LER blowup and HER transverse coupled bunch instability
10/Feb./03 KEKB review
J. Flanagan, H. Fukuma, S. Hiramatsu, K. Hosoyama, T. Ieiri,
M. Kikuchi, H. Nakayama, Y. Ohnishi, K. Oide, M. Suetake,
M. Tawada, M. Yoshioka, L. Wang, S. S. Win
1. LER blowup

2. HER transverse coupled bunch instability1) Installation of new solenoid

1) Observations

2) Field calculation

2) Candidates of source3) Effect of new solenoid

3) Measures4) Effect of wiggler

1. LER blowup
Installation of new solenoid
1) Short solenoids
In 2002 summer, 266 short solenoids were installed.
2) Permanent magnets
175 permanent magnets which cover BPM
have been assembled.
106 pieces were installed at 31th Oct.
Parts of another 175 permanent magnets were made.
Total 350 magnets will cover all BPM in arcs.

Short solenoid

Permanent magnet on BPM (designed by H. Nakayama et al.)(A solution for application of voltage to BPM electrode)Field measurement of permanent magnet
Bz(G)

z

Max. field is about 45 G.

Field calculation
Field strength of solenoids was calculated by 1/8 arc
to estimate coverage of solenoid field more accurately.
Field calculation of solenoid
150Bz at the center of solenoid (Gauss)

Length lager than 20 G
100

91% of drift region
78% of total length

50

0-50

-100-150
0

10000

20000

30000

Distance (mm)

40000

50000

Effect of new solenoid on blowup
Beam blowup at LER (long train)
54/320/4(1train, regular 4 rf bucket spacing)

Beam size@I.P.(micron)

428 Jun 2002
330 Oct 2002
29 Jan 2003
202 Oct 2002
10600

800

1000

1200

1400

Beam current (mA)

1600

1800

Short train
Beam blowup at LER (short train)

Beam blowup at LER (short train)

554/200/4(4train, 200bunch/train, 4 rf bucket spacing)

4/200/4(4train, 200bunch/train, 4 rf bucket spacing)4329/Jan/0330/Oct/02
2

1

Vertical beam size@I.P. (micron)Vertical beam size@I.P. (micron)

4005001000

Beam current (mA)

1500

329/Jan/0330/Oct/02
21000.5

11.5

2

Bunch current (mA)

Equivalent to 1800mA for 1200 bunches

Can we confirm the improvement of the blowup is due toshort solenoids ?
We can not directly prove that this improvement is due toadditional solenoids installed in this summer, because their power
supplies can not be switched off selectively.

How much the length covered by solenoids increased by short solenoids?
Length covered by field larger than Bz(1/8 arc in ring)

Ratio (length w/o short solenoid/with short solenoid)
1.2

250
with short solenoid
without short solenoid

1200

Length (m)

0.8

150
0.6

100
0.4

50

0.2

00020406080

1000204060Bz (G)Bz (G)

Stronger field is better ?

80100Effect of wiggler on blowup

by L. F. Wang

Do electrons inside bends affect to theblowup ?
A simulation shows the density of the electrons
near the center of vacuum chamber in bend islarger than 1/10 of drift space.
Total length of wigglers:100m≈ total length of lattice bends⇒ Turn off wigglers and observe beam size and tune shift.

Emittance (nm)Radiation damping time(transverse) (ms)Bunch length (mm)wig. onwig. off

1930
87

43
5.5

5.4

Beam size

Vertical tune shift

4 spacingWig. 5A< -> 40G54/140/4

Vertical beam size@I.P. (micron)V̲500mA
V̲Tune̲560mA

Vertical
0.01
4/140/4

40.008

wig. off
wig. on

3wig. on0.006

wig. 5A

wig. off
0.004

20.00210-0.002

0
0200

400

600

800

1000

0100

200

300

400

500

BucketBeam current (mA)large difference of beam size

no large difference of tune shift

Effect of damping time on the blowup may be larger than that of cloud.

600

Measurement in Dec. 1999

C type PM high emi. (4)C type PM high emi. wig. off (4)Dec. 199910104/60/44/60/16

8

Vertical beam size@I.P. (micron)Vertical beam size@I.P. (micron)C type PM high emi. (16)
C type PM high emi. wig. off (16)

Dec. 1999

6420864200

50

100

150

200

250

Beam current (mA)

300

350

050

100

150200250

Beam current (mA)no large difference of blowup with and w/o wiggler.

