ARES Upgrade for Super KEKB

KAGEYAMA Tatsuya
for the KEKB RF ARES cavity group• Brief Review• Fundamental Mode Issues• HOM Load Issues• Input Coupler R&D• Summary
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Accelerator Resonantly coupled with Energy Storage
ARES
3-cavity system stabilized with the π/2-mode operation

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Fundamentals of the ARES cavity system

Operation of conventional copper cavities under the heavy
beam loading (2.6A for KEKB LER) would bring on
The longitudinal CBI (Coupled-Bunch Instability, µ = -1, -2, • • • )driven by the accelerating mode, which is to be detuned
downward from the fRF by ∆fa where |dfa| would amount to ~2Xfrev .
cf. frev = 99kHz for KEKB

"f a = #!

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Isin$ s R
P tan$ s%% f RF = # b
2Vc
Q
4 &U

"f a #!

P beamU acc. mode

Fundamentals of the ARES cavity systemcont’dS-cav functions as a kind of EM flywheel.
A flywheel is a device for storing energy or momentum
in a rotating mass, used to minimize variations inangular velocity and revolutions per minute, as in amachine subject to fluctuation in drive and load.

ARES scheme
3-cavity system stabilized with the π/2-mode operation

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Engage C-cav in order to solve the following problems arisingfrom coupling A-cav with S-cav:
 How to stabilize the EM field distribution between A-cav
and S-cav
 How to suppress CBI driven by the parasite(s) on theaccelerating mode

Fundamentals of the ARES cavity systemcont’dQC-cav ≈ 50

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

U s ka2=
U a ks2for π/2 mode

Fundamentals of the ARES cavity systemcont’dU s ka2• The stored-energy ratio of the π/2 mode:=U a ks2• The EM field distribution of the π/2 mode is stable against
detuning of A-cav (= ∆fa) loaded with the beam.
• The amount of detuning of A-cav !
can be reduced as: "f # 2 ="f a
1+ U s /U a

• The parasitic 0 and π modes can be selectively damped(QL ≈ 100) with an antenna-type coupler attached to C-cav.

!

• Furthermore, the damped 0 and π modes are located almost
symmetrically with respect to the π/2 mode. Therefore, their
impedance contributions to CBI can be counterbalanced.
• C-cav functions as a filter to isolate S-cav from A-cav’s HOMs.
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

ARES Operation Status in KEKB
RF Parameters of the π/2 mode:
Us / Ua
R/Q
Q0
Pc
Vc

= 9
= 15 Ω
= 1.1 x 105= 150 kW / cav= 0.5 MV /cav

LER (e+): 20 ARES cavities
 Total Vc = 8~8.5 MV( 0.4~0.425 MV/cav ) Beam current (max.): ~1.8A
 Input RF power/cav: ~300kW
 HOM power/cav:~5kW

HER (e-): 12 ARESs and 8 SCCs
Total Vc = 15 MV
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

= 3.84MV(ARES) + 11.16 (SCC)

Beam current (max.): ~1.1A

ARES cavities in the LER RF section Fuji D7

ARES Upgrade for Super-KEKB
KEKB
HER: 1.1 A by 12 ARESs + 8 SCCs
LER: 2.6 A by 20 ARESs

Super KEKB
HER: 4.1 A by 16 ARESs + 12 SCCs
LER: 9.4 A by 28 ARESs
Coupling Slotbetween A-cav and C-cav

U s ka2=
U a ks2

WG HOM Loadka

GBP HOM Load!

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

We will be probably able to manage
with the current version.
We are planning to remodel the A-cav
part and to upgrade the HOM loads,
whereas the S-cav will be reused.
Furthermore, we will need to build
about 10 sets of ARES cavities toincrease the LER current up to 9.4 A.

ARES Upgrade for Super-KEKB
Fundamental mode issues
2.6 A by 20 ARESs for LER∆fa = -200 kHz
9.4 A by 28 ARESs for LER∆fa = -710 kHz

U s ka2=
U a ks2!

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

CBI driven by the π/2 accelerating mode:∆fπ/2 = -71 kHz if Us / Ua = 9 unchanged. (cf. frev= 99 kHz)We need to increase the stored-energy ratio Us / Ua
by enlarging the coupling slot between A-cav and C-cav.
CBI due to the parasitic 0 and π modes:
The impedance imbalance will not be negligible.
We have investigated the effect of the impedance
imbalance when detuning A-cav by -710 kHz.

CBI due to the π/2 mode
Table 1: RF parameters of ARES cavity for Super KEKB LER.
Us /UaCoupling SlotI
# of Cavities
fRF
h
Vc
Q0
(R/Q)π/2
input βPc
Δ fπ/29
120 × 160

1.11
15
5.4 (2.7)
150−72 (−20)

15
120 × 175
9.4 (2.6)
28 (20)508.887
5120
0.5
1.27
9.4
4.1
210
−45

18
120 × 182

A
MHz

1.32
7.9
3.8
240
−38

Numbers in ( ) for KEKB LER
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

W×Η mm2

MV
×105
Ω
kW
kHz

CBI due to the π/2 mode (cont’d)Growth Rate [ s-1 ]

104

102

µ = !2

101
100

Coupling Impedance of the π/2 modeloaded with 9.4 A for Us / Ua = 9, 15, 18.

