August, 2003 W.S. Graves 1 Electron beam diagnostic methods William S. Graves MIT-Bates Laboratory Presented at 2003 ICFA S2E Workshop DESY-Zeuthen August,

August, 2003 W.S. Graves 1

Electron beam diagnostic methodsElectron beam diagnostic methods

William S. Graves

MIT-Bates Laboratory

Presented at 2003 ICFA S2E Workshop

DESY-Zeuthen

August, 2003


Experimental MethodsExperimental Methods

•Hardware/software controls.

•Thermal emittance measurement using solenoid scan.

•Cross-correlation of UV photoinjector drive laser with 100 fs IR oscillator.

•Streak camera time resolution

•Electron beam longitudinal distribution measured using RF zero phasing

method.

•Slice transverse parameters are measured by combining RF zero phase with

quadrupole scan.

•Will not address undulator diagnostics. See Shaftan, Loos, Doyuran.

Enables injector optimization and code benchmarking.


DUVFEL Facility at BNLDUVFEL Facility at BNL

1.6 cell gun with copper cathode

70 MeVBend

50 m

5 MeV

Bend

DumpDump

Coherent IRdiagnostics

RF zero phase screen

Undulators Linac tanks

Bunch compressorwith post accel.

30 mJ, 100 fsTi:Sapphire laser

NISUS 10mundulator

Photoinjector: 1.6 cell BNL/SLAC/UCLA with copper cathode

4 SLAC s-band 3 m linac sections

Bunch compressor between L2 and L3

Approximately 60 CCD cameras on YAG screens.

Triplet Triplet70-200 MeV


Control systemControl system

Automated measurements very important for gathering large amounts of data and repeating studies.

All sophisticated control is done in the MATLAB environment on a PC. Physicists quickly integrate hardware control with data analysis.

Low level control is EPICS on a SUN and VME crates.


Thermal EmittanceThermal Emittance

xxxN xx 22

Use Lawson’s expression for the RM S width of them om entum distribution of a therm alized beam

factor.t enhancemen field theis and phase, andamplitude RF theare sin and 95MV/m

eV, 59.4eV, 67.4 ,4

,sin

0

rf

rfrf

Cu

rfrfrfCuk

E

he

EhE

2N

2

mc

Emc

E

kx

kx

The norm alized em ittance is

(1)

(2)

(3)


Projected emittance vs charge and FWHMProjected emittance vs charge and FWHM

0 2 4 6 8 100.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74

FWHM (ps)

Em

ittan

ce (

mm

-mra

d)

0 5 10 15 200.61

0.62

0.63

0.64

0.65

0.66

0.67

0.68

Charge (pC)

Em

ittan

ce (

mm

-mra

d)Charge 2 pC

Energy 3.7 MeV

Laser spot 0.5 mm RMS

FWHM 3 ps

Energy 3.7 MeV

Laser spot 0.5 mm RMS

HOMDYN simulations estimate limits on maximum bunch length and charge. Choose working parameters of 2 pC, 2 ps FWHM.

Simulation Simulation

YAG:Ce screen very useful for low charge, high resolution profiles. Screen thickness, surface quality, multiple reflections, and camera lens depth-of-focus and resolution are all important issues.


Solenoid Scan LayoutSolenoid Scan Layout

CCD Camera

YAG screen

Mirror

Solenoid

1.6 cell photoinjectorDipole trim

1cm

65 cm

12 cm33 cm

Can measure•charge•energy•x and y centroid•x and y beamsize•px and py

Telecentric lens magnif. = 1

•YAG:Ce screen very useful for low charge, high resolution profiles.

• Screen thickness, surface quality, multiple reflections, and camera lens depth-of-focus and resolution are all important issues. See SLAC GTF data.


Low Energy Beam MeasurementsLow Energy Beam Measurements

102 103 104 105 106 107 1080

1

2

3

4

5

6

7

8x 10-9

Solenoid Current (Amp)

11

File: a10pc.txt = -30.8 1.7 = 8.11 0.44 mN

= 0.605 0.025 mm-mrad

= 0.773 0.0177 mm' = 2.94 0.0658 mradE = 3.69 MeV

pop02a.bmp

5 10 15 20 25 30

5

10

15

20

25

30

0 50 100 150 200 250 3000

500

1000

1500

2000Hor. RMS width = 39.5 um

Beam size (um)

Inte

nsity

(A

.U.)

pixels

Video processing

•3x3 median filter applied.

