Stability Issues for NSLS-II Infrared

1. L. Carr 17-April-07 NSLS-II Stability Workshop Stability Issues for NSLS-II Infrared

2. Outline Overview: Why is IR so sensitive? Frequency range Position Specification (tighter in vertical than horizontal) Angular Specification – generally less restrictive Summary

3. Infrared Performance Issues: Why is IR so demanding? InfraRed Synchrotron Radiation (IRSR) is used for low-throughput techniques such as microspectroscopy. Spectrometer endstations are based on highly evolved commercial instruments. Instruments already optimized for highest S/N Fourier Transform (FT-IR) interferometers (modulate spectral intensity into AC signal). Detectors achieve “background limited infrared performance” (BLIP)=> Photon Noise is often limiting factor. Spectral range: n (1/l) from 1 cm-1 up to 10000 cm-1 (1 cm < l < 1 mm) IRSR is typically 1000x brighter than standard laboratory IR source … a thermal (blackbody) radiator at ~1200K. background photon noise from 1200K source only factor of ~2x above 300K background. In order to benefit from the brightness advantage, IRSR source noise should be no more than 10X the thermal source noise. Ideally 1X. => thermal source noise serves as reference point.

4. Michelson-type interferometers, typically operating in “continuous” or “rapid-scan” mode. Fourier Transform Infrared (FTIR) Spectrometer fixed mirror source moving mirror (scans back and forth) typical velocity of ~1 cm/s to experiment endstation and detector • Each spectral component receives sinusoidal modulation: FT gets you spectrum: frequency (in Hz) ~ n (in cm-1).

5. Frequency Range Requirement Determined by several factors Desired spectral range (usually spans from 1 to 2 full decades) Mechanical movement of FTIR scanner (available velocities) Digitizing rate capability Detector and amplifier response time => Modulated frequencies span 1 to 2 decades in a single measurement. One more time-scale: sample/reference measurement. Can be < 1 minute or several hours. Result: ~1 Hz up to 1 kHz for far-infrared and THz spectral range. 100 Hz up to 10 kHz for mid-infrared spectral range. sub 1 Hz for all measurements (sample-in / sample-out). SUM: need stable up to ~10kHz

6. Noise Sources in Required Frequency Range Typical SR Noise Sources Mechanical motion (drift, vibrations). < 200 Hz 60 Hz related (electrical pickup). 60 Hz and multiples up to ~720 Hz. Multiples of 720 Hz in RF sidebands. Note: some low frequency noise can be compensated using dynamic beam steering mirrors with feedback. RF (100s of Hz to > 10 kHz) too fast to correct using optomechanics.

7. What motion magnitude can be tolerated? The good news: the effective or apparent source size will always be diffraction-limited. sdiffraction ~ l2/3r1/3 At short wavelength of 2 mm (2x10-4 cm) and r = 2500 cm (NSLS-II), smallest effective beam size is about 500 microns. Model: use Gaussian beam and “aperture” to determine signal fluctuation as function of motion: defines allowable movement. Assume upper noise limit of 1%, set requirement at 0.3% under “worst case scenario”. achieving below 0.1% is still beneficial. Position Stability Requirements

8. Position Stability Requirements Optimal case: beam perfectly centered on all apertures

9. Position Stability Requirements Optimal case: beam perfectly centered on all apertures 10 mm movement cause 0.3% change.

10. Position Stability Requirements Worst case: Sample with sharp edge centered on beam 1 mm movement yields 0.3% change

11. Result: For a symmetrically aligned aperture, beam motion must be kept to below 5% of the effective bunch size for 1% noise. This sets an upper limit of 25 microns and goal requirement of 10 microns. If the aperture can not be symmetrically aligned or experiment can not use a symmetric beam profile, then the constraint becomes 10 times more severe (1 micron for 0.3% noise). This does not reduce noise to background level. It makes it quite tolerable (moderate improvement relative to NSLS). Another factor of 10x smaller would improve S/N for many measurements. Source is always diffraction-limited in vertical, but becomes extended source horizontally. Can tolerate more horizontal movement (at least 3X). SUM: limit beam motion to 1 mm in vertical, ~ 3 mm in horizontal (more forgiving). Position Stability Requirements

12. Noise Example: NSLS Infrared beamline U10 Mechanical & electrical noise below 500 Hz (could be reduced by optical stabilization) “Other” noise (RF?) at higher frequencies “Noise Floor” => intrinsic noise at detector (baseline with no beam)

13. Noise Example: NSLS Infrared beamline U10 Example S/N (red) and Ideal (blue) Loss in S/N due to Beam-related Noise > 10X lossbelow 1.5kHz

14. Apparent Beam Motion Example: NSLS Infrared beamline U10 Position Sensitive Photodetector at endstation, ~ 300 Hz BW EquivalentNSLS-II goal

15. Angular Stability Requirements Main issue is beam spillage at an aperture. Exit (collecting) aperture at dipole extraction serves as limit. Typical mid-IR collection of ~ 10 mrad, assume 0.1% tolerance. Vertical may be more sensitive (downstream aperture). SUM: 10 mrad sufficient for horizontal, suggest 3 mrad for vertical.

16. Infrared Noise Summary Noise from Source & Beamline components limits performance at most IR beamlines. Mechanical movement (mostly below 100 Hz) Electrical (60 Hz multiples) RF (multiple lines, above 500 Hz) Nothing is “magic” about IR requirements: The “competition’s” noise is very low! 1 micron position stability would achieve 300:1 S/N for “worst case". more forgiving in horizontal than vertical (extended horizontal source, assume 3X more tolerance = 3 mm). angular position less critical (several mrads is fine, plan to under-fill optics & avoid beam “spillage”). frequencies to at least 10 kHz. Existing NSLS VUV/IR Mid-IR: effectively lose ~10X of potential S/N benefit. all types of noise, RF sidebands difficult to avoid (occur at many different frequencies). optimal alignment helps, but optimization lost when beam position drifts. Existing NSLS VUV/IR Far-IR: effectively lose up to ~100X noise is both mechanical and electrical. beam stabilization expected to yield significant improvement. Users do not always recognize unusual noise. need independent diagnostic beamport for constant monitoring.