SCRF CAVITY AND RELATED RD ACTIVITIES AT VECC

SCRF CAVITY AND RELATED R&D ACTIVITIES AT VECC S. Som Variable Energy Cyclotron Centre Kolkata, India Presented in IIFC meeting on 27 -Oct-2010 at RRCAT, Indore

OUTLINE OF THIS TALK • MOTIVATION FOR SCRF ACTIVITIES • SCRF CAVITY DESIGN • PROTOTYPE COPPER CAVITY AND ITS RF CHARACTERIZATION • ELLIPTICAL CAVITY SIMULATION • TEST CRYOSTAT DESIGN • ACTIVITIES ON HIGH POWER RF SOURCE & ASSOCIATED DC POWER SUPPLIES

MOTIVATION FOR SCRF CAVITY ACTIVITIES • In view of vast thorium resources in India, the concept of Accelerator Driven Subcritical System (ADSS) gained momentum – more nuclear power generation • ADSS : high energy (~ 1 Ge. V), high current (>10 m. A) proton beam hits the target of heavy element (such as Th, Pu or U etc. ), spallation neutron is produced. Spallation target is surrounded by a blanket assembly of nuclear fuel (such as 232 , a fissile isotope) which breeds to 233 and 90 Th 92 U sustaining fission chain reaction takes place. • XI Plan period: Govt. has been funding for the project on “Design, Analysis and Development of multi-cell SCRF Linac Cavity” at VECC, Kolkata

Design Criteria for Elliptical cavity • Elliptical structures with <0. 5 are inefficient, because of filling and transit time factors • For cavity inner cell design, – Minimize Epk & Bpk – Contradictory requirement – Provide reasonable mechanical stiffness – Maximize R/Q – Achieve a reasonable intercell coupling coefficient Cavity Shape Parametrization

ANALYSIS WITH CST MICROWAVE STUDIO E-field profile along the axis Electric field lines Magnetic field lines Frequency: 704. 4 MHz • Length of a half-cell of the cavity =L/2 =70 mm. • Iris radius = Riris = 50. 4 mm. • Dome (equator) radius = D/2 = 188. 2 mm. • Equator ellipse ratio = A/B = 45 mm/47 mm. • Iris ellipse ratio = a/b = 12. 5 mm/15 mm.

Bead-pull measurement -- special technique adopted Using Phase-shift technique instead of frequency-shift IMPEDANCE PHASE (Degrees) In manufacturing or tuning multi-cell cavity, it is required to investigate the field profile inside the cavity The Field can be sampled by introducing a perturbing object and measuring its change in f 0 The object must be very small so that the field does not vary significantly over its largest linear dimension: it is a perturbation method Phase deviation is much easier to observe than frequency change especially for small perturbation.

CST MWS simulation on FERMILAB data • Length of a half-cell of the cavity = L/2 =70. 34 mm. • Iris radius = Riris = 41. 5 mm. • Dome (equator) radius = D/2 = 194. 95 mm. • Equator ellipse ratio = A/B = 54 mm/58 mm. • Iris ellipse ratio = a/b = mm. /25 mm. Frequency: 650 MHz 14

CST MWS simulation on FERMILAB data

CST MWS simulation on VECC data • Length of a half-cell of the cavity = L/2 =70. 38 mm. • Iris radius = Riris = 43. 5 mm. • Dome (equator) radius = D/2 = 200. 8 mm. • Equator ellipse ratio = A/B = 45 mm/47 mm. • Iris ellipse ratio = a/b = mm. /15 mm. Frequency: 650 MHz 15

CST MWS simulation on HOM For Fermilab data Transverse HOM 1 Fermilab frequency: 970. 68 MHz (2 DSLANS Code) CST MWS simulation on Fermilab data gives Frequency: 974. 8 MHz Effective Transverse impedance: 0. 033 Transverse HOM 2 Fermilab frequency: 974. 03 MHz (2 DSLANS Code) CST MWS simulation on Fermilab data gives Frequency: 978. 9 MHz Effective Transverse impedance: 1. 18

CST MWS simulation on HOM For Fermilab data Transverse HOM 3 Fermilab frequency: 978. 611 MHz (2 DSLANS Code) CST MWS simulation on Fermilab data gives Frequency: 984. 386 MHz Effective Transverse impedance: 2. 56 Transverse HOM 4 Fermilab frequency: 983. 073 MHz (2 DSLANS Code) CST MWS simulation on Fermilab data gives Frequency: 989. 236 MHz Effective Transverse impedance: 0. 86

