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Enhancing Stability at ALS Accelerator Physics Group

Learn about addressing stability issues at ALS, including orbit feedback, energy, and beamsize stability. Explore measures taken to ensure stability, minimize noise, and optimize instrumentation for precise control. Discover user requirements and achieved stability improvements at ALS.

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Enhancing Stability at ALS Accelerator Physics Group

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  1. 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

  2. Aerial view of the Advanced Light Source Christoph Steier, NSLS-II workshop

  3. ALS Parameters and Beamlines 1/10 Electron Beam Size  Christoph Steier, NSLS-II workshop

  4. BPM, Corrector locations • 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 Christoph Steier, NSLS-II workshop

  5. What has been done at the ALS to maximize 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) • 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!) • 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. Feed-forward example: EPU COMPENSATION 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 Mechanically the EPU can move from left to right circular polarization mode in ~1.6 seconds 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. Christoph Steier, NSLS-II workshop

  11. EPU FEED FORWARD ORBIT CORRECTION Orbit Error without Feed Forward Correction 200 Hertz Feed Forward Correction Christoph Steier, NSLS-II workshop

  12. 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

  13. 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

  14. 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

  15. 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

  16. Fast Feedback Layout • Motivation: Fast Orbit stability with passive measures already very good (2-4 microns rms). Improvement into <mm range required active/fast feedback • 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. Christoph Steier, NSLS-II workshop

  17. 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

  18. 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 ½ 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

  19. 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

  20. 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 ½ of SVs used). • System is set up conservatively at the moment – no excitation at higher frequencies. Christoph Steier, NSLS-II workshop

  21. 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

  22. 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

  23. 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

  24. Summary of user input for top-off • DI/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

  25. 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

  26. 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

  27. 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! Christoph Steier, NSLS-II workshop

  28. 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

  29. 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

  30. 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

  31. Magnitude of mode zero motion • ‘Bad’ case corresponded to an energy oscillation of 3*10-5 resulting in position oscillations of about 1.5 mm and angle changes of 4 mrad 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

  32. 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

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