# Presentation: 764280

## Stability Issues for NSLS-II Infrared 

- L. Carr
- 17-April-07 NSLS-II Stability Workshop

## Outline

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

## Infrared Performance Issues: Why is IR so demanding?

**I**nfra**R**ed **S**ynchrotron **R**adiation (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:   (1/) from 1 cm-1 up to 10000 cm-1 (1 cm <  < 1 m)
- 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_.

## Michelson-type interferometers, typically operating in “continuous” or “rapid-scan” mode.

**Fourier Transform Infrared (FTIR) Spectrometer**

- Each spectral component receives sinusoidal modulation:  FT gets you spectrum:  frequency (in Hz) ~  (in cm-1).

## Frequency Range Requirement

**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

## 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.

## What motion magnitude can be tolerated?
The good news:  the effective or apparent source size will always be diffraction-limited.  

diffraction ~ 2/3 1/3 

At short wavelength of 2 m (2x10-4 cm) and = 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.

- The good news:  the effective or apparent source size will always be diffraction-limited.
- diffraction ~ 2/3 1/3
- At short wavelength of 2 m (2x10-4 cm) and **= 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**

## Position Stability Requirements

**Position Stability Requirements**

- Optimal case:  beam perfectly centered on all apertures

## Position Stability Requirements

**Position Stability Requirements**

- Optimal case:  beam perfectly centered on all apertures
- 10 m movement cause 0.3% change.

## Position Stability Requirements

**Position Stability Requirements**

- Worst case:  Sample with sharp edge centered on beam
- 1 m movement yields 0.3% change

## 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 m in vertical, ~ 3 m in horizontal (more forgiving).

- 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 m in vertical, ~ 3 m in horizontal (more forgiving).

**Position Stability Requirements**

## 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)*

## Noise Example: NSLS Infrared beamline U10

- Example S/N (red) and Ideal (blue)

- Loss in S/N due to Beam-related Noise

*> 10X loss**below 1.5kHz*

## Apparent Beam Motion Example: NSLS Infrared beamline U10

- Position Sensitive Photodetector at endstation, ~ 300 Hz BW

***Equivalent******NSLS-II goal***

## Angular Stability Requirements

**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 rad sufficient for horizontal, suggest 3 rad for vertical.

## 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 m).
- angular position less critical (several rads 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.