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= MeV/u* 2

High beta cavity mm 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 6 HFSS Meshing (5  m) HFSS Meshing (20  m) 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 SuperfishCSTHFSS ∆ CST-SF (%) ∆ HFSS-SF (%) Frequency (MHz) H peak (kA/m) E peak (MV/m) Quality Factor ∆ (%) Superfish CST HFSS

“Real” structure 10

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!! 11

Cavity Parameters 12 ISOLDETRIUMF*SPIRAL 2** Frequency [MHz]  (%) L norm (mm) Epeak/Eacc Bpeak/Eacc [G/(MV/m)] Rsh/Q0 [  ]  =Rs∙Q0 [  ] * 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 13 ISOLDE (E acc =6MV/m) P cav (W) R s (n  )Q0=  /R s ∙ ∙ ∙ ∙10 8 ** 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∙10 8 with P cav =7W and E acc =8.5MV/m SPIRAL2**: Q0=10 9 with P cav =10W and E acc =6.5MV/m

Some word about the hot frequency The cold frequency has to be MHz In air:  -32kHz MHz In superconducting mode of operation (shortening of the length of the antenna,…):  -332kHz MHz Other contributions (chemistry,…): ???? ~ MHz skin depth variation:  -11kHz MHz 14

Study of RF tuning plate 15

Frequency with tuner plate Tipgap 70mmTipgap 90mm Tuner plate position +5mm MHz MHz ∆ Tipgap27.55kHz/mm Tuner plate position -15mm MHz MHz ∆ Tipgap20.5kHz/mm ∆ Tuner plate 12.25kHz/mm Total Coarse range=245kHz 5.2kHz/mm Pick up length=-1mm, coupler length=5mm Triumf tuner coarse range 32kHz 16

Measurements vs Simulations 25/03/2009 Tipgap 90 Without tuner plate Tipgap 75 Without tuner plate Tipgap 70 Without tuner plate SimulationMeasurements*SimulationMeasurements*SimulationMeasurements* Long coupler and pick-up MHz (- 32kHz air) MHz MHz** (-77kHz Res) * MHz MHz (- 32kHz air) MHz MHz (-77kHz Res) * MHz MHz (- 32kHz air) MHz MHz (-77kHz Res) * MHz Short coupler and pick-up MHz (- 32kHz air) MHz MHz (-77kHz Res) * MHz MHz (- 32kHz air) MHz MHz (-77kHz Res) * MHz MHz (- 32kHz air) MHz MHz (-77kHz Res) * 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 17

Expected final hot frequency Measured frequency MHz ∆ plate-tuner (pos-15) kHz ∆ tuner central position (-5) kHz Expected frequency= MHz (goal f~ MHz) 18

External Q Let us assume Q 0 =5x10 8 and a condition of perfect coupling (  c =1) Therefore we want Qext of RF coupler of  2.5x10 6 in order to be undercoupled (  c =200 ∆ f  40Hz) (larger bandwidth) Qext Pick-up of  in order to be overcoupled (negligible power flowing from the pick-up) 19 Q load =2.5x10 8

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 20

β measurements 21 RF Coupler Pick-up Network Analyzer Pc Pf Pe Pr Pin Pin = Pf-Pr = Pc+Pe Dividing everything by P e and rearraging, by considering that Note: the system is symmetric so that I can feed from the pick-up and meauring  c

Q ext hot measurements 1)Measuring SWR from S11 2)Measuring S21  pu 3)Measuring Q load 4)Evaluating Q ext 22 cc S21  pu Q load Q0Q0 Q pu QcQc ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ Legend W/o pick-up L pu_in =22mm L pu_in =-1mm Max Error= 3.6%

Q cold measurements as Q0  10 9 ∆f  0.1Hz By feeding the cavity by a rectangular pulse Knowing  c, we get Q0 By switching off I can measure cc 23 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 mm  L insertion  60mm 7500 < Q ext < 5∙10 9 Macor Dust free sliding mechanism

Coupler 25

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 cavities 27

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