The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

COMB-TYPE RF-SHIELD FOR HIGH CURRENT ACCELERATORSY.Suetsugu#, M.Shirai and K.Shibata, KEK, Tsukuba, Japan
Abstract
A new RF-shield structure for high current accelerators
is proposed. The RF-shield has no thin fingers but nested
comb teeth instead at the inner surface of beam duct.
The comb-type RF-shield has a higher thermal strength
structurally compared to the conventional finger-type.
The simulation also indicates its lower impedance.
Especially, the leak of TE mode like HOM through theslits is almost suppressed. A preliminary test with aprototype RF-shield transmitting a 508 MHz microwave
in the atmosphere shows no abnormal heating or arcing.

INTRODUCTION
Several vacuum components like bellows chambers andgate valves in accelerators are usually equipped with theRF (Radio Frequency) shield structures inside. The RFshield bridges the gap to reduce the beam impedance andto avoid the heating of components due to the extra HOM(Higher Order Modes) excited there. The conventional
RF-shield is a finger-type, which consists of lots ofnarrow and thin fingers surrounding the inner surface ofbeam duct [1-4].
Recent high current accelerators, however, put muchseverer conditions than ever before on the RF-shieldstructure. The storage current of ampere orders brings
high wall current density. The high bunch current andthe short bunch length lead to intense HOM excitation.
Especially, the TE-mode like HOM easily couples withthe space between the RF-shield and the outside structure
through the axial slits between fingers. These are likely
to result in the heating of components.
Proposed here is a new RF-shield, that is, a comb-type
RF shield, for high current accelerators. The RF-shield is
no more than the finger-type and will have a higher
thermal strength than the conventional one.
The
calculated loss factor is lower than the conventional
finger-type. In the following, the structure of the combtype RF-shield is described at first and then the some
calculated RF properties and the results of the preliminary
tests are presented and discussed.

STRUCTURE
The inside view of a comb-type RF-shield for a circularbeam duct is presented in Fig.1(a). The RF-shield is nomore than thin fingers but nested comb teeth. In a typicalmodel, the length (a in the sketch in Fig.1, the same way
in the following), the width (t) and the radial thickness ofa comb tooth are 10 mm, 1 mm and 10 mm, respectively.
The gap between adjacent teeth (c) is 2 mm and, therefore,
the gap between the nested teeth is 0.5 mm. At thenominal position, the total length of the RF-shieldstructure (b) is 15 mm. For the circular beam duct with_________________________________________#yusuke.suetsugu@kek.jp

Figure 1(a): Inside view of a comb-type RF shield.

Figure 1(b): Outside of the comb-type RF shield.
The back-up fingers are between the nested teeth.
an inner diameter of 94 mm, for an example, the numberof teeth is 100. The combs will be made of pure copper.
In principle, the high frequency wall current
accompanied with a bunched beam flows via capacitance
between nested teeth (a gap of 0.5 mm). To ensure theflow of DC or low frequency wall current, small fingers(back-up fingers) are prepared at outer half between thenested teeth (see Fig.1(b)). The back-up fingers also
have an important role to block the leakage of HOM as
described later. The back-up fingers will be made ofsilver coated Inconel alloy.
The advantages of the comb-type RF-shield are as
follows: (1) The RF-shield has a high thermal strength.
(2) There is no radial step on the inner surface of beam
duct and the shield has a low impedance. (3) The TE
mode like HOM hardly goes out through the RF-shield
due to the large thickness (10 mm) of the teeth. (4) Thereis no sliding point on the inner surface of beam duct,
which otherwise could be a source of arcing or heating.
(5) The RF-shield can fit for beam ducts with various
cross sections.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

1.2x10104x103

10-1

Loss Factor, k [V C ]

1x10

32x10

3

Comb-type (w/ back-up finger)
Comb-type (w/o back-up finger)
Finger-type

98x10

Comb-type RF-shielda = 10 mm, b =15 mm
c = 2 mm, t = 1 mm

Bz [T]

96x101x103

Finger-type RF-shielda bump with 1 mm height4x109

0345678910Ex at g1
Ex at g2

101210111010

3-3x103-4x10-104x105x10-106x10-10-10

7x10

8x10

Figure 4: Intensities of the axial (z) components ofmagnetic fields for the comb-types (w/ and w/o backup finger) and the finger-type RF-shields.

and the slot length decreases at the same time.
Asincreasing the gap (t), furthermore, the loss factor
increases because the flow of wall current is more steered.

1413-2x10Time [s]Figure 2: Dependence of loss factors of the combtype and the finger-type RF-shield on the bunch
length for a circular beam duct with a diameter of 94mm.

10032Bunch Length, σz [mm]100 100-1x103

2x109

Electric Field, Ex [V m-1]

3x10

Electric Field between Teeth

a = 10 mm
b = 15 mm
c = 2 mm σz = 10 mm
t = 1 mm Bunch Charge = 1 C012345Depth, d [mm]Figure 3: Electric filed intensities along the depth (d)of the gap between the adjacent teeth, g1, andbetween the nested teeth, g2.
On the other hand, the axial stroke of the RF-shield islimited structurally. For the trial model described above,
the available stroke should be +3 mm (expansion) and -4mm (contraction). The bending at the RF-shield of 20
mrad is possible as the usual finger-type one.

