Measurement and Control of Charged Particle Beams in

Measurement and Control of Charged Particle Beams in the Relativistic Heavy Ion Collider Michiko Minty Instrumentation Systems Group Leader Collider-Accelerator Department Brookhaven National Laboratory ESS/AD seminar - April 16 th , 2014

OUTLINE The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

High Energy Colliders in the US RHIC: versatile collider in terms of species (p, d, Cu, Au, U, …) and beam energies (maximum of 100 Ge. V/n for ions, 250 Ge. V for protons); the only high energy polarized proton collider Stanford Linear Collider, e+ e- (1989 – 1998) 1. 2 miles Te. Vatron, p+p RHIC (1987 (>2001) – 2011) 0. 75 miles 2 miles

Relativistic Heavy Ion Collider (RHIC) RHIC COLLISIONS INJECTION ACCELERATION PHENIX STAR LINAC Booster EBIS AGS RHIC consists of 2 separate superconducting accelerators, 2. 4 miles (3. 8 km) long Tandems RHIC beams: 110 bunches, each bunch contains ~1 E 9 ions or 1 E 11 protons RHIC bunches are guided and focused using ~ 1750 superconducting magnets RHIC bunches are very small (~100 mm at interaction points) RHIC bunches circulate ~ 80, 000 times per second

The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

Maximizing the scientific output of RHIC performance (ions or protons) is characterized by the rate at which particles collide, the Luminosity NN Sx ~ N 1 N 2 Sx Sy Ncol Sy f Ncol f is the number of colliding bunches is the collision frequency

Maximizing the scientific output of RHIC performance with protons is also characterized by the beam’s polarization spin Uhlenbeck and Goudsmit (1926): protons possess a spin angular momentum the spin of a proton responds like a magnetic dipole; it precesses in magnetic fields at RHIC we preserve the average orientation of all the proton’s spins, the polarization

The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

Challenges - orbits bunches should collide head-on to maximize collision probability om offsets between the bunches degrade luminosity zo L / L 0 = e -(Dx/4 Sx)2 e 40% loss if Dx = 100 mm (Dy = 0) -(Dy/4 Sy)2 correction factor Dx for position errors beam’s orbits (positions and angles) must be controlled

Challenges The stability of beams in a circular accelerator depends on the so-called “tune” of the accelerator oscillations about the 9 ideal trajectory 10 8 7 11 6 12 13 0 13. 5 1 2 3 4 5 the tune, Q equals the number of oscillations made by a bunch in one revolution around the accelerator we monitor and control the fractional tune Q in this sketch, the vertical (y) tune is Qy = 13. 5 and Q = 0. 5

Challenges Resonances! These characterize the tendency of a system to oscillate at a greater amplitude at certain frequencies of excitation improperly timed pushes … … will not rock the chair properly timed periodic forces … … will rock the chair if the forces are too large … ………… In accelerators, resonances must be avoided

Challenges In an accelerator, resonances can occur if perturbations act on a bunch in synchronism with its oscillatory motion. The errors arise from imperfections (or misalignments) of the magnets m Qx + n Q y = p (m, n, and p are integers) order 0 - driven by resonance diagram dipole magnets first order Qyy Q resonance condition: second order third order Q Qxx the (fractional) tunes should be irrational

Challenges “working point” in RHIC (protons, at full energy) beam 1 zoom beam 2 bounded by strong 3 rd and 4 th order resonances for polarized proton operation, the resonance at 0. 70 is critical during acceleration the operating point is therefore moved during acceleration

The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

WHY AT RHIC time of day 23: 50 yrms during acceleration, run-9 10: 20 14: 30 17: 40 22: 55 correct magnetic field errors - including power supply variations, bit limitations, response starttime and magnet alignment errors acceleration (unavoidable) persistent currents and hysteresis effects thermal effects end time (s)

The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

Precision beam position measurements “stripline” beam position monitor (BPM) v vacuum chamber 23 cm length

added digital equivalent of a single-pole, low pass filter (IIR filter) to effectively average out predominantly ~ 10 Hz variations in the closed orbit yold Xold ynew Xnew precision of average orbit measurements improved by > factor 10

at RHIC we use 600 BPMs (150 /plane) to measure the orbits along accelerator 4 km full scale zoom 400 m full scale +/- 60 microns full scale precision of measurement now ~ 5 mm (smaller than the diameter of 1 red blood cell)

