nlc linac bpms

NLC - The Next Linear Collider Project

Next Linear ColliderBeam Position Monitors

Steve SmithSLAC

Interaction PointBeam Instrumentation Study

June 26, 2002

Author NameDate

Slide #

Next Linear Next Linear ColliderCollider

Steve Smith - 6/26/02

NLC Linac BPMs

• “Quad” BPM (QBPM) – In every quadrupole (Quantity ~3000)– Function: align quads to straight line– Measures average position of bunch train– Resolution required: 300 nm rms in a single shot

• Structure Position Monitor (SPM)– Measure phase and amplitude of HOMs in accelerating cavities– Minimize transverse wakefields– Align each RF structure to the beam– Few 104 devices in the two linacs– Not used in Interaction Region

• “Multi-Bunch” BPM (MBBPM) – Measure bunch-to-bunch transverse displacement – Compensate residual wakefields– Measure every bunch, 1.4 ns apart– Requires high bandwidth (300 MHz), high resolution (300 nm)– Line up entire bunch train by steering, compensating kickers

• Intra-Train Feedback BPM

Author NameDate

Slide #



QBPM Requirements

Parameter Value Conditions Resolution 300 nm rms @ 1010 e- single bunch

Position Stability 1 m over 24 hours (!)

Position Accuracy 200 m With respect to quad magnetic center

Position Range 2 mm

Charge Range 0.5109 to 7.5109 e- per bunch

Number of bunches

1 - 190

Bunch spacing 1.4 ns

Author NameDate

Slide #



• Dipole frequency: 11.424 GHz• Dipole mode: TM11• Coupling to waveguide: magnetic• Beam x-offset couple to “y” port

• Sensitivity: 1.6mV/nC/m (1.6109V/C/mm)

• Couple to dipole (TM11) only

• Does not couple to TM01

• Compact

• Low wakefields

Port to coax

BPM Cavitywith TM110 Couplers

Zenghai Li

Author NameDate

Slide #



Cavity BPM Parameters

Parameter Value Comments

Dipole frequency 11.4 GHz

Monopole frequency 7.66 GHz

Cavity Radius 16 mm

Wall Q ~4000 Ignoring beam duct, etc

Cavity coupling = 3

Loaded Q 1000

Bandwidth 11 MHz

Beam aperture radius 6 mm

Sensitivity 7 mV/nC/m (too much signal!)

Bunch charge 0.7 x 1010 e- Per bunch

Signal power @ 1m - 29 dBm Peak power

Decay time 28 ns

Required resolution = 200 nm

Required Noise Figure 57 dB For = 100 nm, thermal only

Wakefield Kick 0.3 volt/pC/mm Long range

Structure wakefield kick ~2 volt/pC/mm Per structure

Short-range wakefield ~1/200th of structure

Author NameDate

Slide #



Antenna Scan

Author NameDate

Slide #



Antenna Scan - Residuals

Author NameDate

Slide #



Multi-Bunch BPMs

• Stripline pickups

• Report position of every bunch in bunch train

• Used to program broadband kickers to straighten out bunch train

Parameter Value Conditions & Comments

Resolution 300 nm rms At 0.6 x 1010 e- / bunch

for bunch-bunch diplacement frequencies below 300 MHz

Position Range 2 mm

Bunch spacing 1.4 ns

Number of Bunches 1 - 190 @ 1.4 ns

Beam current dynamic range

1109 to 1.4 1010 Particles / bunch

Number of BPMs 278

Author NameDate

Slide #



ATF Bunch Current

Author NameDate

Slide #



Energy from BPMs

• Dispersion:– How energy deviation translates into transverse beam motion

x pp – Where is the dispersion ~ 20 – 80 mm

• BPM stability of ~ 1 m energy measurement of ~ 10-5

• But BPM measures mean position of charge of bunch

• Transverse tails?

• Energy tails?– 1% of charge off energy by 1% error of 10-4.

• Need luminosity weighted energy– Get charge-weighted energy

Author NameDate

Slide #



Luminosity from BPMs

• BPMs measure beam-beam deflection angle • Deflection angle depends on beam size, charge, beam offsets,

angles, bunch length,…

• But luminosity is given by x, y:

L = • Can be difficult to extract true L from deflection.

• Really need luminosity monitor that measures collision processes.

• Deflection measurement is crucial for optimization, feedback, spot size measurement, etc.

yx

beam

cm

D PN

E

H

4

2

Author NameDate

Slide #



Beam-Beam Deflection(electron microscope!)

