IR Demo Diagnostics
The diagnostics for the IR Demo wiggler will be used to turn the laser on, optimize its performance and characterize the optical radiation for users. This documentation covers the following subjects:
Measurements Of Spontaneous Radiation
Requirements: The spontaneous power from the free-electron laser into the lowerst order optical mode is approximately 2 µW when the electron beam is optimized. Initially the power will be as small as one tenth of this. Power at the third harmonic is about half of this value. In order to turn on the laser it is necessary to optimize the spontaneous spectrum and measure the total spontaneous power versus cavity length or the spontaneous power at the wavelength for maximum gain. The total range of the spectrum is up to 10% and the FWHM of the spectrum for an optimum spectrum is 2.5%. Signal to noise must be greater than 10 to 1 for each of these measurements.
Hardware: Four instruments are used to measure the spontaneous power spectrum:
1.) Acton 300i, 0.35 meter scanning monochromator. This device has gratings with 150 grooves/mm, 300 gr/mm, and 1200 gr/mm.
With slits set to 300 µm using the 150 gr/mm grating, blazed at 3 µm, the resolution is better than 0.1% and the throughput is ~1%
2.) A cooled mercury cadmium telluride (HgCdTe) detector. The detector is liquid N2 cooled, D*=3 x 1010 and the active area A < 1 mm2. During spontaneous measurements, the detector output will be measured across a high impedance which will limit the response time to approximately 100 µsec. The signal to be measured will be approximately 2 nA.
3.) A cooled, staring, indium gallium arsinide (InGaAs) detector array. This array is for measuring the third harmonic spectrum. This array should allow acquisition of the 3rd harmonic spectrum in less than 1 second.
4.) A chopper and lock-in amplifier.
Measurements: Several measurements will be carried out:
Power versus angle through mode defining aperture: In this measurement the total spontaneous power through a two waist aperture is maximized. This is done first to optimize the optical transport and then to optimize the optical cavity alignment.
Spectrum of the fundamental using the chopper/lock-in combination with the cooled HgCdTe detector through the monochromator using a 150 grooves/mm grating.
Spectrum of the third harmonic using the InGaAs array and the 300 grooves/mm grating.
Total power versus cavity length.
Power at the gain wavelength versus cavity length.
Optimization of Laser Radiation
Spontaneous power measurements are done on the entire beam from the laser reflected off a burn-through mirror. Once lasing has been achieved, the burn through mirror will be withdrawn and the power will fall on a XXX power meter. This will measure the power on a slow time scale. Other diagnostics will use a 0.1% pickoff to direct laser light onto the diagnostic suite. The spectrum might be as narrow as 0.1% FWHM. The following diagnostics will be used to optimize the laser:
The cooled HgCdTe detector with 50 W coupling. This will achieve the maximum bandwidth for the detector of 100 MHz.
Acton 300i monochromator. With the slits closed down on this monochromator and with the 150 gr/mm, the resolution of this detector is 0.01%.
A Spiricon Pyrocam on a scanning stage. This will be used to characterize the mode quality of the laser and to verify the spot size and set it to proper size using the collimator mirror.
These diagnostics will be used to carry out the following optimizations:
Measure the laser power versus cavity length and set to the proper length.
Optmize the electron beam parameters using the total laser power.
Measure the optical spectrum and optimize the spectral brightness using cavity phases.
Adjust the collimator to get the proper beam size in the laser labs.
Maximize the power and spectral stability using the HgCdTe detector.
Laser Characterization For Users
Users need to know the detailed time structure of the laser on many time scales, the spectral structure and stability of the laser and the transverse mode properties of the laser. The following instruments will be used to characterize the laser:
A XXX thermopile detector for total power measurements. This is an invasive measurement but provides good accuracy. A smaller thermopile detector on the 0.1% pickoff beam can be used for continous power monitoring.
The cooled HgCdTe detector with 50 W coupling. This will achieve the maximum bandwidth for the detector of 100 MHz.
A cross-beam autocorrelator with better than 100 fsec resolution for measuring the microbunch length.
Acton 300i monochromator. With the slits closed down on this monochromator and with the 150 gr/mm, the resolution of this detector is 0.01%. This can be used to monitor the wavelength or for feedback purposes. The repeatibility of the device is 1 nm and the accuracy will be better than 0.1%. The monochromator is computer controlled using EPICS.
A Spiricon 32x32 Pyrocam pyroelectric array on a scanning stage. This will be used to characterize the mode quality of the laser and to verify the spot size and measure the pointing stability of the laser.
InGaAs photodiode for timing measurements.
The diagnostics can provide the following data on the FEL beam:
Total laser power
Laser spectrum
Pointing stability
Autocorrelation width
RMS power fluctuations
Timing jitter with respect to an independent oscillator
Mode quality
CONCLUSIONS
Most diagnostics are commercially available. Specifications are being finalized.
High average current leads to good signal to noise.
Most controls are similar to those already used at TJNAF. Only have to adapt the software and rewrite the database.