•Dark current image subtracted.

•Pixels < few % of peak are zeroed.

Error estimates

Monte Carlo method using measured beam size jitter.


Emittance vs laser sizeEmittance vs laser size

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

1

1.2

Horizontal RMS laser size (mm)

N (m

m-m

rad)

Horizontal

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

1

1.2

N (m

m-m

rad)

Vertical

Horizontal RMS laser size (mm)

FWHM 2.6 ps

Charge 2.0 pC

Gradient 85 MV/m

RF phase 30 degrees

Emittance shows expected linear dependence on spot size.

Small asymmetry is always present.

][93.16.0][ mmmradmm xx ][92.11.0][ mmmradmm yy

eV 0.43

2

N2energy kinetic Averagex

k d

dmcE


Beam size and divergence vs laser spot sizeBeam size and divergence vs laser spot size

Error bars are measured data.

Blue lines are from HOMDYN simulation using RF fields from SUPERFISH model and measured solenoid B-field.

Upper blue line has 1/2 cell field 10% higher than full cell.

Lower blue line has 1/2 cell field 10% lower than full cell.

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

RMS laser hor. size (mm)

RM

S e

-bea

m h

or. s

ize

(mm

)

0 0.2 0.4 0.6 0.8 1 1.20

1

2

3

4

5

6

7

8

9

RMS laser hor. size (mm)

RM

S e

beam

hor

. div

erge

nce

(mra

d)

High

High

Low

LowSize Divergence

][1.112.0][ 01 mmmm xx ][5.39.0][' 0 mmmrad xx


Error bars are measured data points.

Curve is nonlinear least squares fit with βrf and Φcu as parameters: βrf = 3.10 +/- 0.49 and Φcu = 4.73 +/- 0.04 eV.

The fit provides a second estimate of the electron kinetic energy Ek = 0.40 eV, in close agreement with the estimate from the radial dependence of emittance.

0 10 20 30 40 50 60 70 800.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

RF phase (degrees)

N (

mm

-mra

d)

Emittance vs RF phaseEmittance vs RF phase

2N mc

Ekx

04

,sin

e

EhE rfrfrfCuk


-10-8

-6-4

-20 2

-40-30

-20-10

010

2030

400

0.5

1

1.5

2

Time (ps)

Phase matching

angle (mrad)

Sig

nal (V

)

Time profile of UV laser pulseTime profile of UV laser pulse

Phase matching angle of harmonic generation crystals used to produce UV affects time and spatial modulations.

Cross-correlaton difference frequency generation – experiment by B. Sheehy and H. Loos

100 fs IR

5 ps UV

BBO crystal

200 fs blue

Power meter

Note: “Sub-ps” streak camera is

inadequate for this measurement


Above: Laser cathode image of air force mask in laser room.

Below: Resulting electron beam at pop 2.

Above: Laser cathode image with mask removed showing smooth profile.

Below: Resulting electron beam showing hot spot of emission.

Laser masking of cathode image


Streak CameraStreak Camera

100 200 300 400 500

50

100

150

200

250

300

350

400

450

500

Streak image

Time

•Hammamatsu FESCA 500

•765 fs FWHM measured resolution

•Reflective input optics (200-1600 nm)

•Wide response cathode (200-900 nm)

•Optical trigger (<500 fs jitter)

•Designed for synchroscan use. Also good single-shot resolution.

•6 time ranges: 50 ps - 6 ns

50 ps window

Very helpful for commissioning activies and for timing several optical signals.

Limited time resolution below 1 ps.


Streak camera time profiles of laser pulsesStreak camera time profiles of laser pulses

14 16 18 20 22 24 26160

180

200

220

240

260

280

300

320

340

360

sign

al (

arb

units

)

streak delay (picoseconds)32 34 36 38

180

185

190

195

200

205

210

215

220

sign

al (

arb

units

)

streak delay (picoseconds)

6 8 10196198200202204206208210212214216218220222224226228230232234

sign

al (

arb

units

)

streak delay (picoseconds)

Oscillator 796 nm

FWHM 765 fs

Amplified IR 796 nm

FWHM 1.01 ps single shot

UV 266 nm

FWHM 2.40 ps singleshot

Time resolution depends on photon energy: energetic UV photons create photoelectrons with energy spread that degrades time resolution


L1

L2

L3

L4

RF zero-phase time profile

L3 phase = +90, amp. set to

remove chirp

L4 phase = +/-90, amp. set to

add known chirp

L2 phase varies, amp.

fixed

L1 phase = 0, amp. fixed

Chicane varies from 0 cm < R56 <

10.5 cm

100 200 300 400 500 600

50

100

150

200

250

300

350

400

450

100 200 300 400 500 600

50

100

150

200

250

300

350

400

450

100 200 300 400 500 600

50

100

150

200

250

300

350

400

450

L4 phase = -90 degrees L4 phase = +90 degreesL3 corrects residual chirp,

L4 is off

Pop 14 YAG screen

YAG images at pop 14


Quad scan during RF zero phaseQuad scan during RF zero phase

Movie of slice emittance measurement.


