n over 800-2000 eV.  With 10% absorption and 50% quantum efficiency, about 1011 visible fluorescence photons per shot will be generated at each screen.  If ~1% of that light is imaged onto a CCD or CMOS with 10% efficiency, ~107 photons will be imaged per shot.  The screens, fitted to an XYZ manipulator can be positioned in the beam and viewed through a viewport by a camera. 

Total Pulse Energy Monitor
 

  The total energy of the FEL pulse can be measured after the AMO experiment since the target is so “thin” or transparent.  A total energy monitor beamstop at the end of the diagnostics chamber will be used to measure the total energy in each FEL pulse.  The monitor will be retractable for calibration purposes so an additional fixed beamstop will also be provided.  A bolometer has been designed by the XTOD group for use in the FEE diagnostics suite (see PRD 1.5.-009-r0 and subsequent documentation for further details) and we intend to copy their design and duplicate it in the AMO diagnostics.

  
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  Figure 10: Temperature response of the bolmeter (in K) at diffrent ambient temperatures (100K, 150K & 200K).

  The bolometer is essentially a thin piece of silicon that is maintained at a low temperature (~100K) with a thermistor fixed on the back side of the material.  An incoming FEL pulse deposits its energy in the material causing its temperature to rise, which is measured by the thermistor sensor.  With thin enough material the temperature re-equilibrates before the next FEL pulse.  Data acquisition is therefore carried out by digitizing the waveform from the RTD – something that could be sampled at a ~100 MHz for ~1msec – to determine the rate of rise of the temperature following the absorption of the FEL radiation and which is related to the total energy in the pulse.

Wavefront Sensor
 

  The ideal photon beam diagnostic would be a wavefront sensor that measures the shape of the wavefront of the FEL pulse diverging from the focus in the interaction region.  It should then be possible to determine the size, shape and position of the FEL beam focus in the experimental chamber on a shot-by-shot basis.  This information, combined with an approximate measure of the total energy would give the peak intensity of the FEL pulse in the interaction region, the most desirable diagnostic parameter for the AMO experiments. A commercial wavefront sensor for x-ray radiation is available form Imagine Optic, but it remains a research project within the company according to their people.

  
  Figure 11: Diagram of the Shack-Hartmann x-ray wavefront sensor from Imagine Optic.

  The wavefront sensor is a Shack-Hartman detector which samples the wavefront at many locations across the beam using a pinhole array to image the beam onto a detector.  Measuring distortions of the resulting image allows the instrument to determine the shape of the wavefront.  The detector is a CCD with considerable image processing required for each shot to determine the wavefront.  A faster CCD would be required to achieve 120Hz operation.  The pinhole array might be able to be used as a bolometer to determine the pulse energy with some redesign of the instrument.

Imaging CCD
 

  An imaging system is required for the particle imaging experimental chamber that can be read on a pulse-by-pulse basis.  It is of course possible to operate at a lower pulse frequency at the expense of duty cycle if a camera cannot be found to operate at 120 Hz. Images from the camera will need to be saved and later summed with images from comparable shots where the FEL pulse had a similar intensity (focus and power) and the sample was of a comparable nature.  Following data filtering/summing, the images would be processed to determine scattering angles of the radiation.