MAC2010
Feb. 16, 2010

Superconducting Cavity
Yoshiyuki MORITA

Outline•

R&D for Super-KEKB– KEKB superconducting cavities– Beam studies for reverse phase mode
• Voltage and phase settings– Not to change the total voltage• Low current study– Synchrotron tune
– Bunch length– Beam loading• High current study– Transient behavior at cavity trip
• Luminosity run– Stability– HOM damper– Input couplerKEKB superconducting cavity
Designed and achieved parameters
A cross sectional view of the KEKBSuperconducting Accelerating Cavity.
A 509 MHz single cell cavity with a large iris
diameter of 220 mm has ferrite HOM absorbers on
both sides and a coaxial-type power coupler. The
cryomodules of 3700 mm in length are connectedto the beam ducts of 150 mm in diameter.

Parameter

designachieved

unit

Beam Current

1.1

1.4

A

Bunch charge

210nC

RF voltage

1.5

1.2-2

MV/cav.

Beam loading250

350-400

kW/cav.

HOM power

516kW/cav.

Abort statistics @ HER

Trip rate
0.5/day at 1.4A
0.1/day at 0.85A

Eight superconducting cavities in HER

R&D for Super-KEKB
Reverse phase mode

1.56MV×(7-1)

Required total voltage: Lower than the present voltage
13-15 MV→6.7MV
Required total beam power: Higher than the present one
5MW→7.7MW@2.6A→ External Q must be modified (Qext=50,000 at present)→ Input couplers must be exchanged→ High risks for cavity contamination
New operation method proposed
Reverse phase mode
Synchronous phases of several cavities are reversed
The total RF voltage is kept as low as the required value→ Each cavity can have high voltage→ Each cavity can share equal beam loading→ No need for Qext modification

1.18MV×8Vc
Reverse−φs

Normalφs

Phase

Beam studies and test operation
Date

Beam Study

June 26

Low current study@ 150 mA
Synchrotron tune
Bunch length
Beam loading

Nov. 25

High current study
up to 930 mA

Dec. 18

High current study
up to 1 A

Dec. 18-24

Test operation

Phase adjustment for D11B
due to energy change

5S

2SHigh current
test

5S

4S-60MeV
Test operation

Voltage and phase settings
How to set −Φ

Reverse phase−Φs
How to set V and ΦdateΦsenergy

phase shiftΦs

2009/6/26

4S

165.2

82.6

2009/11/252S

168.2

84.1

2009/12/18

5S

161.7

80.8

2009/12/21

4S off res

164.7

82.3

Synchrotron tune: unchanged

Calculated

Total voltage = 1.56 MV × # of cavities + ARES
Several cavities were detuned to vary the total voltage.

Beam loading

Beam loading of the reverse
phase cavity can be controlled.

Reversed phase (Vc=1.56 MV)
Normal phase (Vc=1.18 MV)

D11A trip(15:05)

Recovering (Crab)

σz測定

D11B trip(15:16)

D11B Klystron power

O-phaseOverload
Overload

Reversed phase

Beam abort test(11:36)

D11A trip(12:27,12:37)

Deceleration

Reversed0-phase−φsφsBunch length

Bunch length: unchanged

by Ikeda-sanOverload

0-phase

Reversed phase mode

Reverse phase mode
(light loading)

Normal mode

2009/11/25 by Flanagan-san@ 150 mA
Forward phase

6.74 +/- 0.01

Reverse phase

6.71 +/- 0.01

D11A-RFoff

Beam abort

D11A, Trip

Pin, Pr, Vc, φtun variations
at D11A trip
Beam current ：１A

D11B, Reverse
Φtun: tuner phase

Voltage:Vc

Reflected power: Pr

Vc: D11B:increased
D11C:decreased
Pr: D11B:increased
D11C:decreased
Φtun ~30 deg.
Pin: not significantly
changedReverse
D11C, Normal∆φNormal∆φ−φsφsΔVc at D11A trip (5S-RUN)

Reverse phase:D11B

Abort LOG D10

Normal phase

Abort LOG D11

Voltage variation at D11A trip
The reversed cavity increased the voltage.
Other normal phase cavities decreased voltages.

