Beam-Beam and e-Cloud in RHIC

Oct. 6, 2015

Haixin Huang, Xiaofeng Gu, Yun Luo

22

Electron Cloud Issue in Early Years

The first two attempts to fill both rings with 110 bunches, twice the design number (October 2001). Intensities of the two RHIC rings, named Blue and Yellow are shown in part (a) and the pressure measured at four gauges in an interaction region is shown in part (b).

33Electron Cloud Evidence

Pressure rise in the PHOBOS experimental area after rebucketing with 56 bunches. The beam intensities in the Blue and Yellow ring (a) slowly decay during a store, and the pressure measured in 2 gauges (b) drops sharply after some time. With high pressure the experimental background (c) is increased.

44

Tune Shift Consistent with Electron Cloud

Coherent tunes of the last injected bunch along a train of 110 proton bunches with 108 ns spacing in the Yellow ring. Because of coupling both transverse tunes are visible.

55Reduce Pressure Rise with Beam Scrubbing

Beam scrubbing during the 2007 Au operation. Part (a) shows the Blue and Yellow intensities over 2 hours when the machine was repeatedly filled with the highest available intensities. Part (b) shows the pressure near the warm rf (labeled g4) and the Blue and Yellow polarimeters (labeled bi12 and yo12), the 3 locations with the highest pressure. The dynamic pressure at these locations is reduced by more than an order of magnitude after scrubbing.

66Adding More NEG Pipes in RHIC over Years

Dynamic pressure in the 12 Blue warm straight sections (top), measured by a single gauge in each, while proton beam with 108 ns bunch spacing is filled (bottom). With completely NEG coated pipes, the pressure in 3 sections in 2005, and 5 sections in 2006, remained at 10-11 Torr.

Cure: baking to 200C, NEG(non-evaporable getter) coating(very effective), scrubbing(take long time, but effective)

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p-p lumi is limited by beam-beam. Oncoming proton beam acts as a nonlinear defocusing force which leads to beam decay, emittance increase

It caused incoherent tune shift pending on the betatron amplitudes, which results in much larger tune footprint.

Beam-Beam Effects

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Beam-Beam Limitation in RHIC

• To further increase Luminosity in the p-p run we would like to increase bunch intensity and decrease β*.

• The upgrade of the polarized source is done which increases the Booster input proton current by a factor 3.

• There is no enough tune space between 2/3 and 7/10 to hold the large beam-beam tune spread when proton bunch intensity is higher than 2.0 × 1011.

Vertical Beam Transfer Function

2/3 7/10

Vertical tune

Am

pli

tud

e

99

With linear approximation, electron beams provide equal but opposite focusing force.

Carefully tailored electron beams (same beam size, proper intensity) provide equal but opposite focusing force. A new e-gun with bigger cathode was installed for 100GeV run.

Beam-Beam Compensation Principle

1010

Beam-beam Tune Spread

1.5X1011.

2.5X1011.

2X1011.

3X1011.

1111

Beam-beam Tune Spread (HBBC)

1.5X1011.

2.5X1011.

2X1011.

3X1011.

1212

Beam-beam Tune Spread Reduction (FBBC)

For 2.5X1011 Intensity. Smaller tune footprint but a lot unstable particles (lighter colors)

1313

Beam-beam Tune Spread Reduction

• Simulation condition: bunch intensity 2.0×1011, with half beam-beam compensation (HBBC) and full beam-beam compensation (FBBC). HBBC and FBBC compensate half and full beam-beam parameter respectively.• Head-on beam-beam compensation reduces the size of tune footprint.• Dynamic aperture study leads to use HBBC.

1414

Dynamic Aperture (HBBC vs. FBBC)

Left: HBBC; right: FBBC. The aperture is lower for FBBC case. Choose HBBC instead.

7/10 2/3

15

15

• BPMs in both lenses to bring e- and A- beam in proximity (transverse electron beam position for blue and yellow, electron beam angle steering for yellow)

• 1) BPMs and electron beam; 2) BPMs and ion; 3) eBSD and ion beam;\

• backscattered electron detector to maximize overlap P. Thieberger, BIW12, IBIC2014

eBSD, ion beam and Lisa BPMs, E-beam, E-lens

Beam Alignment between e and p Beams

16

100 GeV pp Stores in 2012 and 2015 (Typical Good)

2012 2015

intensitybb parameter +50%

rms emittance

peak luminosity +120%

300

5

140

12

4

[2012: 9 pb-1/week 2015: 25 pb-1/week]

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Max Beam-Beam Parameter with and w/o e-lens

max,2pp (2012)

max,2pp+pe (2015)

max,2pp (2015)?intensities

beam-beam parameters /IP

• lenses should turn off at max,2pp to avoid additional beam losses

• max,2pp also determines luminosity gain from lattice alone, and additional luminosity gain from e-lenses

• determination of max,2pp would require about several stores of operation without lenses (not done for fear of luminosity loss, only 11-week run)

• Need to estimate max,2pp from available data (still in progress)

18

18

RHIC Operation with e-lenses

1. Turned on for every store (45~60 min.)2. High reliability: available for all stores after running

E-beam current

Intensity

Emittance (IPM)

E-beam current

Emittance (IPM)

Tune

19

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Beam-beam Compensation Demonstrated(HBBC)1) Head-on beam-beam compensation scheme in operation in

2015,consisting of new lattice (RDT compensation) + e-lenses (DQ compensation)

Lpeak +150% Lavg +90% => demonstrated

2) Beam-beam tune spread compensation without additional emittance growth (<400mA), and small additional loss rate (1−2%/h) => demonstrated

3) Resonance driving terms compensation ? => TBD4) Higher /IP possible, presently constrained by injectors

(potential for ~2× L in future runs) => demonstrated

1) More studies for emittance growth with high current (2017) => TBD

6) The tune spread does not go smaller when push e beam intensity higher (FBBC)

=> Tune spread is dominated by nonlinearity in RHIC

Q reductionfrom e-lens

no bb

2x bb

20

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Summary1) Electron cloud has been observed in RHIC

since early operation. Scrubbing has been used when needed (after some new device installation, such as RF cavity, polarimeter target installation). The NEG coating for warm section eventually limited the e-cloud to a manageable level. Still, the STAR solenoid magnet is necessary to have e-cloud there under control.

2) After several years commissioning, RHIC e-lenses work as planned. The average store luminosity is increased by 90%, exceeding the project goal.