# Presentation: 112821

## Stability Issues at the ALS

**Stability Issues at the ALS**
**Christoph Steier**
**ALS Accelerator Physics Group**

-  The Advanced Light Source
-  Introduction
-  Slow/Fast orbit feedback
- Energy Stability
- Beamsize Stability
- Top-off preparation
- RF phase noise

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

**Notes:**
The goal of the superbend project is to create sources of high brightness high energy x-ray beams using superconducting magnets. I will talk about the superbend accelerator physics issues. My talk is structured in the following way.
I will give a brief introduction to the superbend project where I will present the motivation for the superbends and compare the superbends  to other high energy x-ray sources. Next I will discuss the modifications to the ring and how that will change the beam  parameters and particularly how it will impact the  other users. I will discuss the possible operational concerns in particular the impact on injection, momentum acceptance and ramping. Finally I will conclude with some summary statements.

## Aerial view of the Advanced Light Source

**Aerial view of the Advanced Light Source**

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

- 1/10 Electron Beam Size     

**ALS Parameters and Beamlines**

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

-  12 nearly identical arcs – TBA; aluminum vacuum chamber
-  96+52 beam position monitors in each plane (original+Bergoz)
-  8 horizontal, 6 vertical corrector magnets per arc (94/70 total+chicanes)
-  Beam based alignment capability in all quadrupoles
-  22 corrector magnets in each plane on thinner vacuum chamber pieces - FOFB

**BPM, Corrector locations**

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## What has been done at the ALS to maximize stability

_**FEED FORWARD**_** **
-  Insertion device compensation (10 Hz for most IDs, 200 Hz for EPUs)
-  Beta-beating, tune and coupling feed-forward (presents additional challenges to orbit stability!)

**     **_**“**__**PASSIVE”**_
- (i.e. remove the sources)
-  Temperature stability (air below 0.1, water below 0.3 degree peak-to-peak; 0.1 for RF)
-  Minimized water induced vibrations
-  Power supply stability (no switched mode supplies, thick aluminum vacuum chamber in most magnets)
-  Vibration - reduce the effects by mechanical design (ALS has big girders and moderate amplification factors) and remove the source (cryo-coolers).
-  Reduce RF-phase noise (mode-0 noise for IR users)

** ****   **_**FEEDBACK**_
-  Local orbit feedback is not routinely used at ALS
-  Global orbit feedback (1 Hz update rate slow, 1.1 kHz fast)

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Instrumentation at the ALS

**I.     Beam position monitors (BPMs)**
- 	Old in-house design (96) plus J. Hinkson/J. Bergoz multiplexed 	BPMs (currently 50); Bergoz BPMs used in feedback: noise level 	is about 0.3 – 0.5 microns at 200 Hz bandwidth and 200-400 mA; 	current dependence less than 1-2 micron for 200-400 mA
**II.    Photon beam position monitors (PBPMs)**
- 	Several very diverse designs; most are not integrated in accelerator 	control system; some beam-lines use them for local feedback 	(time-scales of feedback range from hours to ms); installed some 	new PBPMs recently (plan to install on most bend magnets)
**III.  Power supplies**
**	**All power supplies at ALS SR are SCR or linear; no switched 	mode. Noise level is typically less than 10-4 integrated over all 	frequencies (some main supplies 10-5). 16-20 Bit control (all 	corrector magnets are 20 Bit); corrector bandwidth >500 Hz.

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Instrumentation at the ALS II

**IV.  Control system**
- 	High level control system has throughput of about 100 Hz and 	delays of less than 10 ms after upgrade.	Low level (fast feedback – 	distributed cPCI crates) runs at 1.1 kHz with standard computer 	and network equipment, network synchronized timing. Extermely 	reliable
**V.    Other**
- Tested some simple methods to measure BPM and magnet motion; plan to incorporate measurement of BPM position relative to common accelerator-experiment ground plate into feedback
- Two synchrotron light monitors (emittance, energy spread); Streak camera (bunch length); LFB, TFB record + grow/damp; Fill pattern (high bandwidth BPM); CSR; fluctuations; nonlinear crystals; ...

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## User requirements

**User requirements**

- Three examples of experiments that currently are the most sensitive:
-  Micro focusing beamlines on bending magnets (e.g. Micro XAS, especially in combination with molecular environmental science samples, i.e. dirt); problem is that sample is very inhomogenous and small source motion causes the spectrum to change significantly. I0 normalization does not help!
-  Dichroism experiments (i.e. on EPUs) measuring very small polarization asymmetries; orbit motion can cause small shifts of the photon energy out of the monochromator, resulting in fake asymmetries.
-  STXM – bend magnet beamline has very high bandwidth beamline feedback – not very sensitive to orbit (but very sensitive to beamsize) variations; undulator beamline does not have fast feedback (heavier mirror) – needs very good fast orbit stability (and beamsize)
- After upgrades to the slow orbit feedback (arc sector, chicanes) and the EPU dipole, quadrupole and skew quadrupole feed-forward, and implementation of fast orbit feedback, even our most sensitive experiments are currently happy with the orbit stability.