µ = !1103µ = !3
9

12

15
Us/Ua

18

Growth rates of the CBIs (µ = -1, -2, -3)
due to the π/2 mode,plotted as a function of Us / Ua .

By increasing Us / Ua from 9 to 15, the growth rate of the severest CBI (µ = -1)
can be reduced by one order of magnitude and down to τ = 1.5 ms, manageablewith an RF feedback system .
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

S. Yoshimoto et al., “The -1 mode damping system for KEKB”

CBI due to the 0 and π modes

The damped 0 and π modes (Ua:Uc = 1:1 when
Δfa = 0 kHz) are located almost symmetricallywith respect to the π/2 mode ( ≈ fRF ).
For KEKB (Δfa = -200 kHz), Re[Z //] has a small
asymmetry with respect to fRF.
|R+ - R-|max ~500 Ω/ARES → τ ~46 ms
cf. τ rad ~23 ms (KEKB)The EM field distributions (Ua:Uc) of the 0 and π modes are
subject to the perturbation of first order Δfa / (fπ - f0).
Δfa = -200 kHz (KEKB)→ -710 kHz (Super-KEKB)

The delicate counterbalancing may be deteriorated in Super-KEKB.
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

CBI due to the 0 and π modes (cont’d)Top:
Re[Z//] of the 0 and π
modes
Bottom:Re[Z//] imbalance R+ - Rwith respect to fRF.
Detuning A-cav downward,
Ua > Uc for 0 mode,
Ua < Uc for π mode.

The fastest growth rate of CBI due to the impedance imbalance of 0 and π
modes when Δfa = -710 kHz :KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

τ = 3.3 ms for Us / Ua = 9 (9.4 A)

cf. τ = 46 ms for Us / Ua = 9 (2.6 A KEKB)

τ = 4.0 ms for Us / Ua = 15 (9.4 A)

to be damped by longitudinal FB kickers

first order term Δfa / (fπ - f0), where fπ - f0 ∝ ka ∝ (Us / Ua)1/2

ARES upgrade for Super-KEKB
HOM load issues
HOM-damped structure for ARES
Based on the conceptual demonstrator ARES96
Carefully designed to be smoothly embedded into the whole ARES scheme
Consists of 4 HOM WGs and 2 GBPs (Grooved Beam Pipe devised at KEK)
4 HOM WGs: Monopole & Dipole(V) HOMs
2 GBPs: Dipole(H) HOMs

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

WG HOM load (KEKB)WG HOM load: Two bullet-shape SiC absorbers at the end of the HOM WG.
 Each absorber directly water-cooled.
 Power capability tested up to 3.3 kW / bullet (26 kW / cavity)with use of an L-band klystron.

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

GBP HOM load (KEKB)
GBP HOM load: 8 SiC tiles per groove, brazed to a water-cooled OFC plate.
 Power capability tested up to 0.5 kW / groove (2 kW / cavity)with use of the same L-band klystron.

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

HOM load status in KEKB

5
Total HOM PowerWG HOM Power
GBP HOM Power
Prediction from Cavity Loss Factor
Prediction from GBP Loss Factor

AC + 2 x GBP
2 x GBPAC + 2 x CBP

0.6

k (fundamental) = 0.12 V/pC

0.5

Loss Factor [V/pC]

Power / Cavity [kW]4

Super KEKB

0.7

3

2

1

0.4

KEKB design

0.3

KEKB at present

0.2
0.1

0
0

200

400 600 800 1000 1200 1400
LER Beam Current (mA)WG and GBP HOM powers per cavity
measured in the KEKB LER, comparedwith theoretical predictions (σz = 7 mm).
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

0

2

4

6
8
Bunch Length (mm)10Loss factors computed for the wholedamped structure (3D) and the GBPs (3D),
together with those for a simplified axiallysymmetric structure (2D) with circularbeam pipes (CBPs).

Prospect of HOM power for Super-KEKB
WG HOM: ~80 kW / cavity

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

9.4 A, σ z = 3mm, Nb = ~5000

7m
m,
N

HOM data taken in KEKB LER
when σz = 7mm and Nb =
1224.

