Superconducting RF cavities for High Current Energy Recovery Linacs

SRF Cavities for High Current ERLs
Rama Calaga
Brookhaven National Lab
September 09, 2005

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Motivation - ecooling@RHIC
• Cooling Au beams at 100 GeV requires ∼ 54 MeV e −
• dCooling
dt
∝ γ 5 2
• Low ɛ x/y , high current (> 200mA) and bunch charge (10-20 nC)
• Replenish e − every cycle - energy recovery linac
SRF GUN
4 x 5 cell cavities − 703.75 MHz
Beam Dump
Compressor
Stretcher
RHIC Beam
RHIC Beam
Cooling Solenoid [30 m]

Prototype ERL - Proof of Principle
Courtsey R. Bowman

Injector & Linac
SRF Injector
SRF Cavity
• Generation of ampere class
beam CW
• Cathode Issues and Isolation
• E z at the cathode
• Low ɛ x/y & δE/E
• Strong Coupling Q ext ∼ 10 4
• High Q ext ⇒ Lorentz force
detuning & microphonics
• HOM Power & extraction
• Multibunch Instabilities (x20
state-of-the-art)
• Field Emission (Q 0 )

SRF Cavity Design Issues

Fundamental Mode - Main Issues
• E peak
E acc
• P cav ∝
(↓),
H peak
E acc
(↓)
Rs
(R/Q)G (↓)
– R s ∝ ω 2 (R s = R BCS + R res )
– R G ∝ const. (dim. ∝ ω)
Q
R BCS [nΩ]
10
8
6
4
2
BNL (0.7 GHz)
TESLA (1.3 GHz)
CEBAF (1.5 GHz)
R res = 3 nΩ
R res = 1 nΩ
• Field sensitivity: a ∝ N 2
k cc
(↓)
0
1.2 1.4 1.6 1.8 2 2.2
Temperature [ 0 K]
Par BNL(HC) CEBAF(HG) TESLA(HG)
Freq. [MHz] 703.75 1497 1300
R
Q ∗ G [Ω2 ] 9 × 10 4 2.1 × 10 5 2.8 × 10 5
E p /E a 1.97 1.96 1.98
H p /E a [mT/MV/m] 5.78 4.15 4.15
k cc 3% 1.89% 1.87%
N cells 5 7 9
N 2
βk cc
8.3 × 10 2 2.6 × 10 3 4.1 × 10 3

HOM Power & Extraction

HOM Power & Kick
P avg = 2k || IQ
k || ∝ 1
√ √
d Nc
R iris σ z
1.5
Single Cell
1.2
BNL
TESLA
CEBAF
10 3 10 -2 10 -1 10 0 10 1 10 2
(CORNELL, 4GLS, KEK)
eRHIC
BNL(ERL, eCooling)
k || (σ)/cell [V/pC]
0.9
0.6
10 2
JLAB(100 kW)
10 kW
0.3
Average Current [mA]
10 1
10 0
10 -1
10 -2
JLAB(1 kW)
JLAB(10 kW)
JAERI
ELIC
LUX
Charge/Bunch [nC]
*** Avg. Power Normalized to 1 V/pC
0.1 W
1 W
10 W
100 W
1 kW
Power
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
k ⊥ ∝ 1
R 3 iris√
dσz N c
k t (σ)/cell [V/pC/m]
8
6
4
2
σ [cm]
Single Cell
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
σ [cm]
BNL
TESLA
CEBAF

HOM Extraction & Damping
Ferrite Absorbers
Broadband (300 K)
Loop Couplers
Resonant Ciruit (2 K)
Parameter BNL(HC) CEBAF(HG) TESLA(HG)
k || (σ z − 1mm) [V/pC] 4.25 10.71 13.14
k ⊥ (σ z − 1mm) [V/pC/m] 0.1 2.24 2.07

Trapped Modes

Design Criteria: Trapped Modes
Frequency Difference
MC
EC
MC
EC
∆f = 30MHz (2.4 GHz)
∆f = 13MHz (1.4 GHz)
Number of Cells

HOMs - Multibunch BBU
Trapped Modes (k cell−cell , N cells , Q ext )
I thr =
e
(
R
Q
−2p r c
)Q e k m M 12 sin(ω m t r )e − ωm
2Qe tr
Parameter BNL(HC) CEBAF(HG) TESLA(HG)
k || (σ z − 1mm) [V/pC] 4.25 10.71 13.14
k ⊥ (σ z − 1mm) [V/pC/m] 0.1 2.24 2.07
Q ext (Dipole) 10 2 − 10 4 10 3 − 10 6 10 3 − 10 7
Single Pass (HG) Vs. Multiple Passes (HC + Magnets)

