SPPS / Laser pump-probe synchronisationmeasurements using a streak camera

SLAC July 28th 2004

Andrew G. MacPhee

Dana Weinstein, Roger Falcone (UCB)Mike Greaves (UCB), Jun Feng, Katia Opachich, Howard Padmore (LBL), Zenghu Chang (Kansas)

• Fast X-ray streak camera program at UC Berkeley / LBL.

• Jitter measurements made at SPPS in January and May with the new camera

• Improvement for Mk. III

Scope of talk

• The secondary electrons emitted from a photocathode have a finite energy distribution, leading to:

– Dispersion of the electrons during transit from photocathode to sweep plates:- degrades temporal resolution.

– Chromatic aberration.

• For high intensity signals, space charge spreads the electron bunch both parallel to and perpendicular to the propagation direction.

– Resolution elements are broadened.

– The sweep rate becomes non-linear.

• We need:

– High extraction field:- to limit dispersion.

– Fast sweep:- for small resolution elements.

– High electron detection efficiency:- to reduce space charge.

Challenges for sub-ps regime

Limits: Dispersion

Dispersion illustrated with low and high extraction fields:Transit time for electrons between cathode and anode as a function of initial electron energy.

The secondary electron energy distribution of gold and cesium iodide excited by Cu-K radiation at ~8keV has width:

E Au: ~3.5eV and E CsI: ~1.7eV

We use CsI as the best compromise between energy distribution, moisture tolerance and sensitivity compared to metals and other salts like KBr.

HIGH FIELD10kV, 0.25mm gap t ~0.110ps

0 1 2 3 48.25p

8.30p

8.35p

8.40p

8.45p

Arr

ival

tim

e at

ano

de (

ps)

Electron energy (eV)

t 110fs

LOW FIELD7kV, 2mm gap : t ~1.2ps

0 1 2 3 478.50p

79.00p

79.50p

80.00p

80.50p

81.00p

Arr

ival

tim

e at

ano

de (

ps)

Electron energy (eV)

t 1.2ps

Streak Camera Lens

-.01

0.00

0.

.00

.01

.05700 .05750 .05800 .05850 .05900 .05950 .06000 .06050 .06100

x (cm) vs distance ref phase= 3.60 ref. pos.= 0.06

0.

.00

.00

.00

.01

.05700 .05750 .05800 .05850 .05900 .05950 .06000 .06050 .06100

r (cm) vs distance ref phase= 3.60 ref. pos.= 0.06

10

10

10

10

10

.05700 .05750 .05800 .05850 .05900 .05950 .06000 .06050 .06100

w (keV) vs distance ref phase= 3.60 ref. pos.= 0.06

Streak Camera Lens

-.01

0.00

0.

.00

.01

4.6000 4.6050 4.6100 4.6150 4.6200 4.6250 4.6300 4.6350 4.6400 4.6450 4.6500

x (cm) vs distance ref phase= 284.76 ref. pos.= 4.62

0.

.00

.00

.00

.01

4.6000 4.6050 4.6100 4.6150 4.6200 4.6250 4.6300 4.6350 4.6400 4.6450 4.6500

r (cm) vs distance ref phase= 284.76 ref. pos.= 4.62

10

10

10

10

10

4.6000 4.6050 4.6100 4.6150 4.6200 4.6250 4.6300 4.6350 4.6400 4.6450 4.6500

w (keV) vs distance ref phase= 284.76 ref. pos.= 4.62

Streak Camera Lens

-.01

0.00

0.

.00

.01

.05700 .05750 .05800 .05850 .05900 .05950 .06000 .06050 .06100

x (cm) vs distance ref phase= 3.60 ref. pos.= 0.06

0.

.00

.00

.00

.01

.05700 .05750 .05800 .05850 .05900 .05950 .06000 .06050 .06100

r (cm) vs distance ref phase= 3.60 ref. pos.= 0.06

10

10

10

10

10

.05700 .05750 .05800 .05850 .05900 .05950 .06000 .06050 .06100

w (keV) vs distance ref phase= 3.60 ref. pos.= 0.06

Streak Camera Lens

-.01

0.00

0.

.00

.01

4.6000 4.6050 4.6100 4.6150 4.6200 4.6250 4.6300 4.6350 4.6400 4.6450 4.6500

x (cm) vs distance ref phase= 284.76 ref. pos.= 4.62

0.

