Chopper system for time resolved experiments with synchrotron radiation.

A chopper system for time resolved pump-probe experiments with x-ray beams from a synchrotron is described. The system has three parts: a water-cooled heatload chopper, a high-speed chopper, and a millisecond shutter. The chopper system, which is installed in beamline ID09B at the European Synchrotron Radiation Facility, provides short x-ray pulses for pump-probe experiments with ultrafast lasers. The chopper system can produce x-ray pulses as short as 200 ns in a continuous beam and repeat at frequencies from 0 to 3 kHz. For bunch filling patterns of the synchrotron with pulse separations greater than 100 ns, the high-speed chopper can isolate single 100 ps x-ray pulses that are used for the highest time resolution. A new rotor in the high-speed chopper is presented with a single pulse (100 ps) and long pulse (10 micros) option. In white beam experiments, the heatload of the (noncooled) high-speed chopper is lowered by a heatload chopper, which absorbs 95% of the incoming power without affecting the pulses selected by the high speed chopper.

The performance of a cryogenically cooled monochromator for an in-vacuum undulator beamline.

The channel-cut silicon monochromator on beamline ID09 at the European Synchrotron Radiation Facility is indirectly cooled from the sides by liquid nitrogen. The thermal slope error of the diffracting surface is calculated by finite-element analysis and the results are compared with experiments. The slope error is studied as a function of cooling coefficients, beam size, position of the footprint and power distribution. It is found that the slope error versus power curve can be divided into three regions: (i). The linear region: the thermal slope error is linearly proportional to the power. (ii). The transition region: the temperature of the Si crystal is close to 125 K; the thermal slope error is below the straight line extrapolated from the linear curve described above. (iii). The non-linear region: the temperature of the Si crystal is higher than 125 K and the thermal slope error increases much faster than the power. Heat-load tests were also performed and the measured rocking-curve widths are compared with those calculated by finite-element modeling. When the broadening from the intrinsic rocking-curve width and mounting strain are included, the calculated rocking-curve width versus heat load is in excellent agreement with experiment.

The realization of sub-nanosecond pump and probe experiments at the ESRF. European Synchrotron Radiation Facility.

We present beamline ID09B that is designed for pump and probe experiments to 50 ps time-resolution. The beamline has been refurbished with a narrow-bandwidth undulator for Laue diffraction and diffraction from liquids. The new undulator has 235 poles, a 17 mm magnetic period and is operated at 6.5 mm gap. It produces a spectral flux of 2.0 x 10(8) photon/0.1% bw/pulse (10 mA) at the fundamental at 15.5 keV and an integral flux of 1.1 x 10(10) photon pulse(-1) in a 2.5% bandwidth. The optics has been renewed with a high-precision toroidal mirror and a cryogenic monochromator. The X-ray chopper used for single pulse selection is also described together with the femtosecond laser. Finally the diffraction from excited iodine molecules in CCl4 is investigated on the nanosecond time-scale. It turns out that the high-angle scattering is insensitive to the thermal chock from the laser: these oscillations are therefore readily used for structure determination. Conversely, the low-angle scattering probes the hydrodynamics of the liquid over longer length scales and the oscillations are believed to originate from thermal stress and expansion of the solvent.