BPM data delivery before Run-10 acquisition rate: nominally 0. 5 Hz nondeterministic After Run-10 acquisition rate: 1 Hz deterministic

Orbit Feedback measurement based on existing beam position monitors using new and improved algorithm for measuring average orbit using original survey (e. g. offset) data deterministic data delivery feedback design orbit correction algorithm (“singular valued decomposition”) extended to application at 1 Hz rate during energy ramp reference orbits specified in terms of BPM data (not corrector strengths) x Mij x=Mq x = vector of ~ 320 BPM measurements M = matrix of transfer functions Mij is the transfer matrix between the ith BPM and jth corrector dipole q= vector of angular deflection of ~ 230 correctors Dq = M-1 Dx

proof-of-principle for orbit feedback using existing infrastructure (2010) energy feedback principle improved (2011) constrain average horizontal corrector strengths use all arc BPMs for energy offset determination implementation of orbit and energy feedback on all ramps (2011) no feedback ~400 mm collisions 250 Ge. V acceleration start ~20 mm with feedback orbits well controlled, reproducible, and well below the 200 mm tolerance

The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

Precision tune measurements apply (using a “kicker”) a broadband excitation near the beam’s natural frequency the fractional tune, Q, is the beam responds at it’s natural resonant frequency, fres frev = (known) revolution frequency “kicker” BPM frequency generator signal processing fres measurement precision: 2 E-5 Q = fres / frev

Precision coupling measurements resonance-free region has Qx ~ Qy … precisely where coupling effects are strongest with Qx ~ Qy and nonzero coupling (C /= 0) beam control in one plane affects the other and produces unexpected results C

run-08 run-11 > factor 10 improvement in measurement resolution

Tune and Coupling Feedback measurement based on direct-diode detection (BBQ = base-band tune) for precision measurements - M. Gasior , R. Jones (2005) feedback design uses methodology of coupling angle measurement – Y. Luo (2004) distinguishes between eigenmodes - R. Jones, P. Cameron, Y. Luo (2005 ) history demonstrated at RHIC in 2006 - P. Cameron et al (2006) successfully applied for all ramp developments in 2009 used regularly by operations for ramp development in 2010 used together with orbit and energy feedback for all ramps in 2011

before: 8 periods 1 period used for BBQ/BTF 8 periods 1 period used for BBTF ………………. . . (repeat) ………………. . ……… 1 (possibly corrupted) period used BBQ/BTF 1 in 16 periods of data (AFE output, I/Q demodulator input) used for BBQ/BTF intermittent corruption of this data due to CPU-limits and data overwrites with BBTF (ADOs removed) after: average of 8 periods used for BBQ/BTF …………………. . . (repeat) ………………. ………

C multiple superimposed ramps tunes and coupling well controlled, reproducibility is excellent

The Relativistic Heavy Ion Collider (RHIC) Maximizing the scientific output of RHIC Accelerator physics challenges Feedback-based beam control orbits tunes and coupling Impact on RHIC performance Summary

Impact on RHIC performance (1) Accelerator availability Time required to successfully accelerate beams to full energy reduced from > 3 days to 2 hours ~ $ 100 k savings for initial beam setup ~ $ 100 k per operational mode change - particle species, energies or optics (3 -4 per fiscal year) ~ $ 100 k eliminated need for dedicated re-optimization efforts at least 1 extra week for physics operation with electrical costs at 25 MW at $60/MW-hr

(2) Operation under extreme conditions: near-resonance acceleration end of acceleration 2/3 resonance DQy= 0. 006 With routine orbit, energy, tune, and coupling feedback on every acceleration of protons to high energies, the vertical tune could be lowered towards dangerous 2/3 orbital resonance (and away from spin resonance at 7/10).

since run-9 25 % increase in relative polarization of each beam equivalent to 14 additional weeks of RHIC operations ($3. 5 M) for same level of statistical uncertainty for physics program

