Transcript Document

Overview of ERL projects: SRF issues and challenges
Overview of ERL Projects:
SRF Issues and Challenges
Matthias Liepe
Cornell University
Matthias Liepe, TTC meeting, Beijing 2011
Slide 1
Outline
• Introduction: SRF for ERLs
– What makes it special / challenging?
• Challenges for…
Outline
– …the cavity
– …the HOM damper
– …the RF power system and control
– …the cryostat and cryoplant
• Summary and outlook
Matthias Liepe, TTC meeting, Beijing 2011
Slide 2
What makes it special / challenging?
Matthias Liepe, TTC meeting, Beijing 2011
Slide 3
Introduction: SRF for ERLs
Introduction: SRF for ERLs
SRF for Future Particle Accelerators
NGLS
BNL ERL
Electron
Cooler and
eRHIC
50 mA
XFEL
17.5 GeV, 800 s.c.
cavities
European
Spallation Source
(ESS)
China Spallation
2.5 GeV,50 mA
Source (CSNS)
1.6 GeV SRF linac
HZB ERL
100 mA
Cornell ERL
5 GeV, 100 mA,
400 s.c. cavities
Accelerator
Driven
Subcritical
Reactor pilot
facility
KEK ERL Light
Source
5 GeV, 100 mA
International Linear Collider
500 GeV, 16000 s.c. cavities
Matthias Liepe, TTC meeting, Beijing 2011
Slide 4
Introduction: SRF for ERLs
Project X (FNAL)
3 GeV, 1 mA
Future: muon collider?
Facility for Rare
Isotope Beams
(FRIB)
Need for Multi-Gev, CW, High
Current SRF Linacs
• Only technology that will allow realizing such linacs in the
foreseeable future is superconducting radio-frequency
– Operated in continuous wave or long pulse mode
– Accelerating high beam currents of many tens of mA
Need lower surface resistance to support efficient cw operation (lower
cryogenic losses), and better control of unwanted cavity-beam
interaction (higher-order cavity modes) to support high beam currents
Matthias Liepe, TTC meeting, Beijing 2011
Slide 5
Introduction: SRF for ERLs
Key technology: Multi-GeV SRF linacs
Example: SRF for the Cornell ERL
ERL main linac: 5 GeV
SRF linac, 100 mA with
energy recovery, 5 kW RF
power per 7-cell cavity
Introduction: SRF for ERLs
ERL injector: 15 MeV
SRF linac, 100 mA
without energy
recovery, >100 kW RF
power per cavity
Cornell Energy Recovery Linac preliminary PDDR
Figure 2.3.29: A cut -away CAD model showing t he main feat ures of t he 12-cavity inject or
cryomodule.
A cut -away CAD model of t he 12-cavity inject or cryomodule is shown in Fig. 2.3.29, which
includes only t he main feat ures of t he module, wit h a closer view shown in Fig. 2.3.30. T he
design incorporat es twelve 2-cell SRF cavit ies, beamline HOM loads, two coax RF couplers
per cavity, a segment ed GRP wit h fixed and sliding support s, gat e valves at each end, and is
10 m long. As a point of reference, Tab. 2.3.7 list s t he beamline component s of t he inject or
cryomodule and t heir lengt hs. In t he sect ions t hat follow, t he det ails of many of t he ERL
inject or cryomodule component s and it s assembly will be described.
T he ERL inject or cryomodule shown in Fig. 2.3.29 is based on t he T T F I I I module st ruct ure.