BNL Cavity

Cavity Design
1
B
alpha
a
Normalized units
0.8
0.6
E pk /E acc
0.4 H pk /E acc [mT/(MV/m)]
Iris Radius - R iris
R/Q [Ω]
0.2
cell coupling
dF/dR equator [MHz/mm]
0
6 6.5 7 7.5 8 8.5 9 9.5 10
Iris Radius [cm]
1
D
A
b
d
Riris
L
Iris Radius, R iris
8.5 [cm]
Wall Angle, α
25 [deg]
Equatorial Ellipse, R = B 1.0
A
Iris Ellipse, r = b 1.1
a
Cav. wall to iris plane, d 2.5 [cm]
Half Cell Length, L = λβ 10.65 [cm]
4
H = D − (R iris + b + B) 4.195 [cm]
Cavity Beta, β = v 1.0
c
Normalized units
Normalized units
Normalized units
0.9
0.8
E pk /E acc
H pk /E acc [mT/(MV/m)]
0.7
R/Q [Ω]
cell coupling
Wall Angle (α)
dF/dR equator [MHz/mm]
0.6
10 12 14 16 18 20 22 24 26
1
0.95
0.9
0.85
0.8
1
0.95
0.9
0.85
α [deg]
Iris Ellipse Ratio (r=b/a)
1 1.5 2 2.5
Iris ellipse ratio
E pk /E acc
H pk /E acc [mT/(MV/m)]
R/Q [Ω]
cell coupling
dF/dR equator [MHz/mm]
dF/dR equator [MHz/mm]
0.8
0.6 0.8 1 1.2 1.4 1.6 1.8 2
Equator ellipse ratio
Equator Ellipse Ratio (R=B/A)

Beam Pipe Transition
• Damping HOMs
20
– Enlarged BP (KEK,
BNL, CORNELL)
– Flutes (CORNELL)
R [cm]
16
12
8
4
12 cm
5 cm 5 cm
20.942 cm
8.5 cm
– Loop couplers (TESLA,
CEBAF)
• Minimize fundamental leakage
(> 10 W ).
• Minimize FPC kick
– Enlarged BP (KEK, BNL)
– Symm. couplers (COR-
NELL)
0
0 10 20 30 40 50
Diameter [cm]
40
36
32
28
24
20
16
12
TM 010
TE 111
TE 211
TM 011
Z [cm]
5.0 x 10 8 1.0 x 10 9 1.5 x 10 9 2.0 x 10 9 2.5 x 10 9
TM 111
Frequency [Hz]
TM 01
TE 11
TM 11
TE 21
Beam Pipe Diameter
• Cold to warm transition
(Counter Flow of He)
300K
4K

BNL High Current Cavity
Main Parameters:
Frequency
703.75 [MHz]
RHIC Harmonic 25
Number of cells 5
Active cavity length 1.52 [m]
Iris Diameter
17 [cm]
Beam Pipe Diameter 24 [cm]
G (Ω) 225
R/Q
403.5 [Ω]
Q BCS @ 2K 4.5 × 10 10
Q ext 3 × 10 6
E p /E a 1.97
H p /E a
5.78 [mT/MV/m]
cell to cell coupling 3%
Sensitivity Factor ( N 2
β ) 833
Field Flatness 96.5 %
Lorentz Detuning Coeff 1.2 [Hz/MV/m]
Lowest Mech. Resonance 96 [MHz]
k || (σ z − 1cm)
1.1 [V/pC]
k ⊥ (σ z − 1cm)
3.1 [V/pC/m]
HOM Power (10-20 nC) 0.5-2.3 [kW]
|E| [MV/m]
1
0.8
0.6
0.4
0.2
Field Flatness
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
z [m]
Superfish
Meas

Cu Prototype & Nb Cavity

HOMs: Simulation & Measurements
Frequency Domain
Time Domain
10 3
TM01x
Monopole Modes
MAFIA
Measurement
10 1
TM010
Monopole Modes
MAFIA
Measurement
Q ext
10 4 0.8 1 1.2 1.4 1.6 1.8 2 2.2
10 2
Z [kΩ]
10 2 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
10 0
10 1
10 -1
10 0
10 -2
Frequency [GHz]
Frequency [GHz]
10 3 0.8 1.2 1.6 2 2.4
Dipole Modes
Dipole Modes
MAFIA
Measurement
10 4 0.8 1 1.2 1.4 1.6 1.8 2
Q ext
10 3
10 2
Z [kΩ/m]
10 2
10 1
10 1
10 0
MAFIA
Measurement
10 0
10 -1
Frequency [GHz]
Frequency [GHz]

Multibunch BBU
TDBBU
MATBBU
X [mm]
Y [mm]
80
60
40
20
0
-20
-40
-60
-80
40
20
0
-20
-40
no HOM spread
3 MHz
5 MHz
10 MHz
0 50 100 150 200 250
no HOM spread
3 MHz
5 MHz
10 MHz
0 10 20 30 40 50 60 70 80
Time [msec]
I thr [A]
I thr [A]
10 2
10 1
10 0
no spread
3 MHz
5 MHz
10 MHz
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
10 -1
10 2
10 1
10 0
no spread
3 MHz
5 MHz
10 MHz
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
10 -1
Frequency [GHz]
Threshold Current > 2 Amps
BNL eCooling Configuration - 4 Cavities - 54 MeV
(Numerical Codes from JLAB)

Conclusion
• The cavity is being prepared for chemical treatment at
JLAB
• Cryostat assembly and cold testing in 2006
• The SRF gun will finish fabrication end of 2006
• The 20 MeV prototype ERL will be operational sometime
in 2007
THANKS & HAVE A NICE LUNCH