W
G:
σ

0.1

Super-KEKB LER

b =1

1

GBP HOM: ~20 kW / cavity

Extrapolation

22
4

10

z =

HOM Power (kW)!"#"$%&"'()*+!!,-"%.*/001!"#"$%&"'()*,234-"%.*,001!"#"$%&"'()*,234-"%.*5001,"#"678"9$"'()*+!!,-"%.*/001,"#"678"9$"'()*,234-"%.*,001,"#"678"9$"'()*,234"%.*5001

GB
W
G:
P:
σ
σ
z =
GB z =
3m
7
P:
m
m,
σ
m,
N
z =
N
3m b =b =48
m, 122
96
4
N
b =
48
96

100

0.1

1
Beam Current (A)10cf. KEKB LER design:
2.6 A, σ z = 4mm, Nb = ~5000

Upgrading WG HOM load for Super-KEKB

WG HOM load for Super-KEKB LER: ~80 kW / cavity
HPT’d up to 26 kW/cavity (3.3 kW/bullet) for KEKB:This limit is only due to the maximum RF power supplied by
the current L-band klystron.
Perform HPT over 3.3 kW/bullet:
We are planning to construct a new test stand for HOM loads
with an L-band klystron capable of more than 10 kW (JFY 2004).
Increase # of absorbers per WG if necessary.

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Upgrading GBP HOM load for Super-KEKB

GBP HOM load for Super-KEKB LER: ~20 kW / cavity
HPT’d up to 2 kW/cavity (0.5 kW/groove) for KEKB.
Need to upgrade the power capability by one order of magnitude.

A new design with directly water-cooled SiC absorbers.

?
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

GBP may evolve into Winged Chamber

SiC

Y.Suetsugu et al., “Development of Winged
HOM Damper for Movable Mask in KEKB”,
Proc. PAC2003.

Beam Chamber
Slot

Winged chamber loaded with directly water-cooled SiC bullets:
Developed under collaboration between the KEKB vacuum
and ARES cavity groups.
Used in the movable mask sections in KEKB since 2002.
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Perform HPT over 5 kW / bullet.

Input Coupler Upgrade
Monitor port for arc detection

Over- and under-cut structure
for impedance matching
Coaxial line WX77D

Door-knob transformer
from rectangular WG tocoaxial line

multipactoring
discharge problem

Ceramic RF window

Coupling loop
ICF-203 flange

Capacitive iris

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Monitor port for arc detection

RF power ( = Pc + Pbeam )~400kW for KEKB
~800kW for Super-KEKB

Input Coupler Upgrade (cont’d)
S-cav (TE013 mode)
Multipactoring-free

RF power

π/2-mode
terminator

RF power
Load

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

HPT setup (old)RF power < 1 MW

HPT setup (new)RF power < 500 kW

Input Coupler Upgrade (cont’d)

input coupler
π/2-mode
terminatorTV camera

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

HPT setupInput Coupler Upgrade (cont’d)

RF Input Power [kW]Limited by the S-cav
wall power.

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Elapsed Time [min.]Input Coupler Upgrade (cont’d)Temperature rise of the coolingwater for the outer conductor

ΔT

Before conditioning
After conditioning

RF input power [kW]TV camera

Single-surface MP on the WX77D outer conductor
when RF input power = 150~200kW.
KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Input Coupler Upgrade (cont’d)

RF power

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Load

HPT setup (future)
RF power up to ~1 MW

Summary

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

By increasing Us/Ua from 9 to 15, the severest CBI (µ = -1) due tothe π/2 accelerating mode can be eased by one order of magnitude
and down to τ = 1.5 ms (9.4 A for Super-KEKB LER), manageablewith an RF feedback system.

The fastest growth time of CBI due to the impedance imbalance
between the parasitic 0 and π modes is estimated as τ = 4 ms
when A-cav is detuned by -710 kHz (9.4 A for Super-KEKB LER).



We need to upgrade the power capabilities of the WG and GBP HOM
loads up to ~80kW and ~20kW per cavity, respectively. The GBP withindirectly water-cooled SiC tiles may evolve into a winged chamberloaded with directly water-cooled SiC bullets.

Aiming at improvements and upgrades for Super KEKB, HPT’ing of
input couplers is being carried out with a new setup using an S-cav.
Our immediate target is to suppress the single-surface MP discharge
on the outer conductor of the coaxial line, which actually increases
the trip rate of the HER RF station D4C.

Summary (cont’d)
R&D Status and Plans
A-cav part: Fabrication of ARES2K4, a high-power demonstrator (JFY2004).
 HPT and beam test of ARES2K4 (JFY2005).
S-cav:
 R&D on high-purity copper plating (ongoing).
HOM Loads:
 Resume the old test stand (Feb. 2003). That is for HPT’ing
the current GBP load over 0.5 kW.
 A new test stand with an L-band klystron supplying RF power
more than 10 kW (JFY2004).

KAGEYAMA, T.
KEKB Review
Feb. 16, 2004

Input Couplers:
 R&D on TiN coating to the outer conductor of the coaxial line
section (ongoing).
 A new HPT setup with use of a S-cav loaded with a 1-MW waterload (JFY2004).
 Prototype couplers for Super KEKB (JFY2004).