All of t he cavity helium vessels are pumped t o 1.8 K (12 Torr) t hrough a common 25 cm inside
diamet er Gas Ret urn Pipe (GRP) which also serves as t he mechanical support from which t he
beamline component s are suspended. To minimize t he heat load t o t he refrigerat ion plant , all
of t he 1.8 K cryomodule component s are surrounded by 5 K int ercept s t o minimize t he heat
leak t o 1.8 K , and t he 5 K int ercept s are likewise surrounded by 100 K int ercept s, which absorb
t he heat load from t he 293 K vacuum vessel. T he GRP is suspended from composit e support
post s t hat are const ruct ed from low-t hermal conduct ivity G-10 fiberglass. T he composit e
post s have int egral met al st iffening disks and rings t hat also serve as t hermal int ercept s at
5 K and 100 K between t he 1.8 K face t hat at t aches t o t he GRP and t he 293 K face t hat
at t aches t o t he vacuum vessel bosses t hat support t he cold mass. T here are st ainless st eel
manifolds of smaller diamet er t han t he GRP running t he lengt h of t he module t hat t ransport
t he supply of liquid helium and t he supply and ret urn of 5 K and 100 K helium gas for t he
t hermal int ercept s. Jumper t ubes wit h 5 mm inner diamet er are connect ed between t he 5 K
and 100 K supply and ret urn manifolds t o t he various t hermal int ercept s wit hin a module. A
shell of 6 mm t hick, grade 1100 aluminum sheet surrounds t he beamline and t he GRP and is
linked to t he 100 K manifold t o serve as a t hermal radiat ion shield between t he 293 K vacuum
vessel and t he cold mass. T he aluminum 100 K shield has apert ures t hrough which t he RF
couplers pass and also has panels wit h inst rument at ion feedt hroughs. T he 100 K shield is
mechanically suspended from one of t he int egral met al st iffeners in t he composit e support
post s. Mult i-layer insulat ion is wrapped around t he ext erior of t he 100 K shield as well as all
of t he 1.8 K and 5 K cold mass component s.
T he magnet ic shielding in t he cryomodule must keep t he field in t he region of t he SRF
Matthias Liepe, TTC meeting, Beijing 2011
Slide 6
Cornell ERL SRF Parameters
Main Linac
Note
15 MeV
5 GeV
Multi-GeV
12
384
~ CEBAF
100
2 x 100
High!
CW Operating gradient [MV/m]
6
16
Limited by Q0
Cavity intrinsic quality factor Q0
11010
21010
Cost driver!!
Total cryogenic load at 1.8K [W]
60
5000
5 MW AC power
5104
>6.5107
High! (no effective
beam loading)
110-4 / 0.1 deg
210-4 / 0.1 deg
Very tight!
Total energy gain [GeV]
Total # cavities
CW beam current [mA]
Cavity loaded quality factor QL
RF field stability
Significant progress has been made during the last year towards achieving these
ambitious goals!
Matthias Liepe, TTC meeting, Beijing 2011
Slide 7
Introduction: SRF for ERLs
Injector Linac
Parameter
ERL SRF related Challenges
The SRF system for high current ERLs is extremely demanding:
Significant progress in these fields is needed for high current ERLs to work!
Matthias Liepe, TTC meeting, Beijing 2011
Slide 8
Introduction: SRF for ERLs
• SRF cavities:
– Continues operation at high fields with low cryogenic losses -> high Q0
– Reliable operation with very low trip-rate
– Very low microphonics levels -> optimized mechanical cavity design
– Design optimized for strong HOM damping
• Higher-Order-Mode damping:
– Strong HOM damping and efficient HOM power extraction for high beam currents
• RF power system and control:
– Low cost, low CW RF power input couplers
– Low cost RF power sources
– Active and fast cavity frequency control
– Very good RF cavity field stabilization at highest loaded Q for energy stability
• Cryostat and Refrigeration:
– Cryogenic system for high cryo-loads
– Cryostat design for low mechanical vibrations and vibration damping
– Cryostat design for excellent magnetic shielding (high Q0)
– Very accurate cavity alignment
Challenges for…
Challenges for…
…the cavity
…the HOM damper
…the RF power system and control
…the cryostat and cryoplant
Matthias Liepe, TTC meeting, Beijing 2011
Slide 9
Challenges for…the Cavity (I)
1. High Q0 at medium (!!) fields
-
GeV scale, CW SRF linacs -> MW-scale cryoplants
Consistent Q0 > 2x1010 highly desirable for cost reasons
-
-
…the cavity
-
Higher Q0 -> higher cost optimal
gradient (2x1010: 15 – 20 MV/m)
Understanding residual
resistance is key! Why does it
fluctuate between 1 and
> 10 nOhm?
