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

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Shunt Impedance – SDTL and CCDTL
11

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. CERN comparison SCL - PIMS
PRF ~ 10% lower
for SCL
(approximate est.)
Single frequency is an additional
13
advantage for Linac4 (160 MeV)

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. 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. The HIPPI structure comparison
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( + 6

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. Comparison of beam dynamics results
++ ( 3+ #+ & 5 + #+ & & (& ((
78 & 9 78 & 9 !26
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; :7 < & & & )
; 6
<) <)
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18

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. 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. (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