Measurement and Control of Charged Particle Beams in the Relativistic Heavy Ion Collider

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 16th , 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 GeV/n for ions, 250 GeV for protons); the only high energy polarized proton collider Stanford Linear Collider, e+ e- (1989 – 1998) 2 miles 0.75 miles 1.2 miles TeVatron, p+p (1987 – 2011) RHIC (>2001)

Relativistic Heavy Ion Collider (RHIC) RHIC COLLISIONS INJECTION ACCELERATION PHENIX STAR RHIC consists of 2 separate superconducting accelerators, 2.4 miles (3.8 km) long LINAC Booster EBIS AGS Tandems RHIC beams: 110 bunches, each bunch contains ~1E9 ions or 1E11 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 RHIC performance (ions or protons) is characterized by the rate at which particles collide, the N1 N2 Luminosity ~ f Ncol Sx Sy N N is the number of colliding bunches Ncol Sy Sx is the collision frequency f

Maximizing the scientific output of RHIC 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

Challenges - orbits bunches should collide head-on to maximize collision probability offsets between the bunches degrade luminosity 40% loss if Dx = 100 mm (Dy = 0) zoom L / L0 = e-(Dx/4Sx)2e-(Dy/4Sy)2 Dx correction factor 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 ideal trajectory 9 the tune, Q 10 8 7 11 6 equals the number of oscillations made by a bunch in one revolution around the accelerator 12 5 13 4 3 0 1 2 we monitor and control the fractional tune 13.5 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 resonance condition: m Qx + n Qy = p order 0 - driven by dipole magnets order 0 - driven by dipole magnets order 0 - driven by dipole magnets resonance diagram resonance diagram resonance diagram resonance diagram resonance diagram resonance diagram (m, n, and p are integers) first order Qy Qy Qy Qy Qy second order third order the (fractional) tunes should be irrational Qx Qx Qx Qx Qx

Challenges “working point” in RHIC (protons, at full energy) zoom beam 1 beam 2 bounded by strong 3rd and 4th order resonances for polarized proton operation, the resonance at 0.70 is critical during acceleration the operating point is therefore moved during acceleration

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

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 ynew Xnew Xold precision of average orbit measurements improved by > factor 10

4 km full scale at RHIC we use 600 BPMs (150 /plane) to measure the orbits along accelerator 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) Mij x = M q x x = vector of ~ 320 BPM measurements M = matrix of transfer functions • = vector of angular deflection • of ~ 230 correctors Mij is the transfer matrix between the ith BPM and jth corrector dipole Dq = M-1Dx

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) ~400 mm no feedback acceleration start 250 GeV collisions with feedback ~20 mm orbits well controlled, reproducible, and well below the 200 mm tolerance

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

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 8 periods ………………………………... (repeat) ……………………………..…….……… 1 (possibly corrupted) period used BBQ/BTF 1 period used for BBQ/BTF 1 period used for BBTF 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: …………………………………….………………..... (repeat) ……………………………..…………………….……… average of 8 periods used for BBQ/BTF

multiple super- imposed ramps C tunes and coupling well controlled, reproducibility is excellent

Impact on RHIC performance (1) Accelerator availability Time required to successfully accelerate beams to full energy reduced from > 3 days to 2 hours ~ $ 100k savings for initial beam setup ~ $ 100k per operational mode change - particle species, energies or optics (3-4 per fiscal year) ~ $ 100k 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 program-med 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.

Acknowledgements BPM support R. Hulsart, P. Cerniglia, A. Marusic, K. Mernick, , T. Satogata, P. Thieberger R. Michnoff Orbit feedback T. D’Ottavio, A. Marusic, V. Ptitsyn, G. Robert-Demolaize Tune/coupling feedback A. DellaPenna, M. Gasior (CERN), L. Hoff, R. Jones (CERN), , C. Schultheiss, C.Y. Tan (FNAL), S. Tepikian P. Cameron, Y. Luo, A. Marusic A. Curcio, C. Dawson, C. Degen, Y. Luo, G. Marr, B. Martin, P. Oddo, T. Russo, V. Schoefer, A. Marusic, K. Mernick, M. Wilinski Chromaticity feedback A. Marusic, S. Tepikian Energy feedback A. Marusic, K. Smith 10 Hz feedback P. Cerniglia, A. Curcio, L. DeSanto, C. Folz, C. Ho, L. Hoff, , C. Liu, Y. Luo, W.W. MacKay, G. Mahler, W. Meng, , C. Montag, R.H. Olsen, P. Popken, V. Ptitsyn, G. Robert-Demolaize, and many others R. Hulsart K. Mernick, R. Michnoff P. Thieberger 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 to automate well-defined processes to reduce sensitivities to external influences WHY compare measurement with desired value apply correction HOW cruise control apply required change in gas difference desired speed cruise measured speed maintain steady conditions GOAL

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

10 Hz feedback at RHIC reduce orbit changes due to triplet magnet vibrations collision point triplet triplet x (mm) time 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 longitudinal motion and may be defined as xx,y = DQx,y/ (Dp/p) where DQ = the spread in betatron tunes within the bunch of momentum spread Dp/p Equivalently the chromaticity may be expressed as xx,y = - (a - 1/g2) DQx,y/ (Dfrf/frf) or xx,y ~ DQx,y/ (Dfrf/frf) where DQ is the change in betatron tune with change in accelerating frequency frf . We use this form for measurement of the chromaticity: we measure the change in tune with applied change in accelerating frequency. Corrections are applied to the sextupoles (no skew sextupoles to date). Chromaticity Feedback: WHY • initially thought to be a requirement for operational tune feedback (tune peak too • broad and flat if x too large) • beam stability requires x < 0 below transition energy and x > 0 above • 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 fD) 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

Measurement and Control of Charged Particle Beams in the Relativistic Heavy Ion Collider