CST MWS simulation on HOM For Fermilab data Transverse HOM 5 Fermilab frequency: 986. 141 MHz (2 D-SLANS Code) CST MWS simulation on Fermilab data gives Frequency: 991. 753 MHz Effective Transverse impedance: 0. 016 Longitudinal HOM 14 Fermilab frequency: 1622. 04 MHz (2 D-SLANS Code) CST MWS simulation on Fermilab data gives Frequency: 1619. 54 MHz Effective Transverse impedance: 9

CST MWS simulation on HOM For Fermilab data Longitudinal HOM 15 Fermilab frequency: 1624. 45 MHz (2 D-SLANS Code) CST MWS simulation on Fermilab data gives Frequency: 1622. 49 MHz Effective Transverse impedance: 24 • 650 MHz, =0. 61 cavity has been studied for Transverse and longitudinal HOMs. • There is no trapped mode with high effective impedance.

DIE DRAWING FOR HALF-CELL CAVITY

NIOBIUM SHEET • • • • Dimension: 600 mm. x 4 mm. Tolerances: 1. 5 mm. x 0. 125 mm. RRR value: 300 or better Surface texture: better than 3. 175 m finish Surface roughness: better than 1. 6 m Deep drawing quality, grain size ASTM#5 or finer, local grain sizes ASTM#4 allowable, min. 90% recrystallized Impurities: Grain size: Typically 50 m H 2 2 Wt. ppm W 0. 007% Yield strength > 50 N/mm 2 Tensile strength > 100 N/mm 2 C 10 Wt. ppm Ti 0. 005% Elongation > 30%, N 2 10 Wt. ppm Fe 0. 003% Vicker Hardness < 50 N/mm 2 ATI Wah Chang, USA. O 2 10 Wt. ppm Si 0. 003% Expected Delivery: March, 2011. Ta 500 Wt. ppm Mo 0. 005% Ni 0. 003%

SCRF CAVITY TEST CRYOSTAT • Design has been done for HORIZONTAL and also for VERTICAL Test Cryostat • Overall dimension for both Horizontal/Vertical Test Cryostat: 2400 mm. length x 1650 mm. Diameter • LHe vessel dimension for both Horizontal/Vertical Test Cryostat: 1050 mm. Length x 512 mm. Diameter The • Different designs of test cryostats have been evaluated. volumes of the helium vessel are 600 litres, 325 litres and 175 litres for design I, II and III respectively. The radio frequency cavity dissipated about 15 watt in the liquid helium system. In comparison the static head load is considerable less.

Scheme Liquid Helium Liquid Nitrogen Cryostat

Mechanical Design - 1 • Cryostat with flat top flange. • Helium tank has a large neck. • Volume of inner helium vessels: 600 lts. • Simple assembly and operation. • R F cavity was placed vertical from top. • Conductive Heat load to the liquid helium vessels from top support is 5. 579 watt. • Heat load to the helium vessels due to radiation using the 10 layer of MLI on helium vessels is 0. 68 milliwatt.

Design - 2 • Helium tank has small neck to reduce heat leak. • Volume of inner helium vessels: 325 lts. • RF cavity was placed vertical from top. • Conductive Heat load to the liquid helium vessels from top support is 5. 462 watt. • Heat load to the helium vessels due to radiation using the 10 layer of MLI on helium vessels is 0. 55 milliwatt.

Design - 3 • Volume of the helium vessel is minimum. • Volume of inner helium vessels – 175 lts • Overall diameter of the cryostat has been increased to accommodate the cavity in horizontal as well as in vertical condition. • The cryostat has provision for operation of the cavity at 2 K also. • Conductive Heat load to the liquid helium vessels from top support is 4 watt • Heat load to the helium vessels due to radiation using the 10 layer of MLI on helium vessels is 0. 49 milliwatt.

Design – 3: Cryostat with vertical and horizontal cavity

Table 1: Design details for inner and outer vessels Design I Inner Vessel Outer Vessel III Inner Vessel Outer Vessel Cylindrical Shell Torispherical head Thickness 3 mm 4 mm 5 mm 6 mm Cylindrical Shell Torispherical head 3 mm Cylindrical Shell Torispherical head Cylindrical Shell Torispherical head 2 mm 4 mm 5 mm 6 mm 3 mm 4 mm 5 mm 6 mm 8 mm External Pressure 11. 8 psi 31. 9 psi 24 psi 52 psi 17. 3 psi 17. 1 psi 21. 7 psi Internal Stress 6870 psi 12289 psi 5078 psi 9065 psi 9028 psi 16055 psi 7657 psi 13607 psi 12. 2 psi 24 psi 22. 5 psi 40 psi 15. 2 psi 17. 1 psi 22 psi 8252 psi 14541 psi 6015 psi 10726 psi 8980 psi 15829 psi 7616 psi 6711 psi 17 psi 21. 2 psi 40. 9 psi 73. 5 psi 60. 6 psi 10 psi 12. 2 psi 15. 3 psi 16. 2 psi 29. 5 psi 24 psi 8789 psi 15646 psi 5823 psi 10344 psi 4387 psi 7776 psi 11349 psi 19368 psi 9431 psi 16749 psi 7077 psi 12550 psi

