Improvement of Movable Mask System forHigh Beam Current Operation in 2002
10/Feb/2003
KEKB VAC K. Shibata

ContentsIntroduction
Mask with Reduced HOM Design
Winged HOM Damper
Other Improvements
Summaries
2003/2/10

MAC2003

1Introduction
Installed Movable Masks (present)
TSUKUBA

D12

Vertical Mask

IRLER
VĻ4
HĻ4HER
VĻ4
HĻ4D03HER D12Installation of Movable Mask
with Reduced HOM Design

Horizontal Mask

NIKKO

D09

Improvements in 2002OHO

LER D03
Installation of Winged HOM
Damper near Movable Masks

Mask HeadVĻ4HĻ4e-e+e-D06+FUJIe/HĻ4

VĻ4

D03 V-MasksChange of the material
from Cu to Ti

IRRemove of Movable Masks

Linac
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Movable Mask with Reduced HOM Design 1– Problem
Overheating of bellows just near the V-Mask in HER at the beam
current higher than 900 mA
Cause
The HOM was excited at the mask head and went along the beamduct.
The TE-Mode HOM went inside of the bellows through slits
between RF shield fingers.
Movable Mask (Ver.4)

Bellows

Mask headCountermeasure :Development of improved movable mask with reduced HOM design(Extension of the ramps of the mask)
2003/2/10

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3Movable Mask with Reduced HOM Design 2 - DesignIn order to reduce the power of TE-Mode HOM, the length of ramps
beside the V-Mask head was expanded from 30 mm to 400 mm.
The height of both mask is same. (15 mm)

Previous Short Mask

New Long Mask

2003/2/10

30 mm

400 mm

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Movable Mask with Reduced HOM Design 3 - Picture
Bellows

Mask Head

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Movable Mask with Reduced HOM Design 4 – Loss Factor
The loss factor of the V-Mask (HER) was estimated from MAFIA T3simulation.
k(l=30 mm) = 0.35 [V/pC]

Loss factor of the HER vertical mask
as a function of the ramp lengthk(l=400 mm) = 0.16 [V/pC]

k(l=400 mm)≈ 0.5k(l=30 mm)
Expectation from k:
The temperature rise of thebellows near the mask should
reduce to 50 %.

Loss Factor k [V/pC]✪#✮0.35
✪#✭
✪#✬

0.16
✪#✫
✪

✪ 30 ✫✪✪✬✪✪

✭✪✪

✮✪✪
400Ramp Length l [mm]2003/2/10

MAC2003✯✪✪6

Movable Mask with Reduced HOM Design 5 – Bz2
Bz represents the TE-Mode HOM excited atthe mask head.

yz·x<Bz2> represents the power of the TE-Mode.

Bz<Bz2> was estimated from MAFIA simulation.
<Bz2(l=400 mm)>
4.5×106<Bz2(l=30 mm)> = 6.8×107

Expectation from Bz:
The temperature rise of thebellows near the mask could
reduce to 7 %.
2003/2/10

Bz2 of the HER vertical mask asa function of the ramp length✱✕✫✪7✱

6.8×10✰✕✫✪✱<Bz2> [T2]

≈ 0.07

Monitoring point✯✕✫✪✱✮✕✫✪✱✭✕✫✪✱
✬✕✫✪✱
✫✕✫✪✱

4.5×107✪
MAC2003✪ 30 ✫✪✪✬✪✪✭✪✪✮✪✪
400Ramp Length l [mm]✯✪✪7

Bellows
Movable Mask with Reduced HOM Design 6 – Temperature✰✪

Short Mask
(l=30 mm)✯✪

Bellows Temp. [deg]Bellows Temp. [deg]✰✪✮✪✭✪✬✪✪✬✪✪✮✪✪

✰✪✪

✲✪✪

✫✪✪✪

HER Current I [mA]Long Mask(l=400 mm)✯✪✮✪✭✪✬✪✪✬✪✪✮✪✪

✰✪✪

✲✪✪

HER Current I [mA]✫✪✪✪

Behavior of bellows temperature against HER beam current

ɺT(l=400 mm) 4.3= 21.0 ≈ 0.2 (I=950 mA)
ɺT(l=30 mm)
Experimental result:
The temperature rise of the bellows near the mask
reduced to 20 %.
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Movable Mask with Reduced HOM Design 7 – Discussion
There is a discrepancy between the expectation and the experimentalresult.
Expectationfrom B
Experimental result
Expectation from kz<All of the TE-mode

ɺT(400 mm)= 0.2
ɺT(30 mm)Part of the TE-mode

The slit length is 20 mmk(400 mm)= 0.5k(30 mm)

Both the TE and TM mode
1.0

The TE-Mode with low frequencies can
not go through the RF shield of bellows.
For Example:<Bz2(400 mm)>f>2 [GHz]≈ 0.22<Bz (30 mm)>f > 2 GHz
Numerical result

0.8=

MAC2003l=30 mm
l=400 mm

0.6
0.4
0.20.00

Measured result
2003/2/10<Spectrum of Bz [a. u.]<Bz2(400 mm)>
= 0.07<Bz2(30 mm)>

51015 20Frequency f [GHz]Spectrum of Bz for l=30 mm andl=400 mm for the HER V-Mask9

Winged HOM Damper 1 – Problem
The TE-Mode HOM exited at the H-Mask went
through the RF shield at the pumping port.

