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RF structure comparison for low energy acceleration. Maurizio Vretenar (CERN).
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1
RF Structure Comparison for Low Energy Acceleration
M. Vretenar, CERNESS-B Workshop
1. Motivations2. Figures of merit3. Structure catalogue4. An attempt of comparison
2
Definitions
“Standard” block-diagram of a high-energy linac:
1. Front-end (ion source, LEBT, RFQ, MEBT);
2. Low-energy, from 3 MeV to some 100-200 MeV → different choices
3. Medium and high-energy (usually superconducting, elliptical).
Whereas front-end and high-energy have a well-established architecture, the low-energy section:
a. includes the “debated” transition NC – SCb. presents a large variety of choices for the accelerating structures
Transition energies:
General consensus on 3 MeV as transition front end – low energy (highest energy achievable without activation of MEBT components).
SC elliptical structures can start at energies in the range 160 – 200 MeV.
3
What architecture for “low-energy”?
Generally no consensus on the type of structure to be used !
Comparing the 2 most recent linac projects (SNS and JPARC) and the 2 European linacs in construction or close to construction (Linac4 and FAIR):
Frequency: basic in the 325-402 MHz range, doubling only in SNS SCL
2.5 MeV 87 MeV 186 MeV
SNS DTL SCL 2f
3 MeV 50 MeV 191 MeV
JPARC DTL SDTL
3 MeV 50 MeV 160 MeV
CERN-Linac4 DTL CCDTL PIMS
3 MeV 70 MeV
GSI-FAIR CH-DTL
Common features: all designs normal conducting, sequence of different accelerating structures
402 MHz1.4mA avg.
325 MHz0.7mA avg.
352 MHz0.02-2.4mA
325 MHz5 µA avg.
4
Figures of merit – Power efficiency
Shunt impedance = efficiency in transforming RF power into voltage on the gap Z = V2/PIs one of the main figures of merit, depends on operating mode and frequency
1. TM-0 modes have Z decreasing with energy.2. TM-π modes have Z increasing with energy.3. TE modes have high Z, decreasing with energy.4. In general terms, Z scales as �f
3 – 200 MeV
TM modes (Linac4) TE mode (GSI)
TE mode structures used (high efficiency) for ions at very low β, recently extended to protons
0-mode
π-mode
5
Beam dynamics constraints
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The “low-energy” linac section is dominated by space charge and RF defocusing forces.
Approximate expression for phase advance in an ideal linac channel
Phase advance per period must stay in reasonable limits (30-80 deg), phase advance per unit length must be continuous (smooth variations).
→ At low β, we need a strong focusing term to compensate for the defocusing, butthe limited space limits the achievable G and l → needs to use short focusing periods N βλ.
Note that the RF defocusing term ∝f sets a higher limit to the basic linac frequency (whereas for shunt impedance considerations we should aim to the highest possible frequency, Z ∝�f) .
6
Mechanical constraints
Variety (cells matched to the particular beta profile) and complexity (need to integrate focusing elements, cooling, vacuum and RF tightness) can considerably add to the cost: the LEGO can be complex and expensive…
CERN DTL design GSI CH-DTL design
7
HIPPI: for a fair comparison of structures
→ Clear need for a zoological classification of linac structures, which requires an agreement on the terms of comparison and on the figures of merit.
→ The HIPPI Joint Research Activity of EU FP6 (=High Intensity Pulsed Proton Injectors), active from 2004 to 2008, has tried this classification at the end of the activity of Workpackage #2 (Normal Conducting Structures). The WP has pushed the developments of structures for Linac4, FAIR linac and RAL upgrades and has published a comparison paper:
Comparative Assessment of HIPPI Normal Conducting StructuresC. Plostinar (Editor), CARE-Report-08-81-HIPPI
http://irfu.cea.fr/Phocea/file.php?class=std&&file=Doc/Care/care-report-08-072.pdf
8
Drift Tube Linac
The workhorse of linacs, is there since 1946.Evolved a lot from the early age (single stem, post couplers,..) Still, quite a lot of development to be done, for:
1. Integrating the focusing elements.2. Optimising the drift tube alignment mechanism.3. Simplify construction (and reduce cost).
Recent R&D work at CERN in the frame of HIPPI, resulted in the construction of a prototype funded by INFN-Legnaro.
The prototype has been successfully assembled, aligned and tested at low RF power. High power tests in spring 2009.