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Achieved orbit stability at ALS

-  Improved fast jitter with fast global feedback (2004)
-  Improved 60 Hz noise with conversion of fast analog FF to digital (ground loop)
-  Improved insertion device FF compensation with better chicane magnets (important in two bunch)
-  Improved slow orbit stability with continuous addition of stable BPMs (ongoing)

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Feed-forward example: EPU COMPENSATION

**Feed-forward example: EPU COMPENSATION**

- Without compensation the EPU would distort the electron beam orbit by ±200 m vertically and ±100 m horizontally.  Using corrector magnets on either side of the EPU, 2-dimensional feed forward correction tables are used to reduce the orbit distortion to the 2-3 m level. Update rate of feed-forward is 200 Hz.

***Mechanically the EPU can move from left to right circular polarization mode in ~1.6 seconds***

- Apple-II type elliptically polarizing undulators are more complex than other IDs
-  The jaws can move in two directions (vertically and longitudinally)
-  The motion in the longitudinal direction is fast (up to 17 mm/s at ALS)
*This makes **orbit compensation** more difficult*

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

- EPU FEED FORWARD ORBIT CORRECTION

- Orbit Error without Feed Forward Correction

- 200 Hertz Feed Forward Correction

** **

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Orbit Correction

- Fast (200 Hz) or slow (10 Hz) local feed forward for all insertion devices (2-d tables for EPUs)
- Fast global orbit feedback (1111 Hz, up to 80 Hz closed loop bandwidth (3 dB))
- Slow global orbit feedback (1 Hz)
- No frequency deadband between feedbacks
- Complete (more correctors) global orbit correction plus local orbit correction at all IDs every 8h after refill.
- Photon beam position monitors at ALS are not used to correct beam orbit – instead they feed back on beamline optics. Bandwidth from h to about 10 kHz (IR beamline)

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Software used for slow orbit feedback

- All ALS high level controls accelerator physics routines are implememted in Matlab
- Orbit feedback is controlled using a GUI which allows to ramp for injection, do single orbit corrections, standardize the lattice, etc.
- Matlab includes all Matrix manipulation tools necessary and has proven to be very reliable
- Code is very flexible (algorithm development is simple and can if urgent need arises even be done during user operation)
- Based on Matlab Middle Layer – now widely used at many (most) light sources

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

- RF-Frequency Feedback

-  Largest long term effect is rain season (plus outside temperature)
-  Short term the fill cycle has a strong effect (heating), but insertion device gap changes are equally important and in an FFT also tidal effects show up

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

- Energy calibration (resonant depolarization)

-  High precision measurement of beam energy is relatively simple at low energy light sources like ALS
-  Allows some conclusions about long term orbit/magnet/ground plate stability
-  Implemented rf-frequency feedback at ALS and verified it with energy measurements

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Fast Feedback Layout

- Design choices:
-  Distances at ALS are relatively large -> distributed system
-  Wanted to avoid expensive specialized hardware (like reflective memory, DSPs)
- Multiplexed (Bergoz) BPMs provide enough bandwidth and low enough noise
-  D/A converter resolution for corrector magnets was upgraded from 16 to 20 Bit.
-  Update rate of system is currently 1.11kHz.

-  Motivation: Fast Orbit stability with passive measures already very good (2-4 microns rms). Improvement into <m range required active/fast feedback

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Computer Hardware of ALS FOFB 

**Use network timing (network is 100 Mbit/s, full duplex, switched), normal PowerMAC/cPCI hardware used in control system upgrade**

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Feedback Implementation Details

**Combination of fast and slow global orbit feedbacks in both planes – no frequency deadband**
**Fast Feedback currently 24 BPMs in each plane and 22 correctors in each plane. 1.11 kHz update rate, bandwidth DC-60 Hz. Only 1⁄2 of singular values used.**
**Slow Feedback 52 BPMs in each plane, 26 horizontal correctors, 50 vertical correctors, RF frequency correction. 1 Hz update rate, about 60% single step gain, bandwidth DC-0.1 Hz. Typically all SVs used.**
**Slow feedback communicates with fast feedback to avoid interference in frequency overlap range. Setpoints/golden orbit used by fast feedback is updated at rate of slow feedback.**

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Orbit feedback performance

-  Fast feedback routinely used in user operation since spring’04 with very positive user response.
-  Extremely reliable. One beam dump and total of 4 (minute long) feedback outages in first 2 years.
-  With slow and fast orbit feedback the ALS achieves submicron stability in the vertical plane:
-  Integrated rms motion 0.01 to 500 Hz in the vertical plane is below 0.5 micron (projected to the 2.25 m beta function, 18 micron vertical beamsize at center of straight)
-   Horizontally the integrated rms motion is now reduced to below 2 microns (at 13.5 m beta function and 300 micron horizontal beamsize).
-  Long term stability (week) is of the order of 3 microns.