.00

.00

.00

.01

4.6000 4.6050 4.6100 4.6150 4.6200 4.6250 4.6300 4.6350 4.6400 4.6450 4.6500

r (cm) vs distance ref phase= 284.76 ref. pos.= 4.62

10

10

10

10

10

4.6000 4.6050 4.6100 4.6150 4.6200 4.6250 4.6300 4.6350 4.6400 4.6450 4.6500

w (keV) vs distance ref phase= 284.76 ref. pos.= 4.62

Corresponds to ~103 electrons in a 25m diameter spot.At anode exit: ~7m long bunch ~100fs

Towards the end of the sweep plates:Pulse stretched to ~1.5ps

Corresponds to ~102 electrons in a 25m diameter spot.At anode exit: ~7m long bunch ~100fs

Towards the end of the sweep plates:Pulse stretched to ~350fs

Limits: Space Charge. Parmela simulations

0.02 SPPS current

0.1 SPPS current

2 pulses, 100fs duration, 200fs separation

• Space charge saturation between cathode and anode:

Child-Langmuir law: IV3/2 d-2

– ~80mA (10kV across a 0.25mm gap with a 25m spot)

– Corresponds to an incident flux at 8kV of ~5x104 photons in a 100fs pulse.

– Certainly possible at LCLS.

– But: temporal resolution will have degraded well before this limit.

• Saturation of the CsI :

– There are ~1011 CsI molecules in a 25micron spot on the cathode (1000Å).

– At the space charge saturation limit above there will be ~106 molecules per liberated electron.

– This is plenty.

– (In the ~ns regime at high flux such saturation effects do occur.)

Limits: Cathode saturation

Limits:

• Space charge

• Dispersion

Design strategy for a sub 200fs camera

• There are five main parts to a streak camera:

– Cathode/anode

– Electron imaging

– Electron detection

– Sweep plates

– Ramp pulse generator

Design strategy for a sub 200fs camera

• Must improve all five components:

– Higher extraction field at the cathode:

• 10kV across a 0.5mm gap compared to 7kV across a 2mm gap.

– Higher spatial resolution:

• ~50m compared to ~140m (over a wider field of view)

– Higer electron sensitivity:

• ~10x.

– Less distortion from the sweep plates.

– Faster ramp pulse:

• ~ 100ps compared to ~300ps

• higher switching voltage ±500V compared to ±300V

• More robust construction

Design strategy: Camera layout

CsI photo-Cathode. -10kV

Anode (0V)

Sweepplates

Magnetic lens Back thinnedCCD

Signal fromexperiment

Tim

e

Space

RecordedStreak image

Design strategy: X-ray photocathode / anode assembly

• Cathode/anode assembly operates at >10MV/m

– within a factor of ~2 of that required for DC operation at ~200fs

• For higher fields the cathode must be pulsed.

– Initial tests with a 40Hz 10kV pulser (active reverse terminated for a matched transmission line) are promising.

– With a 1mm gap, dc emission has been effectively eliminated, whilst the focus appears stable.

– Capacitive coupling to the sweep plates introduces movement in the static image. This will be addressed with improved shielding.

• The cathode substrates are 1000Å silicon nitride windows on a 400m silicon wafer.

Design strategy: Electron imaging system

60 40 20 0 -20 -40 -60 -80 -100 -1200

20

40

60

80

100

120 3.5" lens 2.2" lens

Fie

ld /

Gau

ss

Z-axis position / mm

Lines of equal r×Magnetic vector potential (Gcm2)

Measured field on axis:

Higher resolution, wider field of view, less field extending in the sweep region

Design strategy: Electron distribution in the image plane for the two camera lenses

-0.4 -0.2 0.0 0.2 0.4-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

Po

sitio

n al

ong

slit

/ mm

Image width / mm-0.4 -0.2 0.0 0.2 0.4

-6.0

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Pos

ition

alo

ng s

lit /

mm

Image width / mm

-80 -60 -40 -20 0 20 40 60 800.0

0.2

0.4

0.6

0.8

1.0Off axis / mm : FWHM / m

0.0mm : 17.91 1.5mm : 19.29 2.5mm : 23.41 3.5mm : 32.76 5.0mm : 59.81

Nor

mal

ised

sig

nal i

nten

sity

Position on detector / m-80 -60 -40 -20 0 20 40 60 80

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ised

sig

nal i

nten

sity

Position on detector / m

Off axis / mm : FWHM / m 0.0mm : 38.54 1.5mm : 57.14 2.5mm : 117.95 3.5mm : 238.22 5.0mm : 521.05

Design strategy: Sweep plates

The ramp pulse propagates along the impedance matched microstrip at ~0.7c.As the electron bunch from the cathode passes each ‘leg’ at ~0.2c, it is influenced by

the ramp pulse.

For a linear sweep the electrons must only see the linear part of the ramp at each leg.

The length and geometry of the microstrip and the shape of the ramp pulse is matched to the electron velocity and hence the cathode voltage.

Design strategy: Ramp generator.GaAs photoconductive switch

• Within specification for single shot and 1kHz use.

• The device is stable at +/–500V bias with a 1kHz, 800nm, ~25J, ~65fs, ~3mm diameter spot falling on a 10 diffuser 2cm from the wafer.