Luminosity RUN
Klystron powers at 5S-RUN@1200mA•

Luminosity RUN with reverse phase operation– Dec. 18 to 24– Stably operated• No beam abort caused by the cavity trip
• Beam current: 1200 mA• RF power delivered to beam: ~300kW/caviyty– Energy change from 5S to 4S(off resonance)• D11B: Overload due to beam phase change• Phase adjustment
Klystron power >350 kW
due to beam phasechangeloareOvd

At 4S-RUN

Reverse phase operation in Super-KEKB
Voltage and phase settingsState# of cavities

Normal5Reverse

3

Beam Current

VcΦs

Beam power

Detune

2.6 A

1.3 MV±83°

400 kW

47 kHzMax ΔVc=0.27 MV

Vc=1.3MV

The tuner phase variation was assumed (30 deg./A).
The maximum voltage variation is +0.27 MV at a beam current of 2A.
This variation is acceptable for the superconducting cavities.

Summary of the reverse mode study•

Feasibility studies for reverse phase operation– Voltage and phase settings：Yes we can!
– Synchrotron tune： unchanged– Bunch length: unchanged– Beam loading: well controlled– Beam acceleration: very stable– At cavity RF trips• No uncontrollable behavior observed• Positive voltage variation but acceptable– Luminosity RUN for five days
• No beam abort caused by the cavity trip
• Beam current of 1200 mA stored with beam power of 300 kW

We conclude reverse phase operation for Super-KEKB is possible !

Expected HOM powers in the Super-KEKB
HOM dampers for high power application•

Expected HOM powers– at 2 A, 19 kW+ damper loss (8 kW)– at 2.6 A, 32 kW+ damper loss (13 kW)
Beam current

SBP load

LBP load

(A)

(kW)

(kW)

21215

2.6

2026

Parameters of Super-KEKB for HER
Maximum current: 2.6A ←1.4A
Bunch length: 5 mm
←6-7mm# of bunches: 2500
←1600

1.2V/pC

Beam pipe: 150Φ

We have to develop HOM dampers for higher currents than 2 A !

Development of new dampers for high current application
In order to reduce outgas of the HIPped ferrite material,surface temperature of the ferrite has to be suppressed.
HOM damper design was modified.
・Reduce thickness of the ferrite material form 4t to 3t.
・Make a double winding structure of the cooling channel.

HOM dampers with a ferrite thickness of 3 mm were made and tested.
Temperature rise was reduced by 25%. A double winding structure isexpected to further reduce the temperature rise by 15 %. Dampers of thistype are being fabricated. With those modifications, the LBP dampers canabsorb 26 kW with surface temperature less than 120 ℃.

Absorbed power vs. surface temperature
4t-damper with 8-9 L/min

LBP<18.5kW
SBP<15kW

3t-damper with 8.5 L/min

High power test stand

ANSYS analysis

Development of input couplers for higher beam loadings
Although the present input couplers satisfy RFpower requirements for Super-KEKB, it isdesirable to continue to develop couplers forhigh RF power application.

WG

Door knob

High power coupler

New KEKB high power coupler.
This coupler has a single cooling water path to increasecooling capacity and also to reduce fabrication cost. Acoupler of this type has been fabricated and RF tested up
to 750 kW.
High power test standConditioned up to 750 kW

Summary••••

Feasibility studies for the reverse mode were conducted
We conclude the reverse phase mode can be applied for the Super-KEKB
A modified HOM dampers were designed for high power applications.
– Reduce ferrite thickness from 4 mm to 3 mm
– Make double winding structure– Applied for higher currents than 2A
We continue to develop input couplers for high power application.