Medium field Q slope?
Best surface preparation??
How to preserve high Q0 in
a cryomodule?
a
G. Ciovati, et al., IEEE Trans. Appl. Supercond. Vol. 21, No. 3, 2011
2. Design optimized for strong HOM damping
- Impacts cell shape, number of cells, frequency…
- Important: Optimized shape must be stable under realistic shape
imperfections!
Matthias Liepe, TTC meeting, Beijing 2011
Slide 10
Challenges for…the Cavity (II)
3. Very low microphonics
-
…the cavity
No effective beam loading, so could operate at QL>1x108
But: High QL needs low microphonics to be effective!!
-> Mechanical design for low microphonics! (reduce df/dp
sensitivity to pressure fluctuation in LHe bath)
2ù
é æ
2
ö
Vc
f0
ê1+ ç Df 2QL ÷ ú
DPg =
QL , opt =
4QL R Q êë è f 0
ø úû
2Df
4. Reliable operation with very low trip-rate
-
User facilities (especially x-ray) require uninterrupted beam
Mean time between trip per cavity > months!?
Trips caused by occasional peak detunings and insufficient RF
power
- How frequent? What is the peak detuning over weeks, and
how can it be reduced?
Matthias Liepe, TTC meeting, Beijing 2011
Slide 11
Examples: ERL Cavities
BNL 5-cell, 703 MHz
KEK 9-cell, 1.3 GHz
…the cavity
JLAB 5-cell, 748.5 MHz
Cornell 7-cell, 1.3 GHz
Matthias Liepe, TTC meeting, Beijing 2011
Slide 12
ERL Cavities: Q0 in Vertical (!) Tests
BNL 5-cell, 703 MHz
…the cavity
JLAB 7-cell, 1.3 GHz
KEK 9-cell, 1.3 GHz
Cornell 7-cell, 1.3 GHz
BCP&120C bake
BCP&120C bake
ERL main linac spec
Matthias Liepe, TTC meeting, Beijing 2011
Slide 13
RF Optimization of Cornell’s ERL
Main Linac Cavity (I)
IBBU ~ 1/(worst BBU-parameter)
Franklin Cray XT4
• Dipole mode damping calculated up to
10 GHz with realistic RF absorbers
• Worst mode limits beam current!
Matthias Liepe, TTC meeting, Beijing 2011
Slide 14
…the cavity
Cell shape optimization:
• ~20 free parameters
• Full Higher-Order Mode characterization (1000’s of eigenmodes)
• Verification of robustness of cavity design
RF Optimization of Cornell’s ERL Main
Linac Cavity (II)
Optimize
Cavity
W.R.T. BBU
parameter
Introduce
realistic shape
variations
(400 cavities)
Compute dipole
HOMs to 10 GHz
(1692 modes
/cavity)
Generate
realistic ERL
(x100)
Compute
BBU
current
…the cavity
0.25mm
0.125mm
0.5mm
1mm
Optimized cavity with 0.25 mm shape imperfections supports ERL beam
currents well above 100 mA!
Mechanical Design of Cornell ERL
Cavity for efficient Cavity Operation
• Small bandwidth cavity vulnerable cavity
microphonics (frequency modulation),
especially by helium pressure fluctuations
Stiffening rings can vary from ID at iris to OD at
equator
• ANSYS simulations: large diameter rings
and no rings at all have smallest df/dp
No Rings
ID of rings as Fraction
of Iris-Equator
Distance
Cavity optimized!
No
Rings
Matthias Liepe, TTC meeting, Beijing 2011
Slide 16
…the cavity
• Diameter of cavity stiffening rings used as
free parameter to reduce df/dp
Model of Cornell ERL Main Linac Cavity
Cavity: Current Status
• Residual resistance and medium field Q0 still mostly a
mystery and need much more attention!