300

350

Summary1) Short solenoids and permanent magnets were installed.
2) After the installation the blowup is not seen up to1650mA in fill pattern for physics.
3) We can not prove experimentally that the shortsolenoids are effective or not. If effective, stronger
field may be better to suppress the blowup.
4) Large blowup was observed when the wigglers wereturned off. The result is not understood yet.

2. HER transverse coupled bunch instability
Observations
1) Growth rate
HER horizontal

HER vertical

1200

1200

1000

1000

800
Growth rate (1/s)

Growth rate (1/s)

Jul. 01600

Nov. 00

400

200

Calculation(Resistive wall)

0800

600

Jul. 01400

Calculation(Resistive wall)

Nov. 00

20000100

200

300

400

500

600

700

800

Beam current (mA)

Horizontal > Vertical
Non-linear to beam current.
Almost same value in 2000.11 and 2001.7.

0100

200300400500Beam current (mA)

6007008002) ModeSharp peak appeared at low frequency observing at a fixed position.

EXPERIMENTAL OBSERVATION OF COUPLED BUNCH INSTABILITY

HER [Mode Spectra]
Horizontalx 104

3

Measured3

Verticalx 105

Measured

221

100

Resistive wall (calculation)*

A.UA.U

Resistive wall (calculation)*

200400600

Mode800

1000 1200

Figure 1: Horizontal mode spectraof HER at 700 mA002004006008001000 1200

ModeFigure 2: Vertical mode spectraof HER at 700 mA

Fill pattern: 1 train/1153 bunches/8 ns bunch spacing*Ref. paper for resistive wall calculation: K. Thompson and R. Ruth, "Transverse Coupled-Bunch Instabilities
in Damping Rings of High-Energy Linear Colliders," Phys. Rev. D 43, 3049 (1991).

Another data (Y. Ohnishi)
Wave number (horizontal)
HER Horizontal 500mA

Wave number = 13
Analytic formula (of fast ion): 20

HER Horizontal 700mA

Wave number = 14
Analytic formula: 23

HER Horizontal 700mA3-spacing

Wave number = ~20
Analytic formula: 27

Wave number3) How serious ?
HER horizontal
2000

Growth rate (1/s)

1500

1000500

Design current

00500

1000

1500

Beam current (mA)

Damping time of 0.5ms is necessary for suppressing the CBI at thedesign current of 1.1A.

Candidates of source
1) Resistive wall
Pros - Some data show mode 0 of the CBI.
Cons - Some data do not show mode 0.
Growth rate : Horizontal > Vertical2) IonPros - Some data show mode near ion oscillation.
(But peak position of mode is not dependent
on beam current.)
Cons - Some data show mode 0.
Growth rate : Horizontal > Vertical3) HOM in vacuum chambera) RF cavity ?
b) IR chamberCons - Mode and growth rate in HER are different fromthose in LER.
c) Mask ?
Cons - Almost same growth rate in Nov00 and Jul01.
D9, D12 vertical masks were replaced from Ver.
3.1 to Ver. 4.
d) Other chamber ?

Measures
1) Remove instability source2) Bunch feedback system
Damping time of 0.5ms is needed to suppress theCBI at design current.
As the typical damping time of the present feedback
system is 0.5ms ( PHYSICAL REVIEW ST - AB, 3, 012801 (2000)) , itis expected that the instability is cured by the presentfeedback system up to the design current.

Summary
1) Horizontal coupled bunch instability is observed in HER.
2) There is no consistent model to explain the observation.
3) It will be suppressed by the feedback system up to thedesign current.

In 2000 autumn, 20 G was enough.
Effect of field strength on beam size (LER)(Physics-fill pattern)
6
excitation current of solenoid

1005A
4A
3A
2A
1A
0A

16/74/4ξ =9.57

5yVertical beam size@IP(micron)

80(mA)
I th60

2 trains
60 bunches/train
4 rf buckets-spacing

40

20

Vc=6MV4

32

10-10123Solenoid current(A)

4

5

600

100

200

300

400

Beam current(mA)

500

600

700

Statistics of growth rate at beam abort
Growth rate before beam loss (HER horizontal)

Beam current at beam loss (HER horizontal)

0.005

1000

0.004

beam current (mA)growth rate (turn-1)

800

0.003

0.002

600

400

5 ms2000.001

00Jan/16Feb/8

Mar/3 Mar/26 Apr/18 May/11

date

Jun/4

Jun/27

Jul/20

Jan/16

Feb/8

Mar/3 Mar/26 Apr/18 May/11

Jun/4

Jun/27

Jul/20date

Growth time at beam abort with bunch feedback = 3ms-10ms(@900mA)