ssr2 rf design&mp - indico-fnal (indico)...aug 08, 2018 · gradient range of 5-12 mv/m....

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


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.


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


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


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

10 8/8/2018P. Berrutti | SSR2 RF design&MP

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.


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


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)


ssr2 rf design&mp - indico-fnal (indico)...aug 08, 2018 · gradient range of 5-12 mv/m....

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