– Including performance (degradation) in cryomodules
• Several cavities designed specifically for ERLs (BNL,
KEK, JLAB, Cornell…), i.e. primarily for high currents.
Also some optimization of mechanical design done
(Cornell…)
• Reliability of long term cavity operation at very high QL
needs more study:
– Operate cavities CW for weeks and monitor detuning
-> International ERL cryomodule to be tested at Daresbury in
2012
Matthias Liepe, TTC meeting, Beijing 2011
Slide 17
…the cavity
-> Test cryomodules at 1.8K!
-> Cornell Horizontal Test Cryomodule
Cornell’s Horizontal Test Cryomodule
80K shield
HGRP
Gate valve HOM load
cavity
HOM load
Matthias Liepe, TTC meeting, Beijing 2011
Slide 18
…the cavity
• TTF, JLAB, Fermilab: see occasional significant degradation of
cavity performance once installed in cryomodule. WHY?
• Cornell test cryomodule: show that quality factor can be
maintained after cavity has been equipped with helium vessel,
RF coupler and HOM absorbers
International ERL Cryomodule
Modified
Two 1.3 GHz 7 cell
cavities (fabrication
at test at Cornell)
Cornell-style input
coupler (from ERL
injector)
• International collaboration:
– ASTeC (STFC), Cornell University, DESY, FZDRossendorf, LBNL, Stanford University, TRIUMF
• Test staring in 2012
– Focus on long term cavity operation at high loaded Q
Matthias Liepe, TTC meeting, Beijing 2011
Slide 19
…the cavity
Cornell-style cold
HOM load
Challenges for…the HOM Damper
1. Strong, broadband HOM damping
Q’s of < 10,000 typically needed
2ps bunches excite HOMs to ~100 GHz
2. Efficient HOM power extraction
-
-
High power handling needed: Few 100 W to >1000 W of
HOM power per cavity
Best temperature to absorb power at?
3. Antenna, waveguide or beamline load based?
4. Best RF absorbing material?
-
Graphite loaded SiC, Ceralloy, ferrite, CNT loaded
ceramic?
5. Cost
- 10% to 40% of cavity cost
Matthias Liepe, TTC meeting, Beijing 2011
Slide 20
…the HOM damper
-
Beam Current and HOM Damping
Requirements
CEBAF 12GeV
Project X
XFEL
SPL
BERLinPro
KEK-CERL
Cornell ERL
0.10
1
5
40
100
100
100
0.05
0.06
1
22
150
185
185
1.40E+09
2.00E+07
1.00E+05
1.00E+04
1.00E+04
1.00E+06
5.00E+03
1.50E+09
1.00E+09
1.00E+05
1.00E+07
1.00E+04
1.00E+04
1.00E+04
eRHIC
300
7,500
1.00E+04
4.00E+04
P = k|| IQb
• Risk of resonant
mode excitation and
beam stability
require strong HOM
damping by HOM
damping scheme
Matthias Liepe, TTC meeting, Beijing 2011
Slide 21
…the HOM damper
Project
Average
Beam HOM
Required
current power per monopole Required
[mA] cavity [W] Q <
dipole Q <
• High beam current
requires high power
handling capabilities
of HOM damping
scheme
HOM Dampers
BNL 5-cell: antenna
KEK, Cornell, DESY: Beamline
…the HOM damper
JLAB 5-cell: waveguides
RF absorber Rings
Matthias Liepe, TTC meeting, Beijing 2011
Slide 22
HOM Damper: Current Status
• Lots of activity worldwide
• Some good RF absorbing materials are
available for operation at room temperature
and cryogenic temperatures
– Reproducibility of properties needs to be
addressed
• Cost remains an issues
Matthias Liepe, TTC meeting, Beijing 2011
Slide 23
…the HOM damper
– Antenna HOM couplers
– Waveguide HOM couplers
– Beamline loads
Challenges for…
the RF Power System and Control
-
Desirable to further reduce microphonics
Also needed to compensate Lorentz-force detuning during field
camp up
Tuner design / stiffness also impacts microphonics level
2. Very good RF cavity field stabilization at highest loaded Q
-
Very tight field stability needed at very high loaded Q
3. Low cost, few kW CW input coupler (main linac)
-
Currently 30 to 40% of cavity cost!
