X-ray chopper

chopper design (106 kB

chopper photograph (200 kB)

General Description:
 

The chopper consists of a triangular rotor made of titanium, which has a channel carved into one of its sides. The channel is covered at the tips by small tungsten plates, which produce an entrance and exit slit in the channel. The chopper is installed in the beamline vacuum just before the sample in order to take advantage of the small vertical beam near the focus.  In white beam experiments, the rotating shutter described above provides critical protection from the heatload from the focused beam. The rotor is held in magnetic bearings and rotates normally at 896.6 Hz, the 396th sub-harmonic of the orbit frequency,  which corresponds to a pulse on the sample every 1.11 ms.

The rotor chamber is a 40 mm thick steel cylinder which provides protection against rotor breakdown. The cylindrical chamber is held in a frame, which allows the rotor & chamber to rotate in case of transient transfer of energy from the rotor. The chopper is mounted in a position & tilt alignment stage, which permits centering the chopper precisely on the x-ray beam. When the chopper is stopped, the controller automatically drives into the open position. In fact in static experiments, the beam is always shot through the chopper tunnel.  A PC controls the chopper commands.

The phase jitter is typically 10.5 ns (RMS) at full speed.  In order to do single-shot experiments, the chopper has to be preceded by a ms-shutter. We have therefore installed a ms-shutter in the beamline vacuum in front of the chopper. The triangular cross section of the tunnel in the rotor blade allows to control the opening time by displacing the whole chopper sideways. The opening time at 900 Hz can be as large as 1.5 ms if a bunch out of the single bunch or hybrid mode has to be extracted and closed to 170 ns for single bunch extraction from 16 bunch mode, by expense of reducing the maximum transmission.

Opening times for different modes:

opening times (67 kB)
 

Parameters: