cw heavy-ion linear accelerators
TRANSCRIPT
RISP Workshop on Accelerator Systems, May 7-9, Daejeon, S. Korea
P.N. Ostroumov
Physics Division
May 8, 2012
CW Heavy-Ion Linear Accelerators
May 8, 2012
P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Content
General Layout of a Linac
Stripper and multi-q beam transport
Multi-q LEBT
– 2q experiment at ANL
– Space charge
– Extraction voltage
CW RFQ for heavy-ions
– Frequency, injection/extraction energy
– ANL experience
SC Technology
– Main steps for cavity construction
– Performance: accelerating gradients and residual resistance
– ANL experience
Summary
– Realistic parameters for 200 MeV/u, 400 kW linac
400 MeV/u and 200 MeV/u Linac Options
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Key feature: multiple charge state beams See original papers: Phys. Rev. Letters 86, Pp 2798-2801 (2001). PRST-AB, Phys. Rev. ST Accel. Beams 3, 030101 (2000) PAC-01, Multi-q Beam Transport, p. 3014
Voltage Estimate for 200 MeV/u Linac with 2
Strippers
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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ECR Ion Sources (H+ to 238U)
RFQ
Titanium Stripper 63 MeV/u
QWR
Q = 33+, 34+
Q = 84+, 85+, 86+, 87+, 88+
(20-30) keV/u
~500 keV/u 11 MeV/u
MHB
HWR, TSR
Li stripper
Q = 71+, 72+, 73+, 74+, 75+
HWR, QWR
Total Effective Voltage = 700 MV Compare one stripperoption, Total Effective Voltage=780 MV
200 MeV/u, 400 kW
Stripper Effect on Beam Dynamics
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W= 3.29 MeV/u
sW = 17.6 keV/u
sT = 0.5 mrad
U-238 at 85 MeV/u on 15 mg/cm2 carbon stripper, 1M events
Published in PRST-AB, 7, 090101 (2004)
Multi-q Ion Beam Studies at ANL
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ECR on 100 kV platform voltage
Achromatic bending system
Electrostatic focusing
Phase space plots and beam envelope in the 2Q-LEBT
TRACK
Beam extraction and
Einzel lens
Einzel lens and acc. tube
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Re-Combined Bismuth Ions: 2q versus Single q
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Tuning: minimize angular dispersion – Setting of 4 quads are changed by ~5% - 8% with respect to
pre-calculated values
Transmission is 100%
20 21
20 ,21
(20.9 0.2) , (21.3 0.2)
(42.1 0.4)
I A I A
I A
Published in PRST-AB, 12, 010101 (2009)
Injection of 2Q beam into the RFQ
See the original paper in LINAC-2000, “ Heavy-Ion beam Acceleration of Two-Charge States from an ECR Ion Source”, p. 202
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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RFQ
q/A range is defined by the required accelerated beams (protons, uranium)
Injection energy
– HV power supply for the HV deck at reasonable cost
– Light ions, specifically, protons – space charge issue
• If funds are available, light ion RFQ
– ATLAS injection energy – 30 keV/u
Output energy
– In the original RIA proposal we had ~300 keV/u
– ATLAS: the first CW RFQ in this frequency range ~60-80 MHz: long RFQ was too risky to build, output energy is 295 keV/u
– Based on our experience, now I would recommend ~500 keV/u
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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ATLAS CW RFQ
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Parameter Value
1 Duty cycle 100%
2 q/A 1/7 to 1
3 Input Energy 30 keV/u
4 Output Energy 295 keV/u
5 Average radius 7.2 mm
6 Vane Length 3.81 m
7 Inter-Vane Voltage 70 kV
8 RF power consumption 60 kW
ANL RFQ Highlights
Highly coupled EM structure
– “flat” field distribution, non-operational modes are separated more than by 10 MHz
– “bead-pull” tuning is not required
Conservative design, peak field is 1.5 Kilpatrick
Trapezoidal modulation
– Increases shunt impedance by 60%
A short output radial matcher to form axially-symmetric beam
Fabrication: 2-step brazing in a high temperature furnace
No “cold model”– was directly built from CST MWS geometry
Measured Q-factor is ~92% of the MWS calculated Q for annealed OFHC copper (the tuner ports were covered with blanks using O-rings)
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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SC technology
The most critical technology for CW ion accelerators
– Major cost of a CW Linac
Main parameters for CW operation: 𝑃 =𝑉2
𝑅𝑠ℎ
– voltage (peak fields, design EACC )
– residual resistance
Cryomodule: separate vacuum, vertically loaded, clean room work is minimized
Phase locked operation of cavities: RF coupler, slow tuner, fast tuner, LLRF
Microphonics: can be a voltage limiting factor for SC cavities