4. High CW input coupler (injector)
-
Voltage in injector cavities limited by coupler RF power
5. Reliable, efficient, low cost 5 - 15 kW RF source
-
Need low trip and failure rate
Need lower cost/Watt (<10$/Watt for full system)
Matthias Liepe, TTC meeting, Beijing 2011
Slide 24
…the RF power system and control
1. Active and fast cavity frequency control
RF Power System and Control:
Current Status (I)
Lorentz-force detuning and microphonics compensation at the Cornell ERL
injector module
 Reduces rms microphonics by up to 70%!
Matthias Liepe, TTC meeting, Beijing 2011
Slide 25
…the RF power system and control
• Active compensation of Lorentz-force detuning works well
• Initial steps taking in active microphonics control, but very
challenging and still limited in effectiveness. Note: peak
detuning most important!
S1 Global Cryomodule:
Detuning change during 2 hour operation
He pressure
He Pressure [arb.]
3.65
3.6
3.55
quench
3.5
3.45
3.4
3.35
3.3
0
5000
10000
Time [s] (KEK)
Shin MICHIZONO
Significant differences in df/dp sensitivity! Need more data!
Matthias Liepe, TTC meeting, Beijing 2011
26
…the RF power system and control
3.7
RF Power System and Control:
Current Status (II)
• Tests of Cornell’s novel
LLRF system
– At JLAB ERL-FEL,
CEBAF, HZ-Berlin
horizontal test cryostat
– Demonstrated highly
efficient operation at
record high loaded quality
factors up to 2108
– Exceptional field stability:
σA/A <110-4, σ ~ 0.01
deg
Matthias Liepe, TTC meeting, Beijing 2011
27
…the RF power system and control
• Excellent field stability at very high loaded Q
demonstrated:
RF Power System and Control:
Current Status (III)
KEK, >50 kW, 1.3 GHz
Cornell, 50 kW, 1.3 GHz
2K
KEK, 20 kW, 1.3
GHz
Warm
window
Cold window
5K 80K
300K Coax-waveguide
transition
vacuum
Cornell, 5 kW, 1.3 GHz
bellows
5K 80K
RF power
– Also waveguide couplers (JLAB)
• But: High cost remains major issue
Matthias Liepe, TTC meeting, Beijing 2011
Slide 28
…the RF power system and control
• Various CW RF input coupler developed for ERLs:
RF Power System and Control:
Current Status (IV)
…the RF power system and control
• Solid state amplifier start to emerge as best
choice for a few kW CW RF power source
• Reliable, linear, good efficiency at all power levels
• Cost competitive with IOT, klystron
W. Anders, HZB
Matthias Liepe, TTC meeting, Beijing 2011
Slide 29
Challenges for…
the Cryostat and Cryoplant (I)
-
Large number of significant dynamic heat loads: cavity,
HOM loads, CW input couplers
Cool in series, parallel? How to ensure uniform cooling?
Huge difference in cooling power between RF and beam
on and standby. Cryoplant must have sufficient flexibility!
Optimal temperatures:
-
-
Shield temperature? 80K?
Cavity operating temperature? Large cryoplant stability at 1.6K
and below
Cryoplant contributes >50% to total wall plug power
-
Improvements in coefficient of performance desirable
Matthias Liepe, TTC meeting, Beijing 2011
Slide 30
…the cryostat and cryoplant
1. Cryogenic system for high CW cryo-loads:
Optimization
Challenges for…
the Cryostat and Cryoplant (II)
- How do external vibrations get to the cavities?
- What matters, i.e. drives microphonics?
3. Excellent magnetic shielding
- Excellent magnetic shielding essential for high Q0
in cw operation (B < few mG at cavities)
- How many layers of shield needed?
