High beta cavity simulations and RF measurements

Alessandro D’Elia- Cockroft Institute and University of Manchester

1

HIE-ISOLDE upgrading stages

Stage 1 is shown at the top, while stage 2 can be split into two sub-stages depending on the physics priorities: the low energy cryomodules will allow the delivery of a beam with better emittance; the high energy cryomodule will enable the maximum energy to be reached

M. Pasini, D. Voulot, M. A. Fraser, R. M. Jones, ”BEAM DYNAMICS STUDIES FOR THE SCREX-ISOLDE LINAC AT CERN”, Linac 2008, Victoria, Canada

3MeV/u* 5.5MeV/u* 10MeV/u*

* A/q= 4.5

1.2MeV/u*

2

High beta cavity

3

784.5mm

300mm

Beam

Coupler and Pick up seats

Resonator (/4)

Tools “calibration”

In order to get reliable cavity parameters values from simulations, a comparison between the results coming from HFSS and CST Microwave has been performed using Superfish as a benchmark

4

Superfish vs CST Microwave and HFSS

5

Frequency

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HFSS Meshing (5m)

HFSS Meshing (20m)

CST Meshing

E field*

7

* All field values are normalized to give 1J stored energy in the cavity (CST Normalization)

H field*

8

* All field values are normalized to give 1J stored energy in the cavity (CST Normalization)

Comparison tables

9

Superfish CST HFSS ∆CST-SF (%) ∆HFSS-SF (%)

Frequency (MHz)

101.674 101.666 101.674

Hpeak

(kA/m)16.711 16.76 16.763 0.3 1.1

Epeak

(MV/m)11.38 11.5 11.6 1 1.9

Quality Factor

∆ (%)

Superfish 11795 -

CST 11844 0.4

HFSS 11746 -0.4

“Real” structure

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Remarks

• Never being confident to post-processing results!!

• Even if HFSS and CST results are consistent and very close to Superfish, when we start to complicate our structure (tuner plate, coupler and pick-up), the possibility of having a finer refinement on surface meshing gets HFSS results more reliable

• The above statement are not general!!

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Cavity Parameters

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ISOLDE TRIUMF* SPIRAL 2**

Frequency [MHz] 101.28 141.4 88

(%) 10.3 11.2 12

Lnorm (mm) 30 18 41

Epeak/Eacc 5.4 4.9 4.9

Bpeak/Eacc[G/(MV/m)]

96 99 90

Rsh/Q0 [] 554 545 518

=Rs∙Q0 [] 30.34 25.6 37.5

* V. Zvyagintsev et al., “Development, Production And Tests Of Prototype Superconducting Cavities For

The High Beta Section Of The Isac-ii Heavy Ion Accelerator At Triumf”, RuPAC 2008, Zvenigorod, Russia

** G. Devanz, “SPIRAL2 resonators” talk held at SRF05

Q0 values

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ISOLDE(Eacc=6MV/m)

Pcav (W) Rs (n) Q0=/Rs

5 33 109

7 46 6.6∙108

10 65 4.6∙108

12 79 3.9∙108

15 98 3.1∙108

** G. Olry et al., “Tests Results Of The Beta 0.12 Quarter Wave Resonators For The Spiral2 Superconducting Linac”, LINAC 2006, Knoxville, Tennessee USA

* V. Zvyagintsev et al., “Development, Production And Tests Of Prototype Superconducting Cavities For

The High Beta Section Of The Isac-ii Heavy Ion Accelerator At Triumf”, RuPAC 2008, Zvenigorod, Russia

TRIUMF*: Q0=7∙108 with Pcav=7W and Eacc=8.5MV/m

SPIRAL2**: Q0=109 with Pcav=10W and Eacc=6.5MV/m

Some word about the hot frequency

The cold frequency has to be 101.28MHz

In air: -32kHz

101.248MHz

In superconducting mode of operation (shortening of the length of the antenna,…): -332kHz

100.916MHz

Other contributions (chemistry,…): ????

~ 100.900MHz

skin depth variation: -11kHz

100.905MHz

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Study of RF tuning plate

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Frequency with tuner plateTipgap 70mm Tipgap 90mm

Tuner plate position +5mm 100.684 MHz 101.235 MHz

∆ Tipgap 27.55kHz/mm

Tuner plate position -15mm 100.929 MHz 101.339 MHz

∆ Tipgap 20.5kHz/mm

∆ Tuner plate 12.25kHz/mmTotal Coarse

range=245kHz

5.2kHz/mm

Pick up length=-1mm, coupler length=5mm

Triumf tuner coarse range 32kHz16

Measurements vs Simulations25/03/2009

Tipgap 90Without tuner plate

Tipgap 75Without tuner plate

Tipgap 70Without tuner plate

Simulation Measurements* Simulation Measurements* Simulation Measurements*

Long coupler and pick-up

101.233 MHz(- 32kHz air)101.201 MHz

101.246 MHz**(-77kHz Res) *101.169 MHz

101.013 MHz(- 32kHz air)100.981 MHz

101.000 MHz(-77kHz Res) *100.923 MHz

100. 899 MHz(- 32kHz air)100.867 MHz

100.916 MHz(-77kHz Res) *100.839 MHz

Short coupler and pick-up

101.410 MHz(- 32kHz air)101.378 MHz

101.483 MHz(-77kHz Res) *101.406 MHz

101.191 MHz(- 32kHz air)100.159 MHz

101.240 MHz(-77kHz Res) *101.163 MHz

101.083 MHz(- 32kHz air)101.051 MHz

101.150 MHz(-77kHz Res) *101.073 MHz

* Resonator longer of 0.4mm with respect to the nominal length (135kHz/mm)