Table 2 : Design details for Main Flanges inner and outer vessels Option Flange Thickness Int. stress (UG-34) Ext. pressure (Apx-2) Int. stress (Apx-2) Inner Vessel 30 mm 11326 psi 1179 psi 20215 psi 39 mm 6650 psi 693 psi 11869 psi 40 mm 26737 psi 5191 psi 26445 psi 50 mm 17636 psi 3423 psi 17435 psi 30 mm 11400 psi 394 psi 7773 psi 39 mm 6745 psi 235 psi 4625 psi 40 mm 26345 psi 3464 psi 26540 psi 50 mm 17462 psi 2285 psi 17447 psi 30 mm 11392 psi 556 psi 10953 psi 39 mm 6790 psi 331 psi 6527 psi 55 mm 21365 psi 3888 psi 15879 psi 60 mm 17953 psi 3267 psi 13342 psi I Outer Vessel Inner Vessel III Outer Vessel

Thermal Design To reduce the heat load on liquid helium system: Ø The liquid helium 4. 5 K vessel is shielded with the liquid nitrogen 77 K shell made of OFHC Copper. Ø The liquid helium vessel and liquid nitrogen vessel is covered by reflective multi-layer insulation. Ø The annular space between the outer vessels and helium vessels is evacuated to reduce the heat leak. Ø Liquid helium vessel is supported from top using thin bellows Ø Bottom support is optimally designed glass epoxy Cylinder (G-10). Ø The heat load of the three different types of bottom support to the 4. 5 K system has been evaluated by using ANSYS software Bottom support cylinder Design - 1 Ø Ø Result for thermal analysis Total Heat Load : 0. 224 watt Simple Cylinder Height : 350 mm Diameter : 300 mm

Bottom support cylinder Design - 2 Ø Ø Ø Ø Total Heat Load : 0. 112 watt Slot width : 10 mm Length of slot : 200 mm Vertical Centre Distance Between Slot : 100 mm. No. of Slots in a plane : 2 Nos in 180º No. of plane : 2 Nos Angular rotation between two plane : 90º Bottom support cylinder Design - 3 Ø Total Heat Load : 0. 0918 watt Ø Hole Size : Dia 100 mm Ø Vertical Distance Between two hole : 175 mm Ø Ø No. of holes in a plane : 4 Nos in 90º No. of plane : 5 Nos Vertical Distance Between two Plane : 75 mm Angular rotation between two plane : 45º

Support structure • The whole cryostat is supported on four legs with jack • Four caster wheel mounted on the legs for the easy handling and movement of cryostat. • The S. S. 304 Channel 160 mm X 8 mm has been used with the 6 mm reinforcement plate. • Maximum general longitudinal stress is 3547 psi • Maximum localized stress to cause buckling above the leg top is 632 psi. • Three lifting hook welded with reinforcement plate in the outer vessels for the lifting and handling of whole assembly. • The lifting hook is 20 mm thick S. S. 304 plate with 6 mm. thick reinforcement plate. The maximum stress is 5425 psi

High power RF Source development IOT based RF Amplifier, 60 KWatt at 704. 4 MHz • Compared to klystrons, IOTs exhibits very interesting characteristics in terms of efficiency (> 65% is usually reached), linearity and compactness. • operating at 460 -800 MHz, with powers up to 60 k. W CW , compatible with our requirement. • We have selected TH 793 IOT for 704. 4 MHz and also for 650 MHz operation • Maintenance is simpler -- replacement of the tube only • IOT can be re-gunned twice, at about 60% cost of a new IOT • Output RF cavities are external to tube.

IOT amplifier cavity

DC POWER SUPPLIES

Pulse-Step-Modulation (PSM) Control scheme • Pulse step technology is recommended for RF amplifier kind of load where high efficiency, regulation speed and accuracy, and compatibility with large variation of the load impedance. Moreover, the modular concept with high redundancy (up to 10% defective modules without performance degradation) makes it very reliable, easy to maintain and avoiding of HV crowbar tubes.

HV DC POWER SUPPLY

OTHER DC POWER SUPPLIES FILAMENT P/S CONTROL GRID P/S PROCUREMENT OF ALL P/S IS IN PROGRESS. ION PUMP P/S FOCUS COIL P/S

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