ERL Workshop 2005 D.H. [email protected]
Technology Challenges for RF Guns as ERL Sources
David H. DowellStanford Linear Accelerator Center
ERL Workshop 2005 D.H. [email protected]
Technical Challenges are Everywhere!
RF GunNCRFSRF
CathodesDrive LaserBunch CompressionBeam Transport
ERL Workshop 2005 D.H. [email protected]
Basic Layout of ERL Free Electron Laser
2E 0E
Injector
Undulator
Einj ≈ 10 MeV ∆ ≈ 3 MeVE0 ≈ 100 MeV Edump ≈ 7 MeV
Linac
180o Bend
180o Bend
Einj
Edump
E-beam dump
E0E0 -∆fel
E-gun
Cavity mirror Cavity mirror
Cathode drive-laser
E-Beam make 2 passes thought the linac:1: Acceleration and lasing2: Deceleration and energy recovery
Acc-DecAcc-Dec
∆fel
slice compliments of P. O’Shea, UMd
ERL Workshop 2005 D.H. [email protected]
Basic Components of the RF Gun Injector
RF Gun ~10 MeV Linac 3rd HarmonicLinearizer
Chicane Compressor Merging
Chicane
Main Linac AxisDrive LaserCathode
System
+ Laser Heater to Suppress Beam Instabilities?
ERL Workshop 2005 D.H. [email protected]
The Gun
ERL Workshop 2005 D.H. [email protected]
The Boeing 433 MHz RF Photocathode Gun
ERL Workshop 2005 D.H. [email protected]
Demonstrated Performance of 433 MHz Photocathode Gun, 1992 H-D TestPhotocathode Performance:
Photosensitive Material: K2CsSb MultialkaliQuantum Efficiency: 5% to 12%Peak Current: 45 to 132 amperesCathode Lifetime: 1 to 10 hoursAngle of Incidence: near normal incidence
Gun Parameters:Cathode Gradient: 26 MV/meterCavity Type: Water-cooled copperNumber of cells: 4RF Frequency: 433 x106 HertzFinal Energy: 5 MeV(4-cells)RF Power: 600 x103 WattsDuty Factor: 25%, 30 Hertz and 8.3 ms
Laser Parameters:Micropulse Length: 53 ps, FWHMMicropulse Frequency: 27 x106 HertzMacropulse Length: 10 msMacropulse frequency: 30 HertzWavelength: 527 nmCathode Spot Size: 3-5 mm FWHMTemporal and Transverse Distribution: gaussian, gaussianMicropulse Energy: 0.47 microjouleEnergy Stability: 1% to 5%Pulse-to-pulse separation: 37 nsMicropulse Frequency: 27 x106 Hertz
Gun Performance:Emittance (microns, RMS): 5 to 10 for 1 to 7 nCoulombCharge: 1 to 7 nCoulombEnergy: 5 MeVEnergy Spread: 100 to 150 keV
Applied Physics Letters, 63 (15), 11 October 1993, pp. 2035-2037.
ERL Workshop 2005 D.H. [email protected]
Normal-Conducting RF Photoinjector
Ridged tapered waveguidesfor RF power input
Cooling
Photocathode plate
2.5-cell PI with emittance-compensating magnets (left) and vacuum plenum (right)
complements of D. Nguyen, LANL
RF Photoinjector Summary
ERL Workshop 2005 D.H. [email protected]
RF Fields Should be Fully Symmetric
Coaxial RF Feed
DESY TTFII RF Gun2004 FEL Proceedings
-6-4-202468
-180 -130 -80 -30 20 70 120 170
rf phase (degree)
cylindrical cavity (lc=2.475cm)
racetrack cavity (lc=2.413cm)with d=0.315cmracetrack cavity (lc=2.413cm)with d=0.356cm
Qua
drup
ole ∆(γβ)
r(1
/m)
-8-6-4-202468
-180 -130 -80 -30 20 70 120 170-180 -130 -80 -30 20 70 120 170
rf phase (degree)
cylindrical cavity (lc=2.475cm)
racetrack cavity (lc=2.413cm)with d=0.315cmracetrack cavity (lc=2.413cm)with d=0.356cm
Qua
drup
ole ∆(γβ)
r(1
/m)
-8
d
rr
d
rrDual Feed &
Racetrack Shape
ERL Workshop 2005 D.H. [email protected]
LCLS Gun Design
ERL Workshop 2005 D.H. [email protected]
Rossendorf SRF Guns
Original 1-Cell Design
New 3 ½ Cell Design
ERL Workshop 2005 D.H. [email protected]
Cryomodule Configuration
Cathode isolationValve
Cathode installationassembly
Beam lineisolation valve
Top cover withfacilities feedthru