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Beam spectra with feedback

-  Beam motion with feedback in open (red) and closed loop (blue) at out of loop BPM.
-  Feedback is very effective for moderate frequencies. Right now closed loop bandwidth (3 dB) is about 80 Hz.
-  Correction at low frequencies below the individual BPM noise floor (only 1⁄2 of SVs used).
-  System is set up conservatively at the moment – no excitation at higher frequencies.

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Simulink model of FOFB system

-  Comparison of simulated (Simulink) and measured step response of feedback system in closed loop
- PID parameters were intentionally set to create some overshoot (demonstrating that time constants and performance of system are well understood).

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Frequency Overlap – Master/Slave

- ALS needs slow and fast feedback (do not have enough fast correctors)
- Avoided frequency dead band – fast system not DC blocked
- Synchronization by SOFB updating FOFB golden orbit

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Fast feedback magnets can be noise source

- Strong corrector magnets with high vacuum chamber cut off frequencies can be significant sources of orbit noise
- Observed at several light sources
- Feedback of course will (partially) correct this, but it is much better to avoid effect in the first place
- In case of ALS, problem was not power supply noise, but ground loops which we introduced by analog summing junctions for fast local feedforward (2+2 correctors)
- Switching to a digital feedforward (with same update rate) and eliminating ground loops reduced 60 Hz noise (w/o fast feedback) substantially

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Summary of user input for top-off

-  I/I of 0.3% is small enough
- Injection every 30 s is OK, but should not be much more frequent
- No burst mode (several injections just after each other – 1 Hz)
- Bunch cleaning for two bunch needs to be incorporated in Top-off
- Most experiments do not see injection transients
- Some (especially microscopes with short integration times) do see them and will make use of provided gating signals
- 500 mA is good compromise (minimum upgrade to beamline optics)
- 20 – 30 pm vertical emittance is close to limit for best beamline optics (sagittal focusing)

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Impact of injection transients

- Incoming beam is only small fraction of total intensity
- Its unavoidable oscillations are no problem
- Injection elements also perturb stored beam
- Non-closure of fast bump
- Stray field of pulsed septum magnets
- Potential influence of booster
- Conducted experiments with users and measured transients using BPMs (fast and turn-by-turn)
- Results:
-  Most experiments insensitive to any distortion (protein crystallography, PEEM, most spectroscopy beamlines)
- Very few experiments (STXM, IR) see no-closure of bump and will require gating (multibunch feedbacks help)
- All experiments that see transients can use gating
- Some examples on the following slides

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Effect of the Bumps

- RMS Beam sizes are
- 300 by 23 (later 8) microns

- Transverse feedback system reduces the duration of the transients

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Septum Stray Field Reduction

** ****With full sine current pulse show that slowly decaying eddy currents from first and second half sines mostly cancel. **
**Delayed stray field using ‘full sine’ excitation reduced by factor of 10!**

**Septum Stray Field Reduction**

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## EPU effects

**Variation of on axis field integrals with EPU phase (****causing orbit distortions****). **
**Variations of the (mostly ****vertical) beamsize**** (both with gap and with phase):**
**Due to focusing changes (systematic focusing terms from the bulk of the undulator). **
**Due to coupling terms (skew quadrupole like or solenoid like). **
**Higher order effects impacting the ****dynamic**** (or momentum) ****aperture****, for example due to the field roll-off, which is quite significant and systematic in circular polarization mode. **

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Beamsize Stability

- Receive more inquiries about beamsize than orbit stability!
- Low beam energy, already pretty good orbit stability
- Vertical beamsize variations due to EPU motion were big problem.
- Is caused by skew quadrupole (both gap and row phase dependent)
- Search for root cause still underway.
- Installed correction coils for feedforward based

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## RF phase noise

- Mode 0 motion nowadays is very small – 0.03 degrees rms
- Dominated by noise from master oscillator, rf distribution system, rf frequency correction ... not HVPS
- Fast RF amplitude feedback reduces effect of HVPS to this level
- Use improved master oscillator + filtering at several points in low level RF frequency distribution system

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Magnitude of mode zero motion 

- ‘Bad’ case corresponded to an energy oscillation of 3*10-5 resulting in position oscillations of about 1.5 m and angle changes of 4 rad at source point
- Problem periodically reappeared and needed to be fixed again
- Harmonic cavities lowering mode zero frequency further – into range of amplification due to LFB
- RF frequency feedback introducing DAC noise

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007

## Summary

- Users are very happy with current orbit stability at ALS and handle feedback based on photon beam monitors themselves
- Fast orbit feedback brought significant improvement for frequencies between 0.1 and 80 Hz.
- Preparing for top-off
- Studied and minimized transients with users
- Users helped define scope of upgrade
- For ALS, beamsize stability often is bigger issue than orbit
- Are continuing to improve stability
- Short Term (this year):
- More BPMs for feedbacks
- Faster update rate for fast feedback (goal is 4 kHz)
- Medium Term:
- Slowly installing more photon BPMs (so far not in orbit feedback)
- Started to think about path beyond Bergoz BPMs
- Tested some means of measuring physical BPM positions relative to ground slab – plan to eventually include in feedback

- Christoph Steier, NSLS-II workshop

- April 18-20, 2007