Post on 05-Feb-2016
description
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.