Cavityassembly
InternalHelium dewar
Adjustable supports
Magnetic and thermalshielding Beam tube with
HOM RF pickup
Vacuum vessel
Powercouplers
slide compliments of A. Todd
ERL Workshop 2005 D.H. [email protected]
The “LUX” GunPremise:
Cavity shaping leads to higher impedance and lower thermal loads and stress => can perhaps use OFHC copper instead of Glidcop
Material Courtesy Robert Rimmer
Projected CW fields are
20 / 13 /13 MV/m
slice compliments of A. Todd, AES
ERL Workshop 2005 D.H. [email protected]
Two-Frequency Gun with Emittance Compensation
0 100 200 300 400 5000
0.5
1
1.5
2
2.5
3
z (cm)Tran
sver
se E
mitt
ance
(mm
-mra
d) &
rms
radi
us (m
m) Time Step RMS-emittance vs Z (1 nC, 30ps, rc = 1mm
Transverse emittancerms radius
Elinac = 19 MV/mE beam =62 MeV
Solenoid = 2080 G.E0=106 MV/m, φ0=50 degE3=16 MV/m, φ3 = 1.7 deg
125 cm S-Band TW Section
-20 -15 -10 -5 0 5 10 150
0.2
0.4
0.6
0.8
1
Phase
Slic
e Em
ittan
ce (m
m-m
rad)
Slice Emittance (1 nC, 30ps, rc = 1mm)
z=15cmz=125cmz=450cm
D. Jansen suggests using the harmonic TE-mode to magnetically focus in SRF gun2003 FEL Conference Proceedings
ERL Workshop 2005 D.H. [email protected]
Photocathodes
ERL Workshop 2005 D.H. [email protected]
VacuumValve
Connectionto GunCavity
QE MeasurementLaser (GreNe)
RGA Head
Thin Film Monitor
Sb, K, CsSources
N2Inlet/Outlet
02468
10121416
6/12
/95
7/12
/95
10/2
4/95
11/1
4/95
12/1
3/95
1/3/
961/
23/9
62/
1/96
2/13
/96
2/28
/96
3/19
/96
5/2/
965/
14/9
65/
23/9
66/
3/96
Fabrication Date
Qua
ntum
Eff
icie
ncy
(%)
Nuclear Instruments and Methods, A356(1995) pp. 167-176.
ERL Workshop 2005 D.H. [email protected]
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80 90 100
Time (minutes)
Cat
hode
Tem
pera
ture
(deg
rees
C)
0
1
2
3
4
5
6
Qua
ntum
Effi
cien
cy (%
)
Temperature
Quantum Efficiency
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80 90 100
Time (minutes)
Cat
hode
Tem
pera
ture
(deg
rees
C)
0
1
2
3
4
5
6
Qua
ntum
Effi
cien
cy (%
)
Temperature
Quantum Efficiency
5.50%
6.00%
6.50%
7.00%
7.50%
8.00%
0 20 40 60 80 100 120 140
Time (Minutes)
Qua
ntum
Effi
cien
cy (%
)
0
20
40
60
80
100
120
140
Cat
hode
Tem
pera
ture
(deg
rees
C)
Temperature
Quantum Efficiency
5.50%
6.00%
6.50%
7.00%
7.50%
8.00%
0 20 40 60 80 100 120 140
Time (Minutes)
Qua
ntum
Effi
cien
cy (%
)
0
20
40
60
80
100
120
140
Cat
hode
Tem
pera
ture
(deg
rees
C)
Temperature
Quantum Efficiency
Nuclear Instruments and Methods, A356(1995) pp. 167-176.
ERL Workshop 2005 D.H. [email protected]
Advanced Diamond Amplified Cathode Being Developed at BNL
ERL Workshop 2005 D.H. [email protected]
LCLS QESpecification2x10-5 @255nm
After H-beam cleaning 1.2x10-4
1H: 15 min, 0.7uA
4H: 60 min, 2uA
2H: 60 min, 0.4uA
3H: 40 min, 0.4uAInitial Baked:
230 deg
LCLS QESpecification2x10-5 @255nm
After H-beam cleaning 1.2x10-4
1H: 15 min, 0.7uA
4H: 60 min, 2uA
2H: 60 min, 0.4uA
3H: 40 min, 0.4uAInitial Baked:
230 deg1H:
15 min, 0.7uA4H:
60 min, 2uA2H:
60 min, 0.4uA3H:
40 min, 0.4uAInitial Baked:230 deg
In-Situ Hydrogen Beam Cleaning of Cathode Surface
ERL Workshop 2005 D.H. [email protected]
The Drive Laser
ERL Workshop 2005 D.H. [email protected]
The 25% Duty Factor Drive Laser Used in Boeing/LANLPhotocathode Injector Test*
108 MHz Modelocked Nd:YLF oscillator
54 MHzPockels
Cell
27 MHzPockels
CellFaradayIsolator
LBO Crystal
Nd:YLF Amplifier Heads
OutputPockels
Cell
ToCathode
*D.H. Dowell et al., NIM A356(1995)167-176.
