Beam Dynamics in MeRHIC

Mike BlaskiewiczOn behalf of MeRHIC/eRHIC working group

Outline

• Linac Design and BBU Study• Energy losses and compensation• Electron Errors• Coherent Synchrotron Radiation

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Linac 1

Linac 2

Main ERLs; 6 cryomodules x 6 cavities x 18 Mev/cav = 0.65 GeV per linac

0.75, 2.05, 3.35 GeV

4 GeV

0.1, 1.4, 2.7 GeV

Pre-accelerator 90 MeV ERL

Electrongun 0.1 GeV

• Bunch: Qb=5 nC, σz=2mm

• Einj/Emax = 100MeV / 4GeV

• 3 acc./decel. passes • N cavities = 72 (total)• L module/period = 9.6 / 11.1m• Ef = 18.0 MeV/cav• dE/ds ~ 10 MeV/m

Linac Constant Gradient Quads

(E. Pozdeyev)

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Arcs not shown

Linac Scaled Gradient QuadsLinac 1

Linac 2

Main ERLs; 6 cryomodules x 6 cavities x 18 Mev/cav = 0.65 GeV per linac

0.75, 2.05, 3.35 GeV

4 GeV

0.1, 1.4, 2.7 GeV

Electrongun 0.1 GeV

Gmax ~ 500 G/cmQuad strengthGmin ~ 100 G/cm

Scaling gradient with energy Scaling gradient with energy produces more focusing and produces more focusing and increases BBU thresholdincreases BBU threshold

(E. Pozdeyev)

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Beam Breakup

Modes calculated with CSTSimulations using GBBU20 ms simulation timeSingle mode at a time with

varying spreads in resonant frequency.

One degree (+360n) of betatron phase advance in arcs is optimal.

Constant gradient quads.

(J. Kewisch)

Energy loss and compensation for dog bone design

Racetrack design improves things

Increase in normalized emittance and energy spread are significantly reduced in the new racetrack lattice.

Any corrections should be easier.

x

e

e Icm

cr5

6

2348

55

3/2 242 IcmrE ee

Y. Hao

Beam losses

• Touschek– Total loss beyond ±6 MeV is 200 pA. – Small but, maybe, not negligible. We will look more

carefully.• Scattering on residual gas (elastic)

– Total loss beyond 1 cm aperture at 100 MeV is 1 pA– Negligible

• Bremsstrahlung on residual gas– Total loss beyond ±6 MeV is < 0.1 pA– Negligible

(A. Fedotov, G. Wang)8

MeRHIC - CSR effect after passing 10 arcs with local bending radius of 6.2m and 1 arc with 7.2m (100MeV is not included)

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rms bunch length s=2mm

(no shielding)

s=4mm

(no shielding)

s=2mm

(h=2cm)

s=2mm

(h=1cm)

Energy loss: -E, MeV 8 3.2 2.1e-5 8e-18

-E/E (relative energy loss, at 100MeV – our lowest energy arc)

0.08 0.03

RMS energy spreadErms, MeV 5.7 2.26

E/E)rms (relative energy spread, at 100MeV)

0.06 0.02

Shielding suppression factor =Pcoh (shielded)/

Pcoh( unshielded)

2.6e-6 1e-18

A. Fedotov

Some issues with CSR

• One experiment did not show expected theoretical reduction (with shielding) even in energy loss due to CSR.

• 2. Another experiment studied synchrotron radiation rather than effects on the beam – also some issue were reported, like disagreement with theory for small gap sizes, etc.

• While there seems to be a clear picture about suppression of CSR power loss with shielding, effect of shielding on energy spread is less transparent.

• Transient effects.

• Simple, well-controlled experiment is desired to address these issues. ATF@BNL is ideally suited for such an experiment.

A. Fedotov

ATF@BNL proposal (April 2009)

Team:

A. Fedotov, D. Kayran, V. Litvinenko (C-AD, BNL), P. Muggli (USC), V. Yakimenko (ATF, BNL), others Experimental goal: To have a quantitative study of CSR suppression with shielding due to

vacuum chamber. Measurements will be compared with detailed simulations of CSR which will including shielding and transient effects.

A. Fedotov

Experiment description• Construct and install a system of two vertical plates with controllable gap

between the plates to be placed inside the vacuum chamber of bending magnet.

• Beam parameters will be chosen to enhance CSR effect without shielding.• Energy loss and energy spread will be measured for various values of the

gap between the plates.• Measurements will be done both for Gaussian and square-shape

longitudinal beam profiles.• Measurements will be compared with detailed simulations.

Experiment: approved May 2009Constructed: September 2009Measurements: ongoing

A. Fedotov

Work in progress

V. Yakimenko, APEX09, November 2009

Electron beam fluctuations

The correlation relation leads to a Lorentz distribution frequency spectrum 1/(α2ω0

2+ω2), ω0 is the RHIC revolution frequencyMore realistic than white noise.

Reduction factor R

A quad example:α = 0.06 Q = 0.685 R = 0.06

(C. Montag, M. Blaskiewicz) 14

Ion emittance growth for fluctuations in electron bunch charge/emittance

steering fluctuations

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bbQ

Conclusion

• Main Linac design nearly developed– Constant gradient: weak identical quads, similar arcs,

sufficiently high BBU threshold – Scaled gradient: higher BBU threshold, baseline

• No showstoppers have been found in beam dynamics studies.

• Work to do– Linac details– Ion trapping and countermeasures– Check CSR issues– Electron Noise, what is the spectrum?

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