High beta cavity simulations and RF measurements

Alessandro D’Elia- Cockcroft Institute and University of Manchester

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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

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Superfish vs CST Microwave and HFSS

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Frequency

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HFSS Meshing (5m)

HFSS Meshing (20m)

CST Meshing

E field*

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

H field*

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

Comparison tables

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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

101.248MHzIn 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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RF CouplerPick-up

cavity

tipgap

Network Analyzer

• The Pick-up position is fixed (22mm inside the cavity)• The RF coupler position is varying

Cavity Prototype Measurements

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Measurements on November 2008

* Pick up length=22mm

∆ coupler1=14kHz/mm

∆ coupler2=22kHz/mm

∆ coupler3(from 22 to 64)=5.7kHz/mm

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“positive” structure

“negative” structure

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Tipgap**** 75mm Tipgap 90mm

Short Coupler and pick-up* 101.191 MHz 101.410 MHz

Long Coupler and pick-up** 101.013 MHz 101.233 MHz

∆ Coupler3 4.24kHz/mm*** 4.24kHz/mm

Frequency without tuner plate

* “Short” means coupler length=22mm and pick-up length=22mm

** “Long” means coupler length=64mm and pick-up length=22mm

*** ∆ Coupler3 (measured)=5.7kHz/mm

**** Remind: tipgap is the distance of the bottom plate from the central resonator

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Study of RF tuning plate

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Tuner position +5 Tuner position -15

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Simulation with tuner position +5, tipgap 70

100.684 MHz

1.220.000

Coupler length 5mmPick up length -1mm

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Simulation with tuner position -15, tipgap 70

1.066.710

100.929 MHz

Coupler length 5mmPick up length -1mm

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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 32kHz 23

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) 26

HzQff

Qff

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 ffffQ

Qff

loadload

β measurements

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RF Coupler Pick-up

Network Analyzer

Pc

PfPe

Pr

Pin

Pin = Pf-Pr = Pc+Pe

2

111

c

cfP

Dividing everything by Pe and rearraging, by considering that

puc

e

f

e

PPS

PP and21 2

22

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 S112) 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 if11 if

c

c

c

SWR

SWR

pucloadQQ 10

extQQ0

Legend

W/o pick-up

Lpu_in=22mm

Lpu_in=-1mm

Max Error= 3.6%

Q cold measurements

ffQ

0

0 as Q0109 ∆f0.1Hz

Lt Q

UPdtdU 0

LQt

eUtU0

0)(

0

LLQ

By feeding the cavity by a rectangular pulse

pucpuccavt

L QUP

UP

UP

UP

Q

111

00000

Knowing c, we get Q0

By switching off I can measure

fc

cr PP

2

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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

31• -10mm Linsertion 60mm 5∙109 < Qext < 7500

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 cavities34