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Stability Issues at the ALS

Stability Issues at the ALS

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Stability Issues at the ALS

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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