PROBLEMS AND IMPROVEMENTS OF THE MOVABLE MASK SYSTEM
FOR KEKB
Y. Suetsugu, N. Akasaka, T. Kageyama, Y. Takeuchi, K. Sato and K. Kanazawa,KEK, Tsukuba, Japan

Abstract
The movable mask is a device that cuts off spentelectrons/positrons near the beam orbit and reduces
background of the detector1. For the KEKB (KEK Bfactory), sixteen movable masks had been installed foreach ring. The originally designed masks, however, had
severe troubles of heating, arcing and vacuum leaks.
Several kinds of new masks had been designed partiallyemploying RF technologies in dealing with HOM andwere installed to the ring step by step. The latest masks
are working almost as expected now. Here the problems
of movable masks so far are summarized and the structureof the latest movable masks and their status are presented.

1 INTRODUCTION
Initially sixteen movable masks had been installed foreach ring of KEK B-factory (KEKB), i.e., Low Energy
Ring (LER) and High Energy Ring (HER)1.
Themovable mask is a device that cuts off spentelectrons/positrons just near the beam orbit and reduces
background of BELLE detector. The masks were locatedat the straight and arc sections upstream side of theinteraction region (IR). As the stored currents increase,
however, the originally designed masks began showing
the problems of heating and arcing due to the transient
wall current or the intense RF fields of a trapped mode.
The heating and arcing sometimes caused vacuum leaks.
The trapped mode also excited a strong longitudinal
coupled bunch instability and limited the stored beam
current.
Since the problems were revealed several
improved masks were manufactured and installed. The
history of installed masks is presented in Fig.1. The

Figure1 History of movable masks

finally installed new masks, Ver.4 and Ver.5, are working
almost as expected up to now. Here we summarize theproblems of the old masks (Ver.1 – 3), and then report thepresent status of new ones (Ver.4 and 5).

2 PROBLEMS OF OLD MASKS
The outlook of the originally designed movable mask
(Ver.1) is shown in Fig.2. The mask head is a block
made of Cu-W and inserted from a vacuum flange into thebeam chamber.
The mask head position can becontrolled remotely observing the background of BELLE
and the beam lifetime. The stroke of the mask head is
about 20 mm and a vacuum bellows absorbs themovement. The inside of mask head is cooled by waterflowing though the stem. The spacer fills the space offlange port and makes the chamber inner surface smooth.
The stem, the spacer and the bellows are made of stainlesssteel. The fixed and the sliding shield fingers are at thegap between the spacer and the chamber and between thestem and the spacer, respectively. The shield finger ismade of silver-coated Be-Cu. The shield finger should
prevent the RF fields from intruding inside the structure.
The basic structures for other old masks (Ver.2 and 3) are
the same as Ver.1 mask.
The first essential defect of Ver.1 mask is the sliding
shield finger. It was so weak to ensure the intense RF
fields that it was easily heated and melted.
Theinsufficient cooling structure and the materials with low
thermal conductivities worsens the heating problem.

Figure 2 Structure of Ver.1 mask.

Figure 3 Ver.3.2 mask with SiC HOM absorber.

Fig.3. The Q-value reduced to less than 100. The
problem of exciting beam oscillation was settled after
Ver.3.1. However, after the operation of about 2 months
at the beam current over 700 mA resulted the break of a
SiC block. The absorbed power was estimated as about
500 W for a SiC block. The stored beam current of HER
has been limited by the absorbed power at Ver.3.2 masks.
The third problem, which is still a main problem of
new masks (Ver.4 and 5), is the groove generated on themask head as shown in Fig.4. The groove seems to bemade by the main body of the beam when the orbit wasaccidentally steered or abnormal beam orbit was set. It isalso said that the energy loss due to a power down of theaccelerating cavity leads to the vertical beam oscillation
or beam size blow up and then causes the digging of mask
head. The R&D to prepare the fast and safe beam abortsystem is undergoing now. The change of material of themask head is also discussed.

