ESS Medium Beta Cavity Design and Construction in

ESS Medium Beta Cavity Design and Construction in LASA Saeid Pirani ESS AD Retreat, Lund – November 2016

OUTLINE • ESS medium-beta section • Recent update and status at LASA • Design parameters of medium-beta prototypes o Fabrication – Subcomponents frequency control o Fabrication – Assembly and cavity welding o Fabrication and Pre-Tuning • Fine grain 6 cell cavity vertical test result ESS AD Retreat, Lund – November 2016 2

ESS medium-beta section INFN-LASA activities: MB cavity technical requirements • Series In-kind contribution of 36 medium-beta (MB) cavities, including Niobium procurement, Cavity fabrication in the industry, Documentation, Certification activities, Cold test in a qualified infrastructure, Transportation and Man power for the above activities. • Prototypes Two MB cavity prototypes (same geometry different materials) have been designed, fabricated and will be cold tested at INFN-LASA. ESS AD Retreat, Lund – November 2016 Frequency (MHz) 704. 42 Number of cells 6 Geometric beta 0. 67 Nominal Acc. Gradient (MV/m) 16. 7 Epeak (MV/m) < 45 RF peak power (k. W) 1100 Q external 5. 9 -8× 105 Q 0 at nominal gradient > 5× 109 3

Recent update and status at LASA • PDR, held in LASA on 19 May 2016, of INFN-designed MB cavity : Approved • Suggestions from PDR review committee - Study on Multipacting: Complete - Qext vs Field Flatness: Complete - Study on HOM shifting with mechanical tolerances: Complete • Prototypes - Fabrication (Fine Grain (FG) & Large Grain (LG)): Done - FG cavity vertical test: Done - LG cavity vertical test : soon ESS AD Retreat, Lund – November 2016 4

Design parameters of prototypes Design goal • Plug-compatible • Larger cell-to-cell coupling factor, k>1. 5% • Allowing for a slight modest sacrifice on Epeak and R/Q Mechanical parameter Cavity wall thickness (mm) Stiffening radius (mm) Cavity Internal volume (l) Cavity internal surface (m 2) Stiffness (k. N/mm) Tuning sensitivity KT (k. Hz/mm) Vacuum sensitivity KV for Kext ~ 21 k. N/mm (Hz/mbar) LFD coefficient K L for Kext ~ 21 k. N/mm (Hz/(MV/m)2) INFN Design 4. 5 70 69 1. 8 1. 7 -210 31 -1. 7 Plugcompatible RF Parameters Riris (mm) Geometrical beta Frequency (MHz) Acc. length (m) Cell to cell coupling k INFN design 50 0. 67 704. 42 0. 855 1. 55%↗(+26%) ESS spec. ≥ 47 0. 67 704. 42 0. 855 π-5π/6 mode sep. (MHz) 0. 70↗(+30%) >0. 45 G (Ω) Optimum beta, βopt Max R/Q at βopt (Ω) Eacc at βopt (MV/m) Epeak/Eacc 198. 8 0. 705 374 16. 7 2. 55 ↗ (+7%) 0. 705 16. 7 Epeak (MV/m) Bpeak/Eacc(m. T/MV/m) 42. 6 ↘ (-6%) 4. 95 ↗ (+3%) < 45 Q 0 at nominal gradient >5× 109 Qext 7. 8× 105 5. 9~8× 105 5

Fabrication - Subcomponents frequency control and trimming Dumbbells (DBs) and End groups (EGs) are frequency-controlled before and after trimming HC DB trimmed 6

Fabrication – Assembly, cavity welding Fine Grain Cavity Large grain cavity LG DB 7

Pre-tuning (in air, room temperature, before BCP) • Cavity just fabricated: f=703. 51 , FF=70% • Cavity after final tuning: f=703. 11 MHz, FF=97% MB 001 Field Profile just After Fabrication 6 FF=70% [A. U. ] 5 4 3 2 1 [A. U. ] 0 5 4 3. 5 3 2. 5 2 1. 5 1 0. 5 0 0 5 10 Time [s] 15 20 MB 001 Field Profile After Pre-Tuning 0 5 10 Time [s] 15 Tuning an inner cell 25 FF=97% 20 25 8

Fine Grain 6 Cell Cavity Vertical Test Measurement Results ESS AD Retreat, Lund – November 2016

Cavity properties • Frequency @ 2 K – 704. 213 MHz (as expected) • Length @ room temperature – 1257. 6 mm • Qext for power coupler antenna @ 2 K – 6 E 9 • Qext for pick. Up antenna @ 2 K – 1. 5 E 11 ESS AD Retreat, Lund – November 2016