Deflection coefficient

(for small offsets)

25 radian / nm At Interaction Point

Displacement 100 m / nm At Position Monitor

dq

e+

e-

Author NameDate

Slide #



Beam-Beam Deflection“Guinea Pig” simulation provided by A. Seryi

Initi

al S

lope

Slope at Large O

ffset

Author NameDate

Slide #



Beam-Beam Deflection Slope

40 m focal length!

Author NameDate

Slide #



Power of Deflection Measurement

• Beams see 40 m focal length lens (at < 1 y offset)

• Image distance 4 meters• Magnification of 4m/40 m = 105

• BPM resolution of 1 m – beam offset resolution of 1 m / 105 = 10 pm (!)

• Confounded by – Tails

– Angle jitter

– Rotation

• Dilution of 100 beam–beam offset resolution ~ 1nm

Author NameDate

Slide #



Beam-Beam Scans

• Important diagnostic– Spots sizes

– Alignment

– Waists

– Etc. etc. etc…

• Some Limitations:– Takes lots of time at one measurement per pulse train

– Sensitive to drifts over length of scan

• i.e. low frequency noise, ground motion, damping ring jitter.

• Can we do a scan in one bunch train?– Increase utility of diagnostic by increasing speed

– Reduce sensitivity to drift and low frequency noise

• Yes– Assuming existence of Intra-train IP feedback.

Author NameDate

Slide #



IP Intra-Train Feedback BPM

• Interaction Point Intra-Train Feedback BPM– IP only

– Much like Multibunch BPM

– Critical difference:

• Very short propagation delay

• Close loop in ~ 40 ns < 250 ns train passage time.

– Other differences:

• Half the bandwidth

• Don’t care about bunch-bunch motion

– feedback only makes bunch-bunch motion worse!

Author NameDate

Slide #



Intra-Train Feedback

• Vertical spot size comparable to ground motion

• Small beam size luminosity sensitive to beam jitter

• We can measure extremely small inter-beam offsets via beam-beam deflection

• Can we make a feedback fast enough to correct beam-beam mis-alignment within the 265 ns bunch-crossing time?

• System consists of a fast position monitor, kicker, and feedback regulator.

• Old-fashioned analog feedback.

Author NameDate

Slide #



Intra-Pulse Feedback

Amp

BPMProcessor

IP Round Trip Delay

+

Amp

14 ns

14 ns

Author NameDate

Slide #




• Fix interaction point jitter within the crossing time of a single bunch train (265 ns)

• BPM measures beam-beam deflection on outgoing beam– Fast (few ns rise time)

– Precise (micron resolution)

– Close (~4 meters from IP?)

• Kicker steers incoming beam– Close to IP (~4 meters)

– Close to BPM (minimal cable delay)

– Fast rise-time amplifier

• Feedback algorithm is complicated by:– round-trip propagation delay to interaction point in the feedback

loop

– Response non-linearity of beam-beam defelction for flat beams

Author NameDate

Slide #



Limits to Beam-Beam Feedback

• Must close loop fast– Propagation delays are painful

• Beam-Beam deflection is non-linear– Feedback gain drops like 1/dfor large offsets

– Feedback converges too slowly beyond ~ 30 to make a recover luminosity

• Can fix misalignments of 100 nm with modest kicker power

Author NameDate

Slide #




• Setup critical:– IT Feedback holds outgoing beam to a position setpoint

– Other hardware required to establish/maintain proper setpoint

– Multibunch BPM readout of IT feedback, symmetric pickups

• Higher bandwidth

• Better resolution

• Better stability

• (SLOW)

• Response of beam to FB drive contains diagnostic information– Need deflection vs. bunch digitized and logged

Author NameDate

Slide #



Single-Pulse Beam-Beam Scan

• Fast BPM and kicker needed for interaction point stabilization

• Open the loop and program kicker to sweep beam

• Digitize fast BPM analog output

• Acquire beam-beam deflection curve in a single machine pulse

• Eliminates inter-pulse jitter from the beam-beam scan.

• Noise is all at low frequency

• Use to

• Establish collisions

• Measure IP spot size

• Waist scans

• What else?

Author NameDate

Slide #



Intra-Pulse Feedback(with Beam-Beam Scan & Diagnostics)

Amp

BPMProcessor

IP Round Trip Delay

Digitizer

Beam-Beam Scan& Diagnostics

Ramp

+

Amp

Author NameDate

Slide #



Beam-Beam Scan

Beam bunches at IP: blue points

BPM analog response: green line

Author NameDate

Slide #



IP Beam Position Monitors

• Stabilize collisions

• Measure spot sizes

• Diagnostics

nlc linac bpms

Documents