Beam-based Measurements of HOMs in the HTCAdam Bartnik for ERL Team,

Daniel Hall, John Dobbins, Mike Billing, Matthias Liepe, Ivan Bazarov

Summary

• What I will talk about– Introduction– Our experiment– Raw data

• What I won’t talk about– Detailed analysis of the data (stay tuned…)

New Injector Layout: HTC

From ICMTo Dump

Beam goes this way

Higher order modes

HOMs excited by wake fields

First bunch enters cavity

Bunch excites fields

Future bunches receive kick

Beam breakup in an ERL1. Bunches enter off-axis2. HOM excited3. Bunch gets kicked4. Returns to cavity further off-axis5. Excites larger HOM field6. Next bunch gets bigger kick7. …8. BOOM!

• Instability occurs above a threshold current

Beam breakup in an ERL

• Beam breakup limits currents in an ERL– J-Lab ERL limited to <30 mA by beam breakup

(simulation)– Cornell needs > 100 mA– HTC designed carefully with this in mind

• Question: how can we estimate the threshold current without building an ERL?

Simulations / Merit function

• Simulation– Cavities with given mode ( R/Q , Q, f, …)– Realistic lattice– Add slight randomness to HOM properties– Find some merit function that correlates linearly

with threshold current:• (R/Q)Q/f … (R/Q) Q1/2/f… (??) …

– Measure merit function in real cavity

Goal: Characterize HOMs

• Questions to answer– f– Q– R/Q– Dipole, quadrupole, etc.?

• Measuring R/Q requires a beam-based method

Beam-based method

1. Drive a mode in the cavity

2. Monitor BPM position downstream

3. Turn off driving force

4. Monitor decay of beam’s oscillation– Amplitude = R/Q– Decay constant = Q

5. Position dependence in cavity = monopole, dipole, etc.

Exciting the HOM

• Modulate the bunch charge at frequency fmod

– sidebands:

Time

Beam

cur

rent

Exciting the HOM• Charge modulation via laser modulation

• 1.3 GHz laser– Good: Up to 75 mA current– Good: Easy to search sidebands– Bad: Need to search 0-650 MHz– Prohibitive: Cannot modulate high power laser that fast

• 50 MHz laser– Bad: Only 2 mA– Bad: Laborious to find the sideband exciting the mode– Good: Only search from 0-25 MHz– Good: Can directly modulate the (final) laser beam

Monitor BPM Position

Last easily accessible BPM

3.4 meters drift

Monitor BPM Position

• Spectrum analyzer in zero span mode

• Baseband (0-25 MHz) has poor BPM response and background noise

• Use sideband around higher harmonic of 50 Mhz– 1.3 GHz is convenient, but also has larger background– 1.3 GHz – 50 Mhz = 1.25 GHz was used instead

Monitor BPM Position

1.3 GHz

50 MHz

SpectrumAnalyzer

1.25 GHz

(1.25 GHz – f)

f

BPM signal

PulseGenerator

trigger

Switch Laser

Cathode

BPM180o hybrid

Expected signal

Bunch charge:BP

M S

igna

lBP

M S

igna

l

On resonance:

Otherwise:

Position and amplitude modulation

Only position modulation,• Decay gives Q• Peak amplitude gives R/Q

Scanning details

• Is this no mode, or just a really big/small Q?– Scan with multiple scan lengths / SA bandwidths

• What frequencies do we choose?– Scan takes ~20 seconds– 25 MHz / (40 hours / 20 s) = 3.5 kHz– Eventually settled on 10 kHz steps to speed things up

BPM

Sig

nal

What can we find?

• Small width modes will be missed

• 10 KHz steps– Dfmin ~ 1 kHz

– Qmax ~ 107

• SA bandwidth– Smaller = better noise floor– Larger = faster response (can see smaller Q)– Choose target Q, set bandwidth accordingly

What can we find?

• SA noise floor: P ~ -100 dBm

•

• Noise floor ~ 5 mm @ 2 mA

Machine setting

• No quads• Short bunch length• Position feedback

Charge 0-77 pC

Energy 4.9 MeV

Bunch spacing 20 ns (CW)

Bunch length 2 ps (rms) @ 77 pC

Beam width 3 mm (rms) @ 77 pC

(from simulation)

25 MHz modulator performance

• Laser pulse measurements

• Only 50% modulation depth at 25 MHz

Example data

• Fit to exponential• Get (Q/f) , Df, BPM deflection

Broad scan

• 10 kHz steps• SA bandwidth: 100 kHz (red), 1 MHz (blue)• Found lots of peaks!

Two cavities!

• The beam also passed through the ICM

• Repeat with BPM before the HTC

Last BPM before HTC

Almost all peaks from ICM

BPM after the HTC BPM before the HTC

Modes actually in HTC

One of the peaks in this group

These peaks

Fine scans

• Find peak frequency• Double check expected peak width

Q/f = 3.3x107 GHz-1Q/f = 4.9x106 GHz-1

Position dependence

• On resonance

• Displace beam vertically or horizontally

• Use vertical or horizontal BPM downstream

Example position dependence

Finding the true frequency

fmod = 5.518 MHz, fcenter = 2.5 GHz

• Monitor RF probe on 2nd SA• Vary fmod, record peak height on 2nd SA

• 1.3GHz + n(50 MHz) ± fmodfmod = 10.03 MHz, fcenter = 2.3 GHz

Summary of data taken

• Broad scans, 0.5-25 MHz, 10 KHz steps– Horz off-axis, horz BPM– Vert off-axis, vert BPM

• Traces before HTC at each peak (ICM)• Fine scans around each HTC mode• Position dependence– All combinations (vert/horz off-axis, vert/horz BPM)

• 2nd SA frequency scans for each HTC mode

Summary of resultsfmod

(Mhz)f

(GHz)Q Df

(KHz)BPM Peak

(mm)

3.35976 2.303 8.3x106 0.28 0.22

5.112188 2.495 1.1x107 0.22 1.7

5.269968 1.295 4.3x107 0.030 0.62

5.51796 2.494 2.0x107 0.12 2.2

8.8600 ? 1.5x104

(GHz-1)70 0.025

10.0306 2.290 1.3x107 0.18 0.82

10.6998 2.489 1.7x106 1.5 0.16

24.3003 2.476 4.7x106 0.12 0.13