ESRF interest & commitment to (ultra-) fast science

with X-rays

Kees ScheidtDiagnostics group, Machine division

ESRF interest & commitment to (ultra-) fast science with X-rays

1) Thorough tests of the quasi-isochronous Storage Ring in 1995 :inadequate since incompatible with high-intensity demands

so : typical bunch length remains ~100ps (fwhm) at 5mA single bunch

2) Operation of various filling patterns to satisfy the whole usercommunity, including specialized time-resolved beamlines

3) Development of X-ray time resolved detector forsub-pico second time resolved diffraction experiments

pump

streak- tube

output : ultra-fastmodulation

jitter-freesynchronisation

X-ray Probe100 picosec

laser

100 femtosec

sample

rampHV

ultra-fast X-ray diffraction experimentsthat use the pump-probe technique with :

a 100femtoSec Laser light pump pulse and

a 100picoSec X-ray probe pulse

the ultimate time resolution depends on :

1) intrinsic time resolution of the Streak Camera for X-rays 2) the quality of synchronization

Objective : 1) sub-picoSec time resolution 2) high quality data by accumulation over many shots

the Streak-Tube :improved version ofcommercial model (Photonis)

10 picosec

5mm

460ps fwhm result when accumulating 900 shots

Critical laser issues :

1) Amplitude stability

2) Contrast (or pre-pulse energy variations)

DQE of Streak Camera

1,E-04

1,E-03

1,E-02

1,E-01

1,E+00

5 10 15 20 25 30

Energy in KeV

'Csi-500' 'Ki-1000 'Ki-500'

'KBr-1000' 'Kbr-500' 'Au-300'

Sensitivity for hard X-rays is low due to poor DQEof photo-cathodes

decreasing DQE forincreasing Energy

best : Cesium-Iodide(100)

low : Potassium-Iodide (10) Potassium-Bromide

poor : Gold(1)

Conclusion (1):

Optical synchronization by a GaAs photo-switch between afemtosec-laser and a Streak-Camera attains values <100fs when :

-Laser energy >25uJ per pulse

-Laser stability 0.25% rms

-Laser contrast >104

In these conditions (obtained after delicate adjustments) the time resolution of the system in accumulation mode is determined bythe tube’s intrinsic time resolution, at ~500fs fwhm for UV light

The exact time resolution for hard X-rays was never assessed since various attempts to produce a sub-pico sec event in an appropriatePump-Probe experiments were without success.

Conclusion (2):

The accumulating Jitter-Free X-ray streak camera for sub-pico sec pump-probe experiments was Not a scientific successdue to :

1) The difficulty of conceiving an appropriate experiment

2) The required delicate adjustments on laser, photo-switchand streak-camera making it not user-friendly

3) The particular limitations of a Streak-Tube detector:a) No real space-dimensionb) Low sensitive photo-cathode material