H-mask against beam current

The NEG element was heated and gas (H2) wasdesorbed from the NEG element.

Pressure [Pa]

Abnormal pressure rise near the movable masks in LER (I>1200 mA)
Cause
Behavior of pressure near theInstallation of the special RF shield gasket
The HOM went to the outside of the shieldedregion.
Abnormal pressure rise at the outside of theshielded region was newly observed.

✲✕✫✪✧✱✰✕✫✪✧✱✮✕✫✪✧✱✬✕✫✪✧✱✪✪

Longer ramp of LER mask
LER mask : 130 mm
HER mask : 30 mm✮✪✪✲✪✪✫✬✪✪✫✰✪✪LER Current [mA]Ļ Extension of the ramp of the mask

Countermeasure :Development of the winged HOM damper
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Winged HOM Damper 2 – DesignFeatures
The damper has delta-type wings and two SiC absorbers are
installed inside the both wings.
The wing and the beam chamber are connected with a long slot
(20 mm wide ×707 mm long).
TE modes in a wide frequency range can be absorbed
Theefficiently.
damping of the TE11 mode with a polarization perpendicular to
the wing was about 12 dB for the frequency range from 2 to 6 GHz.
SiC Absorbers

Long SlotWinged HOM Damper
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View from the Inside ofthe beam duct11

Winged HOM Damper 3 – Pictures

H-MaskWinged HOM Damper

The TE-Mode excited at the H-Mask is polarized horizontally.
The TE-Mode polarized horizontally should go through the RFshield at the pumping port at the bottom of the beam duct.
The wing was placed vertically
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Winged HOM Damper 4 – Installation
Two pairs of the dampers were installed near the H-Mask in LER
putting the four movable masks between each pair.
To absorb the TE-Mode excited at the H-Mask, the wings wereplaced vertically and near the horizontal mask.
Every pumping port between a pair of absorbers was screened bya special RF-shield gasket.
~20 m
HV V
Damper1~50 m
H

Positron BeamDamper2

H VV
Damper3

Special RF shield gasket

2003/2/10~20 m
HDamper4

Special RF shield gasket

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Winged HOM Damper 5 – Absorbed HOM power
The absorbed HOM power was estimated from the temperature riseof the cooling water of the SiC absorbers.
Absorbed HOM Power~20 m

HV V
Damper1

1.5 kW

H
Damper2~50 m~20 m

Positron Beam
1.4 mA

3.3 kW

H VV
Damper3

H
Damper4

3.8 kW

2.6 kW

6.4 kW

4.8 kW

Loss factor of H-Mask : 0.6 V/pC (for 10 mm gap)
Power loss for two H-Masks : 20 kW (I=1.4 A)
Absorption rate :Damper1+Damper2
24%
2003/2/10MAC2003

Damper3+Damper432%
14Winged HOM Damper 6 – Pressure near the masks
HV VH

HVV

Monitoring point

Monitoring point✧✱✲✕✫✪✧✱✰✕✫✪✧✱Pressure [Pa]Pressure [Pa]✲✕✫✪✮✕✫✪✧✱✬✕✫✪✧✱✪✪H✮✪✪

✲✪✪

✫✬✪✪

LER Current [mA]✫✰✪✪✰✕✫✪✧✱✮✕✫✪✧✱✬✕✫✪✧✱✪✪✮✪✪✲✪✪✫✬✪✪✫✰✪✪LER Current [mA]

Behavior of pressure near the H-mask against beam current

Result :The abnormal pressure rise near the mask disappeared at theoutside of the HOM dampers.
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Other improvements
Change of the mask head
Problem:
Groove generated on the mask headCountermeasure:
Change of the mask head material from Cu to TiResult:
New groove was generated on 29/Jan./2003.
(The beam energy was changed greatly by accident.)
We need more investigation.
Remove of the IR movable mask
Problem:Heating of the components near the masks
Countermeasure:
Remove of the IR movable mask
Result:
The temperature was reduced .
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Summary and Conclusion
Movable Mask with Reduced HOM Design
The ramps of the mask was expanded from 30 mm to 400 mm.
The temperature rise of the bellows near the mask reduced to 20%.
Winged HOM Damper
The abnormal pressure rise near the mask disappeared outside theHOM dampers.
Detailed analysis is in progress.

Conclusion:
Both the movable mask with reduced HOM design and thewinged HOM damper can relieve the heating of thecomponents due to the TE-Mode.
2003/2/10

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