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RF Structure Comparison for Low
Energy Acceleration
M. Vretenar, CERN
ESS-B Workshop



                     1.   Motivati...
Definitions
“Standard” block-diagram of a high-energy linac:

1. Front-end (ion source, LEBT, RFQ, MEBT);
2. Low-energy, f...
What architecture for “low-energy”?
   Generally no consensus on the type of structure to be used !

 Comparing the 2 most...
Figures of merit – Power efficiency
Shunt impedance = efficiency in transforming RF power into voltage on the gap     Z = ...
Beam dynamics constraints
 The “low-energy” linac section is dominated by space charge and RF defocusing forces.
 Approxim...
Mechanical constraints

Variety (cells matched to the particular beta profile) and complexity (need to
integrate focusing ...
HIPPI: for a fair comparison of structures
→ Clear need for a zoological classification of linac structures, which require...
Drift Tube Linac
                               The workhorse of linacs, is there since 1946.
                            ...
CCDTL and SDTL
   CCDTL=Cell-Coupled Drift Tube Linac               SDTL=Separated Drift Tube Linac
Main idea: as β increa...
SDTL & CCDTL power distribution
               3 MW klystron                                          2.8 MW klystron
    ...
Shunt Impedance – SDTL and CCDTL




                                   11
Pi-mode : SCL and PIMS
        Side Coupled Linac structure           Pi-Mode Structure
Both operating in π-mode, PIMS at ...
CERN comparison SCL - PIMS


                                    PRF ~ 10% lower
                                    for S...
TE mode: CH-DTL

                    Low and Medium - β Structures in H-Mode Operation                    $               ...
CH-DTL: mechanics and beam dynamics


                                                       Strong effort at GSI for the
...
The HIPPI structure comparison

                                    !       quot;
      ##               !        $

     ...
Comparison of Shunt Impedances




 Calculated ZTT values per meter (“real estate”) scaled down to take into account addit...
Comparison of beam dynamics results

   ++       ( 3+         #+   &        5 + #+ &       &             (& ((
           ...
Superconducting options
 A triple-spoke SC linac for the energy range 100-200 MeV has been
 analysed in HIPPI as a possibl...
Other ideas…
IH Accelerating Structures with PMQ Focusing for Low-energy Light Ions
S. S. Kurennoy, S. Konecni, J. F. O'Ha...
(my personal) conclusions

Some personal conclusions, based on the experience of HIPPI and of the Linac4
  R&D, not requir...
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ESS-Bilbao Initiative Workshop. RF structure comparison for low energy acceleration.

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RF structure comparison for low energy acceleration. Maurizio Vretenar (CERN).

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ESS-Bilbao Initiative Workshop. RF structure comparison for low energy acceleration.

