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

1.00E+09

1.00E+10

1.00E+11

1 10 100 1000 10000

Brf[Gauss]

up'Qf

SY2

N5C

4M2-302

FT-small

FT-large200V/div, 5ms/div

High Loss Effect

Magnetic Alloys

Ferrites

2000 Gauss

シャントインピー

ダンス

に比例

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 output1.4X0.7X0.8m

J-PARC600kW tube AMP500kW output1.4X1.0X2.4m

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

current @ 2-3MHz

• Cavity shroud is completed. Start to install cores. • Core impedance : about 135 /core@3.5MHz• 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 impedance@ low frequency because of uncut cores

• Expected voltage @1MHz : 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/LRp: shunt

Parallel inductor for J-PARC

Inside of PRISM AMP

-4.00E+02

-2.00E+02

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

0 2 4 6 8 10 12

RXZ

-3.00E+02

-2.00E+02

-1.00E+02

0.00E+00

1.00E+02

2.00E+02

3.00E+02

4.00E+02

5.00E+02

0 2 4 6 8 10 12

RXZ

-4.00E+02

-3.00E+02

-2.00E+02

-1.00E+02

0.00E+00

1.00E+02

2.00E+02

3.00E+02

4.00E+02

5.00E+02

6.00E+02

0 2 4 6 8 10 12

RXZ

-4.00E+02

-3.00E+02

-2.00E+02

-1.00E+02

0.00E+00

1.00E+02

2.00E+02

3.00E+02

4.00E+02

5.00E+02

6.00E+02

0 2 4 6 8 10 12

RXZ

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 ?

uQf (é•ê´ëÃÉRÉAÇÃì¡ê´Åj

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6

frequencyÅ@ÅiMHÇöÅj

uQf (GHz)

uQf(FT3L,13um)uQf(FT3L,18um)uQf(FT3M,18um)

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 re

sonate.• 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 0.4-0.5 m

• 45 kW/gap, 11.25 kW/core• 0.32W/cc (av.), 0.54 W/cc (Max.) < 1 W/cc

Water I nWater Out1. 9m1. 2m

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