– QWR: Centering drift tube; mechanical damper, RF power
– HWR, TSR: RF power, PZT
EM design, mechanical design and engineering analysis
Fabrication technology
RF surface processing
Recent cavity test results at ANL: Voltage, EPEAK , BPEAK , residual resistance
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Design: EM Optimization
Reduce EPEAK/ EACC , BPEAK/ EACC
– Conical center and outer conductor
– Triple spokes: conical spokes
Maximize RshG
Aperture: from beam dynamics, for RIA it is in the range from 30 mm to 40 mm
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Accelerating Field Quality
QWR: beam center steering, quadrupole component of the E-field
– Shaping of the drift tubes to compensate magnetic force with electric force
– Displacement of the cavity axis: works well for fixed velocity profile
HWR: quadrupole component of E-field
– Elliptical aperture
– “Donut” shape of the drift tube (higher shunt impedance)
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Mechanical design and Engineering Analysis
Compact mechanical design to maintain a high real estate accelerating gradient;
Provide coupling ports enabling advanced RF surface processing techniques (electropolishing and high pressure water rinsing);
Integrate a coupling port for a RF coupler; Facilitate the integration of several cavities and their sub-systems (RF
coupler and tuners) into the cryomodule; Provide a means for cavity alignment in the cryomodule; Ensure that the stresses in the niobium and the stainless steel parts are
below the maximum allowable limits; Satisfy pressure vessel requirements according to the ASME code Minimize the sensitivity of the resonant frequency to fluctuations in helium
pressure Ensure that the slow tuner operation provides a sufficient tuning range and
that the correlated cavity deformations remain well below the plastic limit; If necessary, integrate a fast tuner with a required tuning window; Create a complete set of fabrication drawings.
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Fabrication
Niobium sheet forming
Niobium machining
Wire EDM
Brazed NB-SS transitions
Electron Beam Welding
– BCP weld preparation
– Pre-weld manual HPR on weld surfaces, class 1000 bag; un-bag in chamber
SS vessel installation
Wire EDM
Brazed NB-SS transitions
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Stainless Steel LHe Vessel, Electron Beam Welding
72.75 MHz QWR, βOPT=0.077
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Surface Processing
Ultrasonic cleaning
– 1 hour in DI water with 60oC Liquinox
Electropolishing after all mechanical work complete
– 150 microns in two 6-hour procedures
High-pressure water rinsing
– 4 hours total @ 11 lpm using 0.04 micron filtered DI water (1 hour per coupling port)
Drying and Clean Assembly
– 24 hours drying @ class 100
– Assembly in class 100 area
Bake at 600oC for 10 hours for hydrogen degassing
Light EP, HPR
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Electropolishing
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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72 MHz QWR
Cavity #1 Cavity #2
RF Coupler and Slow Tuner
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Tested at 4 kW, 72 MHz
Extremely reliable operation for more than 25 years
Test Results of 72 MHz Cavities
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Microphonics in 72 MHz cavity
3 kW RF power provides
40 Hz window for 0.1 mA beam
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Before centering
After centering
High Performance 325 MHz HWR: Fabrication Status
High performance HWR – 325 MHz, βOPT=0.29
Expected parameters – Surface resistance at 3.75 MV: below 5 n, 2K
– Peak fields: 170 mT, 110 MV/m
– Accelerating voltage: 6.5 MV
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
High Performance Cryomodules at ANL
7 QWRs provide 15 MV 7 QWRs will provide 17.5 MV (design)
Operational since July 2009 Can be operated up to 26 MV
Scheduled for installation in 2013
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P.N. Ostroumov CW Heavy-Ion Linear Accelerators RISP Workshop
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Design Parameters for CW Heavy Ion Linac
Realistic design parameters for the 200 MeV/u, 400 kW RIA
– 2 stripper option
– Total number of SC cavities: ~190
– Total number of cryomodules: ~30
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Year 1999 2003 2012 ILC pulsed
EPEAK, MV/m 21 27.5 60 70
BPEAK, mT 75 82 90-120 140
Operational T, K 4 4 & 2 2 2
Res. Resistance, n 25 25 & 10 4-10 4.7