4. Accurate cavity alignment (0.5 – 1mm)
Matthias Liepe, TTC meeting, Beijing 2011
Slide 31
…the cryostat and cryoplant
2. Cryostat design for low mechanical vibrations
and vibration damping
Cryostat and Cryoplant:
Current Status
– Need to explore operation below 1.8 K
• Test / prototype
modules important
to verify module
cryogenic manifold
sizing and layout
5K supply
5K distribution to
heat exchanger
Matthias Liepe, TTC meeting, Beijing 2011
80K supply
80K distribution to
heat exchanger
Slide 32
…the cryostat and cryoplant
• Experiences with DESY, LHC, JLAB, and SNS cryoplants
provide excellent opportunities to learn from
Mechanical Coupling Characterization Measurements
with a Modal Shaker at Cornell Injector Module
Excitation
Point
Excitation
Force
Coupler
Waveguide
110 N
(25 lbs)
No
No
Coupler
110 N
(25 lbs)
No
No
Cryomodule
Saw-Horse
Support
110 N
(25 lbs)
Yes
No
Helium Gas
Return Pipe
Support
110 N
(25 lbs)
Yes
Yes
Beam Line
10 N
(2 lbs)
No
No
Helium
Supply/Return
110 N
(25 lbs)
No
No
• Ground vibrations
and other mechanical
vibrations do not
strongly couple to the
SRF cavities
• Main contribution to
cavity microphonics
comes from fast
fluctuations in the
He-pressure and the
cryogenic system
Matthias Liepe, TTC meeting, Beijing 2011
Slide 33
…the cryostat and cryoplant
Detectable On
Cavity RF
Frequency
(>0.1Hz modul.)
Detectable
With Cavity
Accelerometer
Cryostat and Cryoplant:
Current Status
Axial magnetic field on axis, Bz
50
4.8
3
0
7.4
-3
0
-2
-50
-100
-150
-157
-200
-226
-250
-35
-30
-25
-20
-15
-10
-5
0
Distance from Iris [inch]
1.00
B < 3 mG
Cornell ERL injector
cryomodule: Cavity
string is aligned to 0.2
mm after cool-down!
X position [mm]
Bz [mG]
-28
0.50
0.00
-0.50
X1 [mm]
X3 [mm]
ERL Injector Cooldown
WPM Horizontal
X4 [mm]
X5 [mm]
-1.00
4/29/08 0:00
4/30/08 0:00
Matthias Liepe, TTC meeting, Beijing 2011 Date-Time
5/1/08 0:00
5/2/08
0:00
Slide 34
…the cryostat and cryoplant
• Sufficient magnetic
shielding and cavity
alignment has been
demonstrated
z
Summary and outlook
Summary and outlook
Matthias Liepe, TTC meeting, Beijing 2011
Slide 35
Summary and outlook (I)
• Challenges that have been resolved:
• Challenges that need some additional work:
– Long term cavity operation at high loaded Q with very low trip
rates
– Microphonics reduction by passive and active means
– Broadband HOM dampers
– Low cost, reliable RF power sources (few kW range)
– Cryostat design for large number of significant dynamic loads
supporting wide range in loads
Matthias Liepe, TTC meeting, Beijing 2011
Slide 36
Summary and outlook
– Cavity design for strong HOM damping
– Operation at very high loaded Q (5x107 to >1x108) with excellent
RF field stability
– Cryomodule providing excellent cavity alignment and magnetic
shielding
Summary and outlook (II)
– Reliably achieving high Q0>2x1010 at medium fields
– Reducing cost of lower CW power RF input couplers
(few kW range)
• How you can help:
– Routinely test cavities at 1.6K, 1.8K, and 2K
– Test full modules at 1.6K, 1.8K, and 2K
– Study microphonics in cryomodules, especially long
term, sensitivity to LHe pressure…
– Test operation of cryoplants below 1.8K
Matthias Liepe, TTC meeting, Beijing 2011
Slide 37
Summary and outlook
• Challenges that need much more work:
The End
Thanks for you attention!
Matthias Liepe, TTC meeting, Beijing 2011
Slide 38