(3) acceleration/deceleration A dedicated study was performed to confirm the degree of residual polarization loss during acceleration executed with complete suite of feedbacks demonstrating fully automated beam control, enabled an otherwise impossible experiment

Summary The resolution of all measurements (beam position, energy deviation, tune, coupling, and chromaticity) has been improved by more than a factor of 10 and is nearing the limitations of the instrumentation Control of the parameters affecting beam properties during acceleration in RHIC has transitioned from being pre programmed to based on measurements of the beam’s properties Feedback-based beam control is now the norm: all beams in RHIC are now established using orbit, tune, coupling and energy feedback. Precision control of these parameters has expanded the parameter space accessible during acceleration. This allows for more extreme operating conditions and is now essential for polarized proton operation.

BPM support Acknowledgements P. Cerniglia, R. Hulsart, A. Marusic, K. Mernick, R. Michnoff, T. Satogata, P. Thieberger Orbit feedback T. D’Ottavio, A. Marusic, V. Ptitsyn, G. Robert-Demolaize Tune/coupling feedback P. Cameron, A. Della. Penna, M. Gasior (CERN), L. Hoff, R. Jones (CERN), Y. Luo, A. Marusic , C. Schultheiss, C. Y. Tan (FNAL), S. Tepikian A. Curcio, C. Dawson, C. Degen, Y. Luo, G. Marr, B. Martin, A. Marusic, K. Mernick, P. Oddo, T. Russo, V. Schoefer, M. Wilinski Chromaticity feedback A. Marusic, S. Tepikian Energy feedback A. Marusic, K. Smith 10 Hz feedback P. Cerniglia, A. Curcio, L. De. Santo, C. Folz, C. Ho, L. Hoff, R. Hulsart , C. Liu, Y. Luo, W. W. Mac. Kay, G. Mahler, W. Meng, K. Mernick, R. Michnoff , C. Montag, R. H. Olsen, P. Popken, V. Ptitsyn, G. Robert-Demolaize, P. Thieberger and many others Run coordinators M. Bai, K. Brown, H. Huang, G. Marr, C. Montag, V. Schoefer Operations G. Marr, V. Schoefer , R. Smith, J. Ziegler Management W. Fischer, T. Roser

Accelerator Reproducibility parameter no feedback

Feedback WHY to automate well-defined processes to reduce sensitivities to external influences HOW compare measurement with desired value apply correction cruise control desired speed difference apply required change in gas measured speed GOAL maintain steady conditions cruise

tune/coupling feedback at RHIC kicker BPM frequency generator signal processing phase lock loop QF QD conversion to currents model Qx Qx Qy desired Qx desired Qy

10 Hz feedback at RHIC reduce orbit changes due to triplet magnet vibrations collision point x (mm) triplet time

10 Hz feedback at RHIC history < run-9 feedback on relative beam positions run-10 new 10 Hz feedback, proof-of-principle with new correctors high speed daughter cards for BPMs dedicated networking digital signal processing run-11 routine application

higher integrated luminosity essential for (possible future) RHIC operation with near-integer tunes

Chromaticity Feedback The chromaticities, xx and xy represent the coupling of transverse and lo motion and may be defined as xx, y = DQx, y/ (Dp/p) where DQ = the spread in betatron tunes within the bunch of momentum Equivalently the chromaticity may be expressed as xx, y = - (a - 1/g 2) DQx, y/ (Dfrf/frf) or xx, y ~ DQx, y/ (Dfrf/frf) where DQ is the change in betatron tune with change in accelerating fre We use this form for measurement of the chromaticity: we measure the tune with applied change in accelerating Corrections are app Chromaticity Feedback: frequency. WHY sextupoles (no skew sextupoles to date). 1) initially thought to be a requirement for operational tune feedback (tune peak too broad and flat if x too large) 2) beam stability requires x < 0 below transition energy and x > 0 above 3) dynamic aperture issues if x too large

Chromaticity Feedback: HOW tune/coupling feedback chromaticity feedback Measurement: Vary rf frequency (specifically, add a frequency modulation of amplitude Dfrf with periodicity f. D) and measure tunes With feedback (and T/C feedback too), the corrections sent to the quadrupoles (the “filtered tunes”) are used as input to the measurement algorithm