  1. 1. RF Structure Comparison for Low Energy Acceleration M. Vretenar, CERN ESS-B Workshop 1. Motivations 2. Figures of merit 3. Structure catalogue 4. An attempt of comparison 1
  2. 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 – SC b. 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. 2
  3. 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): 2.5 MeV 87 MeV 186 MeV 402 MHz SNS DTL SCL 2f 1.4mA avg. 3 MeV 50 MeV 191 MeV 325 MHz JPARC DTL SDTL 0.7mA avg. 3 MeV 50 MeV 160 MeV 352 MHz CERN-Linac4 DTL CCDTL PIMS 0.02-2.4mA 3 MeV 70 MeV 325 MHz GSI-FAIR CH-DTL 5 µA avg. Common features: all designs normal conducting, sequence of different accelerating structures Frequency: basic in the 325-402 MHz range, doubling only in SNS SCL 3
  4. 4. Figures of merit – Power efficiency Shunt impedance = efficiency in transforming RF power into voltage on the gap Z = V2/P Is one of the main figures of merit, depends on operating mode and frequency TE mode (GSI) TM modes (Linac4) π-mode 0-mode 3 – 200 MeV 1. TM-0 modes have Z decreasing with energy. TE mode structures used (high 2. TM-π modes have Z increasing with energy. efficiency) for ions at very low β, 3. TE modes have high Z, decreasing with energy. recently extended to protons 4 4. In general terms, Z scales as f
  5. 5. Beam dynamics constraints The “low-energy” linac section is dominated by space charge and RF defocusing forces. Approximate expression for phase advance in an ideal linac channel π q E0T sin(− ϕ ) 3q I λ (1 − f ) 2 2 σt q Gl kt2 = = − − Nβλ 2 mc βγ mc 2λ β 3γ 3 8πε 0 r0 mc 3β 2γ 3 3 ! ! quot; # 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, but the 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) . 5
  6. 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… GSI CH-DTL design CERN DTL design 6
  7. 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 Structures C. Plostinar (Editor), CARE-Report-08-81-HIPPI http://irfu.cea.fr/Phocea/file.php?class=std&&file=Doc/Care/care-report-08-072.pdf 7
  8. 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 Tolerances 0.058 0.1 mm X (horiz) 0.073 0.1 mm Y (long) 0.029 0.1 mm Z (vert) 1 1.506 3.0 mrad Y (yaw) 0.8 1.795 3.0 mrad Z (roll) 0.6 Ez 0.4 • All drift tube positions within tolerances 0.2 8 First bead-pull measurements of Ez 0 0 120 Position
  9. 9. CCDTL and SDTL CCDTL=Cell-Coupled Drift Tube Linac SDTL=Separated Drift Tube Linac Main 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) 9 CCDTL (CERN) SDTL (JPARC)
  10. 10. SDTL & CCDTL power distribution 3 MW klystron 2.8 MW klystron 1.3 MW klystron 3 cells/tank 5 cells/tank 0.8 MW 0.8 MW 1 MW 1 MW 1 MW JPARC SDTL Linac4 CCDTL initial Linac4 CCDTL final configuration 10
  11. 11. Shunt Impedance – SDTL and CCDTL 11
  12. 12. Pi-mode : SCL and PIMS Side Coupled Linac structure Pi-Mode Structure Both operating in π-mode, PIMS at the basic frequency, SCL at 2*basic frequency Coupling Cells Bridge Coupler Quadrupole SCL long chain of cells (>100) fed by a single klystron, 2 PIMS fed by 1 klystron (splitting) 12
  13. 13. CERN comparison SCL - PIMS PRF ~ 10% lower for SCL (approximate est.) Single frequency is an additional 13 advantage for Linac4 (160 MeV)
  14. 14. TE mode: CH-DTL Low and Medium - β Structures in H-Mode Operation $ %& ' ( )) H 110 H 210 ( < 100 - 400 MHz R f ~ 100 MHz * + #, F β < 0.12 < β ~ 0.03 ~ Q # LIGH T ! β. quot; - IO NS /% & H 11 (0) H 21 (0) NS ( 0) ! IO HE AV Y β. 1 D T L 2(0 # % # < 300 MHz f 250 - 600 MHz ~ < < β ~ 0.6 , 3 β ~ 0.3 # 4 14
  15. 15. CH-DTL: mechanics and beam dynamics Strong effort at GSI for the engineering of the CH structure (internal triplet, coupling cell, drift tube supports). 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 15 errors.
  16. 16. The HIPPI structure comparison ! quot; ## ! $ quot;& % '( ) * & +& (+ , , )-# .+ / ( 0 ) quot;& ! quot; $quot; 1 ! 2+ 3& ) 4 5 2& quot;& 1 quot;* 2& 16 ( + 6
  17. 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. 17 PIMS – 30% reduction.
  18. 18. Comparison of beam dynamics results ++ ( 3+ #+ & 5 + #+ & & (& (( 78 & 9 78 & 9 !26 :7 < & & & ) ; :7 < & & & ) ; 6 <) <) !# $ quot; %& ! #' ! #' ()* + , -, $ %& ! #' ! #' ./ $ %& ! #' ! #' 00. / $ %& ! #' ! #' 00. / $ %& ! #' ! #' 18
  19. 19. Superconducting options A triple-spoke SC linac for the energy range 100-200 MeV has been analysed in HIPPI as a possible option for Linac4 and SPL: A 90 - 160/180 MEV SPOKE LINAC AS AN OPTION FOR THE CERN LINAC4 /SPLquot; Jean-Luc Biarrotte, Guillaume Olry, CNRS, IPN Orsay - CARE-Note-2006-008-HIPPI 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 Spoke Spoke SCL Version 1 Version 2 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 % HIPPI Triple-spoke cavity HIPPI comparison of spoke and SCL prototype built at FZ Jülich, now under test at IPNO 19
  20. 20. Other ideas… IH Accelerating Structures with PMQ Focusing for Low-energy Light Ions S. 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. 20
  21. 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. 21

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