eRHIC Main Linac Design

E. Pozdeyev + eRHIC team

BNL

May 19, 2008 E. Pozdeyev, BNL 2

Outline and Design Parameters

PHENIX

STAR

e-ion detector

eRHIC

Four recirculation passes

Main ERL (1.9 GeV)

Low energy recirculation pass

Beam dump

Electronsource

Possible locationsfor additional e-ion detectors

Energy, GeV 10

Bunch spacing, ns 71

Bunch intensity, 1011 1.2

Beam current, mA 270

rms, μm, normalized 80

Rms bunch length, cm 1

Polarization, % 80

• Npass = 5 up + 5 down

dE/ds = 8 – 8.5 MeV/m

• L = 230 m

May 19, 2008 E. Pozdeyev, BNL 3

Linac Design

cryomodule

D F D11.9 m 1m

D FF F

• Nfc = 6 (per module)

• N3h = 2 (per module)

• N modules = 18 dE/ds = 8 – 8.5 MeV/m

• Ef = 19.5 MeV/m

• E3h = 19.0 MeV/m

• G = 340 Gauss/cm

• Lq=20 cm

• μ0 = 90º

703.75 MHz5 cell, 1.4 m with dampers

2.1 GHz5 cell, 0.75 mwith dampers

quads

Cryomodule

May 19, 2008 E. Pozdeyev, BNL 4

Optical Functions and Beam Size

-functions in the linac (m)5 up + 5 down,

unity recirculations (not shown)

Beam size in the linac (mm)5 passes up

unity recirculations (not shown)

May 19, 2008 E. Pozdeyev, BNL 5

Multipass transverse BBU

Beam Breakup as a function of the HOM frequency spread• 72 modes per cavity• simulated and measured modes in copper model with HOM absorbers• 5 random seeds x 2 HOM orientations = 62 fHOM distributions• no specific optimization of beam optics to maximize BBU threshold

I=270 mA

May 19, 2008 E. Pozdeyev, BNL 6

Energy Loss / Spreadcaused by the longitudinal wake

Monopole wake field simulated by ABCI.Fundamental wake is the convolution of the cosine wake with the charge distribution.Supposedly, the fundamental wake is recovered.

Loss factor (no fundamentalwake): k|| = 0.57 V/pC

Average energy loss per e: dEloss = -12.3 MeV

Full energy spread: dEspread = 21.3 MeV

May 19, 2008 E. Pozdeyev, BNL 7

Compensation of the energy spreadEnergy loss (12.3 MeV) can be compensated only by off-phasing or by anadditional cavity without recovery. The energy spread can be reduced if the beam phase width is increased and beam is matched to the RF wave.

Smal d, Large dE

Large d, fits the RF wave -> small dE

The optimized phase width can be estimated as

The energy spread compression can be estimated as

V

Ef

1arccos

RFb

i

f

i

i

f

lE

E

For Ei = 21.3 MeV and V=100 MeVestimated f ~ 38º.

The initial bunch phase width for the fundamental RF is ~ 35º.

Longer wavelength RF is required to reduce the energy spread.

dE compensation has to be done at lower energies (~100 MeV). The lowfrequency RF can be used up to E ~ 100 MeV.

May 19, 2008 E. Pozdeyev, BNL 8

Compensation of the energy spread

λ = 1.7 m (~175 MHz)V = 100 MeVm56 = -60m566 = -235(in RF degrees)

Energy spread compressed by5.8 times (21.3 -> 3.68 MeV)

Compensated energy spread as a function of RF frequency.

Note 3rd harmonic RF can increasethe suppression ratio.

May 19, 2008 E. Pozdeyev, BNL 9

Minimum Turn-On time

sec16.0min, onturnt

Assuming the maximum current ramp rate is limited by the available RF power

RF

revbaponturn

revapbeam

revn

revi

n

i

n

i

n

ni

n

ibeam

RFbeam

P

TVInt

Tt

IVnP

Tt

InnnIS

nprogressioarithmetict

ITiII

IIVIIVP

PP

apapap

ap

ap

max,2

min,

2

0

0

2

11

2

11

d

dd

d

2

)1(d

d)1(

2

0

Assuming V=20 MV, PRF=10 kW, Trev=13 μs, Ibeam=280 mA, nap=5

May 19, 2008 E. Pozdeyev, BNL 10

R&D items

• Strong ions beam cooling (CEC, for example) can reduce the required electron current and alleviate intensity related effects - R&D on the ion beam that can benefit e-linac design

• Compact, multi-cell cryomodule without sacrificing HOM damping efficiency

• Other linac optics options (smaller -function, “concentrated” 3rd harmonic in designated cryomodules, etc. )

• Increase BBU threshold

• Lower frequency RF to increase the bunch length and possibly drop the 3rd harmonic

May 19, 2008 E. Pozdeyev, BNL 11

Other Linac Setup scenarios

2 x 200 m SRF linac10-12.5 MeV/m4-5 GeV per pass

5 (6) verticallyseparated passes

eSTAR

ePHENIX

New tunnel construction can be expensive. Linac can be constructed in RHIC tunnel.