Right side

Circle is matched, normalized phase space area at upstream location.

Ellipse is phase space area of slice at same location.

Straight lines are error bars of data points projected to same location.

Left side

Beam size squared vs quadrupole strength.

Each plot is a different time slice of beam.

Software is used to time-slice beam.

Collaboration with Dowell, Emma, Limborg, Piot


Slice emittance and Twiss parametersSlice emittance and Twiss parameters

Note strongly divergent beam due to solenoid overfocusing at tail, where current is low.

Space charge forces near cathode caused very different betatron phase advances for different parts of beam.

is parameter that characterizes mismatch between target and each slice.

= ½ (0 – 2 0 + 0)

0, 0, 0 are target Twiss param.

, , are slice Twiss param.

Beam Parameters: 200 pC, 75 MeV, 400 fs slice width


Different slices require different solenoid strengthDifferent slices require different solenoid strength

HeadTail HeadTail HeadTail

HeadTail HeadTail HeadTail

Increasing solenoid current

Vertical dynamicsLattice is set to image end of Tank 2 to RF-zero phasing YAG.

Particles in tail of beam are diverging, and in head converging.

Head has higher current and so reaches waist at higher solenoid setting.

Current projection

Time


Solenoid = 98 A

Solenoid = 108 A

Solenoid = 104 A

Slice emittance vs solenoid strength. Charge = 200 pC.

Solenoid

Eyn

Alpha

Beta

98 A

3.7 um (3.2)

0.4 (1.0)

1.3 m (1.3)

104 A

2.1 um (2.8)

-6.9 (-3.6)

9.8 m (6.8)

108 A

2.7 um (2.7)

-9.0 (-9.6)

45 m (36)

Projected Values

(parmela in parentheses)

Data

Parmela


Slice parameters vs chargeSlice parameters vs charge10 pC

100 pC 200 pC

50 pC

Low charge cases show low slice emittance and little phase space twist.

High charge cases demonstrate both slice emittance growth and phase space distortion.


Longitudinal structureLongitudinal structure

100 200 300 400 500 600

100

200

300

400

50 100 150 200 250 300

50

100

150

0 2 4 60

50

100

150

200

Time (ps)

Cur

rent

(A

)

File: csr01, FWHM = 2.2 ps

Analysis of RF zero phasing data can be complicated by modulations in energy plane.

See contribution from T. Shaftan for detailed description.


RF zero phasing

•Method uses accelerating mode to “streak” the beam by increasing the energy spread (chirping).

•Uncorrelated energy spread is ~5 keV and coherent modulations can be ~20 keV. Streak chirp must be much larger than this.

•Time and energy axes are difficult to disentangle.

RF deflector cavity

•Transverse momentum is ~ 1 eV/c. Relatively small deflecting field gives excellent time resolution.

•Less sensitive to coherent energy modulations.

•Can obtain simultaneous time/energy/transverse beam properties when combined with dipole in other plane.

RF zero phasing vs RF deflectorsRF zero phasing vs RF deflectors


Concluding RemarksConcluding Remarks

•Experience seems to indicate that most differences between experiment and simulation are due to experimental inaccuracies.

•Beam can be used to diagnose many hardware/applied field difficulties.

•“Easy to use” control system integrating realtime analysis and hardware/beam control very important.

•With adequate diagnostics, meeting beam quality and FEL performance goals is straightforward.

•See work by Shaftan, Loos, Doyuran of BNL on undulator diagnostics and trajectory analysis.

August, 2003 W.S. Graves 1 Electron beam diagnostic methods William S. Graves MIT-Bates Laboratory Presented at 2003 ICFA S2E Workshop DESY-Zeuthen August,

Documents

cell field

cell gun

low charge

cell bnlslacucla

measured data points

measured beam size jitter

emittance vs laser sizefwhm2

rf fields