RF System for Bunch Rotation C. Ohmori （ KEK)

Contents PRISM RF –Introductions –Present status –RF for 6 cell ring –Upgrade plan High Duty RF system for FFAG –Medical application –Experiences from J-PARC

Requirements for RF High voltage at 3.8 MHz –Total 2-3 MV –200 kV/m –8 straights for RF

Saw-Tooth RF –Linear RF bucket –Composed of 3 harmonics Requirements for RF

MA cavity for PRISM High field gradient at low frequency Wideband (low Q) Thin cavity (about 30 cm / cavity ) Use the maximum size for MA cores (1.7m X 1m) Very low duty RF system –Small tetrodes for the end stage –Small APS (anode power supply)

High Field Gradient : around 200 kV/m few MV RF for quick phase rotation (around 1.5 us) Dedicated system for pulse operation (low duty : 0.1%)

Characteristics of Magnetic Cores 200V/div, 5ms/div High Loss Effect Magnetic Alloys Ferrites 2000 Gauss 電圧に比例

Dedicated system for low duty AMP –Use small tubes –Works for short moment; 1-2 us X 1 kHz For 1 kHz repetition, need to minimize RF-ON time –99 % of time: zero anode current, 99.9%:zero RF output Cavity loss : few kW Tube loss : few ten kW APS –Old fashion to minimize cost: Crowbar, 3-phase Full- wave rectification J-PARC :1MW system, no crowbar, switching with IGBT –Supplies power to 4 AMPs, several MW in total.

Tube ON ＲＦＯＮ Cathode current

100kW tube AMP, >1MW output 1.4X0.7X0.8m J-PARC 600kW tube AMP 500kW output 1.4X1.0X2.4m APS, 1X1.5X2.0m APS for J-PARC, 4.5X2X2.7m Dedicated RF system for low duty

STATUS of PRISM RF RF frequency 5 -> 3.8 MHz (larger circumference) Tested at 2-3 MHz with a test cavity Achieved 42 kV/gap by test cavity Tube AMP:60A(design value)->70A as a RF 2-3MHz Cavity shroud is completed. Start to install cores. Core impedance : about 135  Number of cores: 4 instead of 6 (design : 6 cores, total 1k  )

6 cell PRISM –Test using  beam –1(or 2,3) MHz, 100 kV/m –Saw-tooth PRISM –3.5 MHz, about 200 kV/m –Saw-tooth Upgrade Plan –Cost, Higher field gradient

6 Cell ring To test bunch rotation using  beam Low energy, low RF frequency Few ten kV is required to observe bunch motion. Problems –Low low frequency because of uncut cores Expected : 352  X 60 A= 21 kV –A Solution : use 2 RF AMPs to drive 1 cavity. Expected voltage : 352 X 120 A =42 kV to prove 100kV/m Another solution : use parallel inductor scheme established through J-PARC R&D for high frequency.

Hybrid RF system Proposed by A. Schnase. Combination of MA cavity with a resonant circuit composed by inductor and capacitor. Developed for J-PARC RCS cavities. f=1/2  √LC 1/L=1/Lcore+1/Lind J-PARC: add C and L to control Q and f PRISM : add L to control f Q=Rp/  L Rp: shunt

Parallel inductor for J-PARC Inside of PRISM AMP

Total C =100pF Hybrid (13 uH) Total C =200pF Hybrid (+30uH inductor) Expected impedance with parallel inductor

PRISM CAVITY for 6 cell ring 4 Cores, around 3 MHz Expected cavity impedance : 500  More than 35 kV (>100kV/m) seems possible (70A X 500  ).

Saw-Tooth : Dual Drive System RF Cavity will be a wideband cavity. –But, bandwidth of AMP is still limited (1/RC). To obtain high RF voltage, a large drive voltage is required for CG-Cathode. Usually, AMP is driven from CG or Cathode. Drive from both CG and Cathode is possible in case of short pulse operation. –Narrow bandwidths are enough for both CG and Cathode. -> save the cost for Driver AMP –But, need test.

Upgrade Plan High Field Gradient Cost reduction

Improvements of cavity impedance Hybrid cavity with ceramic cavity Improvements of cavity cores –X 2 by annealing under magnetic field for thinner ribbon –Small cores : OK –Large core ?

How to improve MA consists of Fe, Si, B, Cu and Nb. Amorphous ribbon (<20  m) is annealed and crystallized. Combination of magnetic field during annealing and thinner ribbon (13  m) The small crystal has an axis magnetized easily. By the special annealing, the axis is equal. But relation between core impedance and this effect is not clear. Small cores : proved by Hitachi Metal Large core : need big magnet and special oven. => Appling FY2007 JSPS grant to produce these special core in KEK. B-H curve of MA core produced by annealing with/without magnetic field. (by Hitachi Metal)

Ceramic Cavity Proposed by J. Griffin. Low frequency air core cavity with a ceramic capacitor to resonate. Titanium Oxide capacitor was tested. 50kVDC (tested with 60kVDC). 1200pF. Electrodes are O.D. 121mmX I.D. 52mm, Thickness of ceramic is 14mm. Supported by JSPS money. Purpose : bunch rotation with low rep. rate. Small test cavity :30 cm X 30cm 4 k , Q=700, fres=7 MHz High power test is planned. High rep. rate : Hybrid cavity system with MA cavity. –Effective Q => below 100.

conclusions We start assembling PRISM rf cavity Demonstrate > 100 kV/m in April/May Also plan to test saw-tooth RF For general application, design of FFAG cavity is presented.

High duty RF system for FFAG General applications (beam acceleration ) High duty (100 %) Preliminary design based on experiences at the J-PARC ring RF –Direct water cooling –High average power

Direct Water Cooling Effective Cooling Scheme Adopted for J-PARC Ring RF –1W/cc, more than 1000 H test run COSY, CERN-LEIR, HIMAC-MA Design for FFAG cavity

Power Consumption for FFAG cavity 10MeV->140 MeV 1.5->4 MHz 6 kV/gap, 100 % duty 4 X PRISM-size cores –1.7 m X 1m X 3cm –Size of Cavity :2m X 1.2 m X m 45 kW/gap, kW/core 0.32W/cc (av.), 0.54 W/cc (Max.) < 1 W/cc

50cm

Technical key issues Good immersion –Mechanical strength –Cause of pin holes Good coating –J-PARC : epoxy+glass fiber Careful treatment of core surface –Scratch of MA surface may make local hot spots, destroy coating and reduce core impedance –Dusts on MA surface may make very local hot spots and destroy coating