ERL Workshop 2005 D.H. [email protected]
Drive laser should be diode pumped
HR
HR
Collimated 20 WLaser Diode Bars
Nd:YVO4 Slab
slide compliments of M. Shinn
ERL Workshop 2005 D.H. [email protected]
New Drive Lasers Should be Shaped Spatially and Temporallyand Fully diode-pumped, fiber oscillator & amplifier
Ti:Sapph Oscillator
Dazzler
Stretcher
Amplifier
Compressor
ω−tripler
800 nm9 nJ100 fsec
5 nJ200-400 psec
25 mJ170-350 psec
15 mJ0.1 - 30 psec
266 nm1.8 mJ
temporal
risetime: 20-80% 2.5 psec10%-90% 4 psec
12.3 psec FWHM
Ti:Sapph OscillatorTi:Sapph Oscillator
DazzlerDazzler
StretcherStretcher
AmplifierAmplifier
CompressorCompressor
ω−triplerω−tripler
800 nm9 nJ100 fsec
5 nJ200-400 psec
25 mJ170-350 psec
15 mJ0.1 - 30 psec
266 nm1.8 mJ
temporaltemporal
risetime: 20-80% 2.5 psec10%-90% 4 psec
12.3 psec FWHM
Courtesy of Yuzhen Shen, Carlo Vicario, B. Sheehy, XJ Wang-10 0 10 20
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Time (ps)
CrossCorrStreak CamSc'd Spectrum
ERL Workshop 2005 D.H. [email protected]
Beam Transport(some comments)
ERL Workshop 2005 D.H. [email protected]
1300 MHz (third harmonic)energy spectrum programming
for bunch compression
Three dipole magnetic buncherand diagnostics
Linear Bunching for High Peak Current
Proceedings of the 1995 Particle Accelerator Conference, Dallas, TX, USA, pp.992-994.
Nuclear Instruments and Methods, A393(1996)pp. 184-187.
ERL Workshop 2005 D.H. [email protected]
Pulse Compression Boosts Peak Current From 45 to 280 Amperes
0
20
40
60
80
100
120
140
160
180
0 100 200 300 400Time (ps)
Before CompressionAfter Compression
55 ps
9 ps
Nuclear Instruments and Methods, A393(1996)pp. 184-187.
ERL Workshop 2005 D.H. [email protected]
Emittance Compensation in the Three-Dipole Chicane
Adjust path lengths in second and third dipoles
Adjust pole face rotations
30o
60o
30o
-19.5o -19.5o
384 mm 384 mm
Original configurationn = 1/2 dipoles
ERL Workshop 2005 D.H. [email protected]
Increase path length 4%
Entrance pole angle at -15.6 deg
Compress a 3nC, 1 πmm-mrad microbunch from 7.5mm (50amps) to 0.39mm (1000amps). The emittance is reduced by increasing the third dipole path length 4% and rotating entrance pole face -15.6o. Net bend angle is 1.2o.
2 1 0 1 2
4
2
0
2
2 1 0 1 2
4
-2
0
2
Uncompensated Phase Space Compensated Phase Space
Emittance = 16 π mm-mrad Emittance = 6 π mm-mrad
Div
erge
nce
(mra
d)
Div
erge
nce
(mra
d)
x (mm) x (mm)Proceedings of the 20th International Free-Electron Conference, Williamsburg, VA, USA.
ERL Workshop 2005 D.H. [email protected]
Summary and Discussion•RF guns:
•NCRF under construction by AES for LANL•SRF guns:
•Rossendorf 0.5 cell, Version 1, 1.3 GHz•Rossendorf 3.5 cell, Version 2•BNL/AES 0.5 cell, 700 MHz
•Cathodes: •Requires visible sensitivity•High-QE, Diamond amplified?•Cathode survival at high current, vacuum, etc.•Thermal emittance
•Drive Laser:•3D Pulse shaping required•Direct-diode pumped•Fiber oscillator and amplifiers (future)
•Beam Transport, emittance presevation:•Wakefields, space charge, etc.•Asymmetric bends