth starting from 100 kHz up >1 GHz should be feasible

• 1-10 Ohm loading, differential L~70 nH

taken from: M. Gasior, AB-Note-2003-082-BDI

Provided byR. Steinhagen

100 kHz 1 GHz

Scale this version to fit to the SPS vacuum chamber

EO-PBM – Electro-Optical Pick-Up

• Working principle similar to LCD/TFT screen: particle beam modulates crystal birefringence (double refracting) → intensity of two laser beams A & B, position ~ (A-B)/(A+B)

• Pro: very wide-band signal, no beam power issues, true DC response (alt. AGM?)• Only lab tests for the full assembly until now• Hope to have a in-vacuum prototype ready for post-LS1 → to be published

SPS-LSS4.421SPS-ECA4

P A

P A

p-beam

532/1550 nmLaser

MM or SM (not matched)

OM4 (matched)

DAQ: Scope &Multiband-Inst._Mon.

Σ & Δ

tunnel

EO-Hybrid Analog FEx2

loca

lly s

tab

ilise

dΔ

T<

0.1

-1°C

(Courtesy R. Steinhagen)

(Courtesy R. Steinhagen)

On-going activities

Pick-up design• 3D-EM simulations and documentation of long strip-line design• Verification using HFSS and CST Microwave/Particle Studio• Evaluation of “terminated” strip-line performance (also thermal simulation including losses)• Optimization for length reduction/tapering of electrodes

Closed Orbit Suppression suppression of common mode signal to avoid amplifier saturation and good usage of dynamic range (digitization)

• Fact-finding: programmable step attenuators with required resolution 0.1 dB/step• Preparation of hardware & firmware development (for use with VME form factor)

Coaxial transmission lines• New smooth-wall coaxial transmission lines installed during LS1(lower dispersion than corrugated

cables)• Characterisation measurement method utilizes “synthetic TDR” via commercial VNAs (high sensitivity)• Equalize cable length (equal pairs of same length for pick-up A and B outputs)

Possible Roadmap

New Pick-up design• Finalize EM-design until end 2013 + down selection which PU to be realized• Produce design drawings in 2014-2015• Prototyping and lab testing 2015-2016,

to be ready for installation at the latest in LS2

Closed orbit suppression• Start HW development after fall 2013• Possible synergies with other active projects, e.g. damper HW upgrade project• Finalize HW and firmware in 2014• Ready for tests with beam in the SPS after LS1

Coaxial transmission lines• May require new analog compensation filters• Adapt analog front-end for optimum signal levels to fast ADCs

THANK YOU FOR YOUR ATTENTION!

Button electrodes - Variant of electrostatic electrodes

taken from: Robert E. Shafer, BEAM POSITION MONITORING

Button electrode response to a single Gaussian beam bunch:

signal current flowing onto (behind) the electrode

charge on the inside surface of the electrodes of length and azimuthal width

beam velocity

current of a centred pencil beam

voltage onto termination R

termination resistance

for circular electrodes with small capacitance C, the factor should be set equal to the electrode area divided by the beam-pipe half aperture .

bipolar doublet, occurs at about

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nal a

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itude

(no

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/ V