Figure 3 presents the change of electric filed intensities
in the gaps between the adjacent teeth and the nested teeth,
g1 and g2 in the sketch, as a function of depth, d [mm].
The electric fields in the figure are the peak values justafter a bunch with a length (σz) of 10 mm and a charge of1 C passed at the center of duct with a diameter of 94 mm.
In the gap g1, the intensity of electric filed decreases
rapidly as the position goes outside.
Plotted is thetransverse electric field but the other components also
behaves similarly. Since the gap g1 is the only way for
the HOM to go outside since the back-up fingers (seeFig.1(b)) block it in the gap g2, that indicates the HOM
will hardly leak outside through the thick teeth.
In the gap g2, on the other hand, the maximum electricfield does not change so much with the depth. The mode
has a frequency of about 14 GHz and seems an eigen
mode excited in the gap. The mode, however, damps
rapidly with time.
The intensity decreased to a half
within 0.3 ns.

HOM outside of RF-Shield
RF PROPERTIES
Loss Factors
Using the MAFIA simulation code, the loss factors ofthe comb-type and the finger-type RF-shield were
calculated for several conditions. The outer half of thenested teeth part is short-circuited assuming the back-up
finger. The dependence of the loss factors, k [V C-1], forboth types on the bunch length, σz [mm], are shown inFig.2 for the beam duct with a diameter of 94 mm. Theloss factor of the comb-type is smaller than that of thefinger-type by a factor of 3 or 4. The loss factor reduced
as contracting the RF-shield. That is because the lengthof nested part increases, that is, the capacitance gets large,

The HOM intensities outside of the RF-shield were
calculated for both types of RF-shields.
The timevariation of the axial component of magnetic field (Bz) ispresented in Fig.4. The intensity for the comb-type withback-up fingers is much smaller than those for the fingertype by one order of magnitude. Since the Bz represent
the TE mode like HOM, it can be said that the TE mode
like HOM is well damped by the thick comb-type RFshield.
Note here, however, that the Bz for the case of combtype RF-shield without the back-up fingers is almost thesame as that of finger-type. That means that the leakageof TE mode like HOM is comparable to the case offinger-type if the comb-type has no back-up fingers. The

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Figure 5: Set up of heating test transmitting a 508
MHz microwave.
0.035

(b)

0.03

Upstream
Downstream

0.02

odT/dt [ C s-1]

0.025

0.015
0.01

In the experiment, the input power was stepped up
gradually up to 400 kW at the flat top of wave form. At
400 kW, for an example, the RMS current density islarger by 1.5 times than those of the designed beam
current in the Super KEKB [6]. The experiments were
performed changing the length of the RF-shield (b) from
12 mm to 18 mm.
The temperature rises at several power levels arepresented in Fig.6 for the case b = 15 mm. Two data arethose at the upstream and the downstream side of theopposed comb teeth. The dashed line is the calculated
temperature rise considering the joule loss of about 100
mm centered on the RF-shield. Although the data werescattered due to the small temperature rise, the measured
temperature rise was almost in agreement with theexpected ones. The reflected power was always less thanabout 2 % of the input power, which was comparable tothat from the dummy load itself.
After the experiments the inside and outside of the RFshield was checked and no trace of abnormal local heatingor arcing was found. The arcing in atmosphere, however,will not likely occur compared to the case in vacuum. It
should be also noted that the frequency of HOM will behigher than that used in the test. No arcing in the present
test, therefore, should be considered as a minimal result.

0.005
00100

200300400500

Input Power [kW]Figure 6: Temperature rises of RF-shield part as afunction of the input power of 100 – 400 kW, where
the dashed line is those calculated from the joule loss
at 100 mm region centered on the RF-shield.

mode excited between the gap g2 (see Fig.3) seems to go
through the gap and emitted to outside. To suppress
sufficiently the HOM outside the RF-shield, the back-up
fingers, or some other measures to short-circuit the nested
teeth just outside, will be indispensable for the comb-type
RF-shield in high current machines.

PRELIMINARY HEATING TESTSThe heating of the comb-type RF-shield by wall current
was examined transmitting a high-power microwave inatmosphere [3]. Figure 5 shows the partial layout of theexperimental setup including the trial model of the RFshield. Utilized is a 508 MHz klystron used for the RF
accelerating cavities in a KEKB. The TE10 mode in therectangular waveguide was transformed to the TEM modein the coaxial line by a coaxial coupler. The CW
microwave is transmitted through the model and finallyabsorbed in the dummy load. The size of the comb-type
RF-shield is the same as described above.
Thetemperatures at 8 points near the root of the teeth and 2
points on the body of the duct were measured by thermocouples.

SUMMARYThe simulation and the preliminary test of the combtype RF-shield gave promising results as described above.
The availability to the accelerators, however, should bestudied using a real beam, where the frequency spectrumof the excited HOM and the wall current is different fromthe preliminary test.
Two prototypes of the bellows
chambers with this new RF shield are under
manufacturing and will be installed in the KEKB positronring this year to see the basic properties of the comb-type
RF-shield.

ACKNOWLEDGMENTS
The authors especially express their deep appreciation
to T.Kageyama, K.Ebihara and H.Nakanishi of KEKB RF
group for their valuable suggestions and kind instruction
during RF experiments.

REFERENCES[1] LEP Vacuum Group, Vacuum, Vol.41, p.1882 (1990).
[2] W.Unterlerchner, Vacuum, Vol.41, p.1920 (1990).
[3] Y.Suetsugu, K.Ohshima and K.Kanazawa, Rev. Sci.
Instrum., Vol.67, No.8, p.2796 (1996).
[4] Y.Suetsugu, K.Kanazawa and K.Ohshima, Vacuum,Vol.47, No.6-8, p.629 (1996).
[5] K.Kanazawa, S.Kato, Y.Suetsugu, H.Hisamatsu,
M.Shimamoto and M.Shirai, NIM-PR-A, Vol.499,p.66 (2003).
[6] Y.Ohnishi, KEK Proceedings 2002-17, p.1 (2003).