3 NEW MASKS
3.1 Ver.4 mask
Ver.4 movable mask was designed for LER arc section.
The structure is quite different from the old ones. The
schematic structure of a vertical mask and the installedmasks in the tunnel are presented in Fig.5 and Fig.6,respectively. The mask head is not a small block as

Figure 4 Groove on the mask head.
Once the sliding shield finger is broken, the RF field caneasily enter inside the spacer because of its coaxial linestructure. An extensive power of the entered RF field
was dissipated at the bellows in a resonance condition.
Furthermore, the broken fingers can be a seed of arcing
between the spacer and the stem. We had experienced
vacuum leaks four times with Ver.1 and 2 masks (seeFig.1).
Two cases are at the gap between stem andspacer, and other two cases are at the bellows. For Ver.3(3.0 – 3.2) the sliding shield fingers are moved towardoutside as shown in Fig.3 and the current density of it wasreduced.
The problem of fingers, that is, of excess
heating and vacuum leak, was solved after Ver.3 mask.
The second serious defect of Ver.1, 2 and 3.0 is that
there is no effective absorber for the trapped mode of themask. The Q-values of the major trapped mode of Ver.1
and 2 are several thousands. Even if the sliding fingers
are not broken, the intense trapped RF field remains
around the mask. Actually, we had sometimes observed alongitudinal coupled bunch instability when the mask
head approached the beam. For Ver.3.1 and 3.2 the SiC
blocks were equipped as a HOM absorber as shown inFigure 5 Layout of Ver.4 mask.

Figure 6 Ver.4 mask installed in the tunnel.

3.3 Ver.5

Figure 7 Structure of Ver.5 mask.

Two Ver.5 masks are developed and installed at theinteraction region of LER. Due to the spatial limitationof the interaction region we cannot use Ver.4 mask there.
The structure (cross section) of Ver.5 mask is shown inFig.7. The basic structure is a plunger used for the tuningof accelerating cavities. The mask head was made ofpure copper.
The mask damps the RF field heavily
through its coaxial wave-guide structure loaded with acylindrical SiC absorber (Fig.8). The Q-factor of Ver.5
mask for the major trapped mode is calculated as about 10
and the design power capability of the SiC absorber is up
to 10 kW. Before installation, the mask received a highpower test using the 1296 MHz microwave up to 4.2 kW.
At the beam current of 500 mA the absorbed power isestimated as about 800 W. The Ver.5 mask should haveno problem, therefore, even for the design current
operation at 2.6 A.
The grooves on the mask head,
however, should be checked regularly.

4 SUMMARY

Figure 8 SiC absorber for Ver.5 mask.
before but a bent chamber (Mask chamber in Fig.5). The
driving supports move the chamber and the chamber in
itself works as a mask. The chamber is made of purecopper. The stroke is about 20 mm. The movement of
the mask chamber is absorbed by two universal bellows(Bellows chamber) connected at the both side of it. The
universal bellows has the RF shield structure same asother bellows for KEKB2. The most important feature ofVer.4 mask is that there is no trapped mode although theHOM is generated since the cross section of the beamchamber does not change in this mask region. The Ver.4mask has shown no serious problem in the operation at thebeam current over 700 mA for more than 6 months.
However the temperatures of a corrugation of the bellows
located near the masks are relatively higher than others.
That temperature rise seems to be due to the HOM
generated at the mask and the scattered synchrotron
radiation. We have to keep watching the temperature aswell as the grooves at the mask head.

The old movable masks (Ver.1 – 3) for KEKB has
some serious problems in high current operation. The
heating trouble and the excitation of the longitudinal
coupled bunch instability had limited the stored beam
current, that is, the luminosity. The two types of new
movable masks (Ver.4 and 5) have been developed toovercome the situation. They were installed to LER andshow no essential problem except for some heating at thebellows near the Ver.4 mask. All of movable masks forboth rings will be replaced by new ones until the summer
shutdown this year.

ACKNOWLEDGEMENTS
The authors would like to thank Prof. S.Kurokawa, Prof.
M.Yoshioka, Prof. Oide and many other staff of theAccelerator Laboratory in KEK for their continuous
encouragement and cooperation.

REFERENCES[1] Y.Suetsugu et al., “Development of the Movable
Mask System for KEKB”, in proceedings ofEPAC2000, Vienna, June 2000, p.2301.
[2] Y.Suetsugu et al., ”Design studies on a vacuum
bellows assembly with radio frequency shield for theKEK B factory”, Rev. Sci. Instrum., 67, 2796, 1996.