** These new measurements have been done in a much noisy environment that explain the 13kHz of difference with respect to the previous ones

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Expected final hot frequency

Measured frequency 101.150 MHz

∆ plate-tuner (pos-15) - 130 kHz

∆ tuner central position (-5) - 122.5 kHz

Expected frequency = 100.897 MHz(goal f~100.900 MHz)

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External Q

Let us assume Q0=5x108 and a condition of perfect coupling (c=1)

Therefore we want

• Qext of RF coupler of 2.5x106 in order to be undercoupled (c=200 ∆f 40Hz) (larger bandwidth)

• Qext Pick-up of 1010 in order to be overcoupled (negligible power flowing from the pick-up) 19

HzQ

ff

Qf

f

loadload

4.01 0

0

Qload=2.5x108

Q measurements

• Hot measurements are important to test and calibrate the coupler and pick-up before going to cryostate

• Very difficult to get reliable measurements allowing for such a high Qext values

• Cold measurements are needed for the final characterization• It is not possible going through standard frequency domain measurements

as

• Two different strategies for hot and cold measurements

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00

0

with 1

fff

fQ

Qf

fload

load

β measurements

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RF Coupler Pick-up

Network Analyzer

Pc

PfPe

Pr

Pin

Pin = Pf-Pr = Pc+Pe

2

1

11

c

cfP

Dividing everything by Pe and rearraging, by considering that

puc

e

f

e

P

PS

P

P and212

2

2

2

21)1(

421

S

S

c

cpu

Note: the system is symmetric so that I can feed from the pick-up and meauring c

Qext hot measurements1) Measuring SWR from S11

2) Measuring S21 pu

3) Measuring Qload

4) Evaluating Qext

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c S21 pu Qload Q0 Qpu Qc

1.019 - - 5636 11380 - 11168

1.73 2.21∙10-1 0.055709 3902 10870 1.95∙105 6283

1.84 1.67∙10-2 0.000306 3944.5 11204 3.66∙107 6089

1.84 3.52∙10-5 1.36∙10-9 3944.5 11202 8.24∙1012 6088

1.0157 1.75∙10-2 0.000307 5643 11376 3.70∙107 11200

0.9574 2.31∙10-1 0.056495 5504 11085 1.96∙105 11578

1 if1

1 if

c

c

c

SWR

SWR

pucloadQQ 10

extQ

Q0

Legend

W/o pick-up

Lpu_in=22mm

Lpu_in=-1mm

Max Error= 3.6%

Q cold measurements

f

fQ

0

0 as Q0109 ∆f0.1Hz

Lt Q

UP

dt

dU 0 LQ

t

eUtU0

0)(

0

LL

Q

By feeding the cavity by a rectangular pulse

pucpuccavt

L QU

P

U

P

U

P

U

P

Q

1

11

00000

Knowing c, we get Q0

By switching off I can measure

fc

cr PP

2

1

1

c

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We can use for c value the one we got from the hot measurements or we can feed the cavity by a rectangular pulse, in the steady-state

Coupler

24• -10mm Linsertion 60mm 7500 < Qext < 5∙109

Macor

• Dust free sliding mechanism

Coupler

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Internal Reflections

Conclusions

• E-m design of the high beta cavity is finished

• The machining of the copper part is finished

• Measurements show a very good agreement with simulations

• First prototype of the tuner already available, sputtering on the end of June

• Mechanical design and fabrication of the coupler is started, deliviring date, end of July

• Starting the design of the low beta cavities27

Reviewer comments

28

The cavity parameter list represents a challenging but attainable specification and would represent the state of the art in sputtered heavy ion linac performance

CERN has applied for EuCARD to develop quarter wave sputtering using a magnetron or other techniques and this R+D would benefit HIE-Isolde but also the world community as new material searches are ongoing to improve cavity performance while reducing production costs.

A study of the power dissipation in the RF coupler at different coupling factors is needed in order to define the heat deposited to the Helium space

The tuning sensitivity of the bottom tuning plate at 13kHz/mm should be adequate to meet the coarse tuning range required in the cavity. However it may be preferable to reduce this sensitivity somewhat to allow a reduced specification on the tuner resolution since in this case a tuner position change of only 1 micron corresponds to more than the expected full cavity bandwidth. A factor of two reduction would help. This would also reduce the rf currents on the tuning plate where thermal conduction may be limited due to a marginal pressed contact