SSR2 – RF Design&MP
Paolo Berrutti
Outline
2
• Introduction
• Cavity EM parameters
• Cavity geometry
• Multipacting studies and mitigation
• Summary
8/8/2018P. Berrutti | SSR2 RF design&MP
Introduction
3
• SSR2 is part of the SC LINAC of PIP-II, it will accelerate the H- beam from
the end of SSR1 (MeV) to the beginning of the first 5-cell cavity section (185
MeV).
• The total number of SSR2 cavities needed is 35 divided into 7 cryomodules,
having 5 resonators each.
• SSR2 beam pipe aperture is 40 mm
• Peak fields limitations are 40 MV/m for electric and 70 mT for magnetic field.
• Focusing is achieved with solenoids, having integrated quadrupole and
dipole correctors coils.
PIP-II technology map
8/8/2018P. Berrutti | SSR2 RF design&MP
Cavity design and EM parameters
4
Parameters SSR2 v 2.6
Optimal beta βopt 0.475
Geometrical beta βg 0.402
Aperture [mm] 40
Frequency [MHz] 325
Effective length 2βoptλ/2 [m] 0.438
Epeak/Eacc 3.38
Bpeak/Eacc [mT/(MV/m)] 5.93
G [Ohm] 115.2
R/Q [Ohm] 296.6
Max Epeak [MV/m] 40
Max Bpeak [mT] 70
Max energy gain [MeV] 5.17
Max gradient [MV/m] 11.8
• Value of beta has been optimized for
PIP-II.
• The RF design has been finalized.
• EM parameters allow 5.17 MeV
energy gain at max peak fields.
• 3D EM fields are shown below E on
the left and H on the right.
8/8/2018P. Berrutti | SSR2 RF design&MP
Single spoke resonator geometry COLD I
5
• The lengths are
reported in the table
below.
• All dimensions are for
the cold cavity (2K)
Parameter Length [mm]
L_cav 500
R_cav 271.6
R_spoke 130.71
D_aperture 40
Gap_to_gap 185.9
W_spoke 72.26
R_cav
Gap_to_gap
D_ap
erture
L_cav
R_spoke
W_spoke
Z-Y cavity cross section
8/8/2018P. Berrutti | SSR2 RF design&MP
Single spoke resonator geometry COLD II
6
• The lengths are
reported in the table
below.
• All dimensions are for
the cold cavity (2K)
Parameter Length [mm]
R_cav 271.6
CP_to_center 337
CP_diam 76.90
H_spoke 133
R_c
av
CP
_diam
H_spoke
CP_to_center
X-Y cavity cross section
8/8/2018P. Berrutti | SSR2 RF design&MP
MP simulations
• CST particle studio is thesoftware used, it requires theuser to create a shell all aroundthe cavity volume to have alayer of emitting material.
• The electron sources have tobe placed on the inner cavitysurface, input particle energyusually ranges from 2 to 6 eV,the emission angle can be setto random.
• Different Secondary EmissionYields (SEY) have been appliedto the cavity walls in order tounderstand how strongly themultipacting depends onmaterial properties.
Particle sources
for a single
spoke cavity are
highlighted in
red.
8/8/2018P. Berrutti | SSR2 RF design&MP7
Growth rate and MP locations
• When MP occurs the number of electrons in
the cavity increases exponentially with time,
the growth rate is the exponential coefficient
of the best fit of number of particles vs. time
expressed in 1/ns.
• If MP resonant condition is satisfied, the
number of particles will increase every half
RF period.
• MP areas migrate and evolve with the gradient: the resonant condition needs a certain
field amplitude to be sustained, increasing the gradient the MP moves from higher field
regions to lower field regions.
H field E field MP @ Eacc=4.4 MV/m MP @ Eacc=10.4 MV/m8/8/2018P. Berrutti | SSR2 RF design&MP8
MP SSR1
• Since several SSR1 resonators have been tested and multipacting
barriers have been experimentally observed; the results of multipacting
simulations of SSR1 have been compared with the data collected
during the vertical tests of the cavities. The sum over all the cavities of
the time spent to process a barrier is overlapped with the GR results for
discharge cleaned material.
SSR1 Eacc in operating condition from 3 to 10.5 MV/m
8/8/2018P. Berrutti | SSR2 RF design&MP9
Multipacting mitigation
Most severe multipacting take place near transition of cylindrical part to end walls. Several design options of this transition were considered. Most significant improvement was achieved after introducing additional step in the transition are.
Modification of this transition reduce multipacting in operating range of cavity fields.
SSR2 v1
SSR2 v2.6 SSR2 v2.6
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Multipacting Growth Rate
When the multipacting condition occurs one can define the growth rate as the exponential coefficient of the particle number vs time. N(t)=N0eαt
The old SSR2 design (left) showed high growth rate for all the gradient range used in PIP-II.The new design (right) has a considerably lower growth rate throughout the whole gradient range.
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SSR2 – Multipacting comparison
The secondary electron multiplication, δ=N(t+τ)/N(t), significantly reduced in the operating gradient range of 5-12 MV/m. Comparison with SSR1 cavity demonstrates that multipacting in the modified SSR2 cavity version 2.6 can be processed way easier than in SSR1 cavity.
Parameters SSR2 v1 SSR2 v2.6
Optimal beta 0.471 0.475
Epeak/Eacc 3.45 3.38
Bpeak/Eacc
[mT/(MV/m)]
6.107 5.93
G [Ohm] 112.98 115.2
R/Q [Ohm] 289.94 296.6
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Summary
13
• SSR2 EM parameters satisfy PIP-II project needs
• Multipacting has been mitigated: SSR2 cavity shows MP levels lower than
SSR1 (built and tested)
8/8/2018P. Berrutti | SSR2 RF design&MP