+V/2

+V -V

-V/2

R R

RR

Photoconductiveswitch

MicrostripSweep plate #1

MicrostripSweep plate #2

50 50

Laserpulse

t

V

Bias

Ramp

Flyback

{

0

V/2

-V/2

Schematic of the +ive and –ive ramp pulses

Design strategy: Ramp generator

Signal switched into a 50 loadfor a range of applied voltages

< 100ps rising edge Damage observed in early switches with no insulation or diffused Ge layer

Semi-insulating GaAs

2400Å SiO2

3000Å Au400Å Ge100Å NiSI GaAs wafer

Laserpulse

• Germanium/gold alloy electrode contacts

• 100Å Ni wetting layer

• Rapid anneal at 420C for 30s

• Diffusion of germanium into the GaAs

Measured camera performance compared to specification: Static resolution

• FWHM ~ 4pixels

• 13m pixels → ~50m image

• 2× magnification → ~25m resolution at cathode

0 2 4 6 8 10 12

0

2

4

6

Pos

ition

(m

m)

position (mm)

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(au

)

Position (mm)

Static CCD imageof 25m cathode slot

Measured camera performance compared to specification: Dynamic resolution with UV pulses

0 2 4 6 8 10 12

0

2

4

6

P

ositi

on (

mm

)

position (mm)

0 10 20 30 40 50 600.0

0.2

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Inte

nsi

ty (

au)

Time (ps)

0 10 20 30 40 50 60

600

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1000

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1400

1600

Pos

ition

on

CC

D (

pixe

ls)

Time (ps)

• Sweep rate: ~60fs/pixel– Linear over full timing window

• Dynamic image FWHM ~ 14pixels– ~800fs FWHM typical, ~600fs min

SPPS / Laser Jitter measurement

Inherent jitter between SPPS and the laser limits pump probe measurements to ~1ps resolutionOnline monitoring with the X-ray streak camera provides a timing fiducial for every shot.Full advantage of the ~80fs SPPS pulse can be realised.

10Hz Jitter measurement between SPPS X-ray pulses and the Ti: Sapphire laser phase locked to the SLAC RF signal

Laser timing fiducials

SPPS X-ray pulse

Fit Gaussian to all three pulses:The RMS deviation in the separation of the two laser spots over 1000 shots is ~100fs

Jitter measurements recoded at SPPS in January

0 20 40 60 80 100-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Aut

ocor

rela

tion

r kLag (s)

Autocorrelation plot of the jitter over 100 secondswith 0.1s lag intervals

Autocorrelation: rk = Autocovariance Variance

Variance is the average squared departure from the mean.Autocovariance is the average product of departures at times t and t+k95% Confidence band at ± 2 / N test for randomness

0 20 40 60 80 100

-1000

-500

0

500

1000

t (

fs)

Time (s)

Jitter measured in the X-ray hutch at 10Hz for 100 seconds.Laser vs X-rays: uncertainty 120fs.

0.01

0.1

1

5

20

4060

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99

99.9-1500 -1000 -500 0 500 1000 1500

-1500 -1000 -500 0 500 1000 15000

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Cou

nts

Bin (ps)

Cum

ulat

ive

Cou

nts

Histogram and probability plots of the jitter show an otherwise normal distribution in the correlated data

Jitter measurements recorded in May (over ~8minutes)

0 100 200 300 400 500-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

280fserror bar

t (

ps)

Time (s)

Jitter measured in the X-ray hutch at 10Hz for 500 seconds.Laser vs X-rays: uncertainty 140fs.

0 100 200 300 400-0.2

0.0

0.2

0.4

Aut

ocor

rela

tion

Lag (s)

Autocorrelation plot of the jitter over 500 secondswith 0.1s lag intervals

Comparison between data recorded in January and data recorded in May

0 20 40 60 80 100

-0.2

0.0

0.2

0.4 Jan May (x2, -0.15)

Aut

ocor

rela

tion

Lag (s)

Autocorrelation plots of Jan and May dataMay data scaled x2 and offset -0.15

Histogram and probability plots of the jitter show an otherwise normal distribution in the data.Base width ~3ps, fwhm ~1ps.

0.01

0.1

1

5

20

4060

80

95

99

99.90

-1500 -1000 -500 0 500 1000 15000

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Co

un

tsBin (fs)

Cu

mu

lativ

e C

ou

nts

Correlation between streak and eo measurements

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

Str

ea

k-jit

ter

(ps)

EO-jitter (ps)

0 1000 2000 3000 4000 5000-1.5

-1.0

-0.5

0.0

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1.0

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Jitte

r (p

s)

Shot number

streak eo

Next steps

• We have demonstrated reliable jitter measurements at the 100fs level.

• We should be able to improve this to ~10fs.

• To achieve this we need to improve the dynamic image resolution.

– Change sweep mechanism to eliminate distortion.

To go any further with streak cameras we will need to:

• Eliminate dispersion limitation

• Eliminate space charge limitation

New CCD camera for streak camera readout

• 12-bit 5kHz real time centroiding on laser and X-ray pulses (binned to two rows)

• ~30Hz sustained frame rate with 1400x1400 10micron pixels

• Aiming for 120Hz frame rate with 1k x 1k 4 port sensor (25MHz pixel clock)

• Useful for 2D X-ray imaging too.