Measured TolerancesX (horiz) 0.058 0.1 mmY (long) 0.073 0.1 mmZ (vert) 0.029 0.1 mmY (yaw) 1.506 3.0 mradZ (roll) 1.795 3.0 mrad
• All drift tube positions within tolerances
First bead-pull measurements of Ez0
0.2
0.4
0.6
0.8
1
0 120Position
Ez
9
CCDTL and SDTL
CCDTL=Cell-Coupled Drift Tube Linac SDTL=Separated Drift Tube LinacMain idea: as β increases, can have longer focusing periods with same phase advance →
take the quadrupoles outside of the drift tubes.Advantages:1. Smaller diameter of the drift tube, potential for higher shunt impedance.2. Quadrupoles between tanks can be EM, accessible for replacement and interventions.3. Drift tubes w/o quadrupoles have less stringent alignment tolerances, drift tube
adjustment system can be simpler and less expensive.2 options:-Short tanks connected by coupling cells, no or minimum power spitting → CCDTL (CERN)-Longer tanks fed separately, splitting from RF source. → SDTL (JPARC)
CCDTL (CERN) SDTL (JPARC)
10
SDTL & CCDTL power distribution
0.8 MW 0.8 MW
3 MW klystron 1.3 MW klystron
1 MW 1 MW 1 MW
2.8 MW klystron
JPARC SDTL Linac4 CCDTL initial Linac4 CCDTL final configuration
5 cells/tank 3 cells/tank
11
Shunt Impedance – SDTL and CCDTL
12
Pi-mode : SCL and PIMS
Quadrupole
Coupling CellsBridge Coupler
Quadrupole
Coupling CellsBridge Coupler
Quadrupole
Coupling CellsBridge Coupler
Side Coupled Linac structure Pi-Mode StructureBoth operating in π-mode, PIMS at the basic frequency, SCL at 2*basic frequency
SCL long chain of cells (>100) fed by a single klystron, 2 PIMS fed by 1 klystron (splitting)
13
CERN comparison SCL - PIMS
PRF ~ 10% lowerfor SCL
(approximate est.)
Single frequency is an additional advantage for Linac4 (160 MeV)
14
TE mode: CH-DTL
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CH-DTL: mechanics and beam dynamics
Need special “KONUS” beam dynamics (no synchronous phase particle, acceleration around the crest of the wave, internal rebunching on some gaps) to have a zero RF defocusing component → can afford the long focusing periods required by the TE mode of operation, but the beam has to spend long time in non-linear regions of phase space.
Emittances at FAIR linac (no errors)
Present simulations indicate that the CH can accept a high space charge beam (large bunch current), but show some beam loss (up to 5%) in presence of errors.
Strong effort at GSI for the engineering of the CH structure(internal triplet, coupling cell, drift tube supports).
16
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17
Comparison of Shunt Impedances
Calculated ZTT values per meter (“real estate”) scaled down to take into account additional losses:DTL – 20% reduction. CH-DTL – simulations in good agreement with measurements, 5% reduction. CCDTL – 17% reduction.SCL – 20% reduction.PIMS – 30% reduction.
18
Comparison of beam dynamics results
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19
Superconducting options A triple-spoke SC linac for the energy range 100-200 MeV has beenanalysed in HIPPI as a possible option for Linac4 and SPL:
SUPERCONDUCTING SPOKE LINAC DESIGN AS AN ALTERNATIVE OPTION FOR THE CERN LINAC4 HIGH ENERGY PART E. Sargsyan, A. Lombardi, CERN - CARE-Note-2006-009-HIPPI
HIPPI Triple-spoke cavity prototype built at FZ Jülich, now under test at IPNO
Spoke Version 1
Spoke Version 2
SCL
frequency 352.2 352.2 704.4 MHz V0 7.28 6 - MV synchronous phase -20 -20 -20 deg Wout 163.9 159.8 163.4 MeV no. of cavities/tanks 14 16 20 no. of cryomodules 7 8 - total length 22.4 25.6 28.7 m �x growth 2.2 2.3 2.3 % �y growth 3.5 3.5 4.7 % �z growth 4.4 5.3 3.1 % rbeam, max/raperture 0.315 0.319 0.505 transmission 100 100 100 %
A 90 - 160/180 MEV SPOKE LINAC AS AN OPTION FOR THE CERN LINAC4 /SPL"Jean-Luc Biarrotte, Guillaume Olry, CNRS, IPN Orsay - CARE-Note-2006-008-HIPPI
HIPPI comparison of spoke and SCL
20
Other ideas…
IH Accelerating Structures with PMQ Focusing for Low-energy Light IonsS. S. Kurennoy, S. Konecni, J. F. O'Hara, L. Rybarcyk, EPAC08
Proposed for deuterons, 1 – 4 MeV
An interesting idea, combining the high shunt impedance of TE modes (in this case IH, TE110) with a classical beam optics ensuring low beam losses and minimum emittance growth.
Possible now because we have compact permanent quadrupoles (PMQ) that fit into small drift tubes and we have 3D RF simulation codes that allow designing complicated structures.
To be investigated for the energy range 3 – 50 MeV, applied to the CH.
21
(my personal) conclusions
Some personal conclusions, based on the experience of HIPPI and of the Linac4 R&D, not required to be objective…
If I would have to build a linac in the range 3-200 MeV, with average current in the mA range:
1. In the short term → take the Linac4 (and SNS/JPARC) architecture, in case reconsidering CCDTL/SDTL vs. DTL for a new “low-cost” DTL design.
2. In the medium term → consider a spoke option for the 100-200 MeV range, which needs R&D on the cavities.
3. In the long term → consider possible alternatives or improvements to the DTL, (TE-mode structures with PMQs, multiple cavities with power splitting, ...)
A final warning: for a linac with duty cycle of 5-10%, cost considerations indicate that the optimum transition energy warm to cold is in the range 80 – 180 MeV.