Power Rise @ 2 K 100 W CW cryo power, i. e. 16, 7 MV/m @ Q 0 5 e 9

Radiation

KL (Hz/(MV/m)2) Static Lorentz Force Detuning Free cavity 5 0 -5 -10 -15 -20 1. 0 E-04 1. 0 E-03 1. 0 E-02 1. 0 E-011. 0 E+001. 0 E+011. 0 E+021. 0 E+031. 0 E+04 Kext (k. N/mm)

HOM • The monopole mode close to 5 th machine line - Directly measured at 2 K, F = 1741. 8 MHz (>19 MHz from machine line at 1761. 05 MHz) - Close to expected value, <1 MHz to simulation at 1742. 4 MHz. ESS AD Retreat, Lund – November 2016 14

Special thanks to my colleages in LASA-INFN for their frindly and colaborative attitude Thanks for your attention ESS AD Retreat, Lund – November 2016 15

ESS AD Retreat, Lund – November 2016 16

Study on Multipacting • Used Fish. Pact code • For INFN MB cavity, the two-point multipactings are found in small regions near equator (<<1 mm) Typical SEY Pen HC/End HC First order MP occurs in the Eacc range of 7 MV/m to 14 MV/m, with maximum final impact energies around Eacc = 11 MV/m, 30. 2 e. V and 33. 3 e. V for inner cell and end cell, respectively. • Max final impacting energy after 20 impacts for LASA MB cavity 35 Inner cell 30 Pen HC End HC Ef(e. V) 25 20 15 10 5 7 9 11 Eacc (MV/m) 13 15 # The impact energy for the 1 st order two-point MP is in the range of 30 - 50 e. V, irrespective of RF frequency. Although two-point MP’s are ubiquitous, they are not expected to limit the cavity performance owing to the fact that the impact energy is very close to the first cross-over of the secondary emission yield (SEY) curve, which shifts up after the surface is “cleaned up” by processing. ----- R. L. Geng # Geng, R. L. " Proc. of the PAC’ 2003 (2003): 264 -268. 17

Study on Qext vs Field Flatness 8. 1 E+05 1. 0 E+06 7. 6 E+05 9. 5 E+05 7. 1 E+05 9. 0 E+05 Qext • Four common types of field distribution acchieved by stretching or squeezing cells • With field flatness, FF≥ 90% the Qext is expected in range 7 to 9 × 105 6. 6 E+05 6. 1 E+05 8. 0 E+05 cells 3, 4 squeezed cell 1, 6 stretched 0. 910 1. 010 Field Flatness 8. 5 E+05 7. 5 E+05 0. 822 0. 872 0. 922 Field Flatness 0. 972 1. 022 1. 4 E+06 8. 5 E+05 8. 0 E+05 7. 5 E+05 7. 0 E+05 6. 5 E+05 6. 0 E+05 5. 5 E+05 5. 0 E+05 4. 5 E+05 1. 3 E+06 1. 2 E+06 Qext 5. 6 E+05 0. 810 cells 3, 4 stretched cells 1, 6 squeezed 1. 1 E+06 1. 0 E+06 9. 0 E+05 Cell 1 stretched Cell 6 squeezed 0. 50 0. 70 0. 90 Field Flatness 1. 10 8. 0 E+05 Cell 1 squeezed Cell 6 stretched 7. 0 E+05 0. 50 0. 70 0. 90 Field Flatness 1. 10 18

Study on Qext with tuning • Since the end cells are not the same as inner ones, stretching or squeezing whole cavity will lead shorter length change on end cells, thereby affecting the frequency and field flatness. • Stretching cavity leads decreasing Qext, while squeezing leads increasing Qext. The sensitivity is about 0. 18 E+5/mm 1 mm squeezed cavity by Freq. tuner FF trend during stretching -4. 00 704. 900 704. 700 704. 500 704. 300 704. 100 703. 900 703. 700 703. 500 -2. 00 0. 00 Length variation 2. 00 4. 00 8. 30 E+05 8. 10 E+05 Qext FF trend during squeezing Cavity tuner effect on frequency and Qext Frequency Length changes on each cell by squeezing the cavity 1 mm 7. 90 E+05 7. 70 E+05 7. 50 E+05 7. 30 E+05 -4. 00 ESS AD Retreat, Lund – November 2016 7. 10 E+05 -2. 00 0. 00 Length variation 19

Tuning after Bulk BCP • Cavity just fabricated: f=703. 51 , FF=70% • Cavity after tuning: f=703. 11 MHz, FF > 97% ESS AD Retreat, Lund – November 2016 20