The 11th SympoSium on Accelerator Science and Technology, Harima Science Garden City, 1997

Supereonducting Single Cell Cavity Thst for Neutron Science Project at JAERIJoichi KUSANO, Nobuo OUCH!, NobuoAKAOKA*, Kenji SAITO**, Shuichi NOGUCHI**,Ken MUKUGI***, Kazuo HASEGAWA, Motoharu MIZUM010
Japan Atomic Enetgy Research Institute
2-4 Shirakata, Tokai-mura, Naka-gun, lbaraki-ken, 319-11 Japan* Nippon Advanced Technology Co. Ltd** KEK: High Energy Accelerator Research Organization*** Mitsubishi Electric Corp.
Abstract

ity system has newly started for a design study of the systemand for a cavity test facility installation.

JAERI has been proposing a high intensity proton linear accelerator for the Neutron Science Project exploring basic researches and nuclear waste transmutation technology
based on a next generation spallation neutron source.
Superconducting (SC) cavity is a main option for highenergy portion of the acceleratm: The SC cavity development has been started since 1996 with an installation of test
stand, design of cavity shape and fabrication of single cell
cavities of f3=0.5 in collaboration with the KEK. This report
describes the performance of the SC cavity test facility at
JAERI and the result of the frrst SC cavity vertical test.
1. Introduction
In these years, JAERI has been planning the Neutron

Science Project (NSP) for exploring basic researches combined with the OMEGA program in JAERI based on a next
generation spallation neutron source driven by the high intensity proton linac[l]. The design concept of the high intensity
proton accelerator has been considered to achieve the various
beam operation modes in the NSP.
A conceptual layout of the NSP accelerator is shownin Figure 1 and preliminary parameters for the accelerator are
given in Table 1. The development of superconducting rf cav-

Table 1 A preliminary specification of the JAERI NSPLinac
Energy:Accelerated particle :Average current:

1.5 GeVH+,H·
1st stage: 1 mA
2nd stage : 5.33 mA maximum
Peak current :Nominal30 mA
Low enetgy part :Normalconducting linac:200 MHz
High enetgy part :Superconducting linac : 600 MHz
Beam operation mode : 1st stage: Pulse mode operation
2nd stage: CW I Pulse operation
Repetition rate :50Hz
Macro-pulse width :2 ms (at 1mA operation) to CW
Chopped-pulse width : 400 ns (interval 270 ns)
Chopping factor :60%

2. Superconducting rf cavity

Superconducting (SC) rf cavity is a main option forhigh energy portion of the NSP accelerator. Resulting withbasic studies for the structure of high ene~gy part of the accelerator, several favorable characteristics were pointed out with
SC option in comparison with normal conducting cavity option. ·There are 1) a high electrical field gradient for beam acceleration which conducts ashort length of the acceleratm; 2) high quality
factor of -10 9 which gives a low operationcost and 3) acceptable wide beam tube aper
ture of 100 to 180 mm in diameter which inI S ( +,H·) RFQ
II
1CXlMIN
troduces a low beam loss rate operation.
1.SGIII~- ·~,-100m~'"
In the CW electron acceleratm; technolo(Highppart)
Short
Pulse
·(low ppart)-aoo m~ storage Ringgies of SC accelerators are established. The
Fig. 1 Conceptual Layout of NSP Linac
experiences of design, manufacturing and...

-240-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

operation for the electron SC accelerator are accumulated foryears such as KEK-TRISTAN and other many laboratories[2].
The electron SC accelerator has single velocity of light speed(~=1). This condition leads uniform cavity shape and uniform cryomodule design for all accelerating cavity structures.
In the proton accelerato~ however, velocity varies sequentially from lOOMeV to 1.5GeV (0.43 < ~ < 0.92). Accordingly, the length of the cavity also changes by ~A. in eachregion. Main concern is the mechanical strength of the cavity
under the vacuum load for the enetgy range of 100 to 200MeV
because of the more flatter shape than that of the electron accelerator.

The test stand installed is consists 1) an ultra pure water generating system(UPW), 2) a high pressure rinsingsystem(HPR), 3) an ultra high vacuum pumping system(UHV)and 4) a cryostat for vertical test. Additionary, a large furnacewith clean vacuum is installed at the test stand which can beused since September 1997. The layout of the test stand isillustrated in Fig. 4. The summarized characteristics of thetest facilities are shown in Table 2.

3. The R&D work

The R&D work for SC accelerator has been startedsince middle of 1995 in collaboration with the KEK SC group
to make a design and demonstrate the cavity characteristics
and to establish a cavity test facility.

Design study of SC cavities
The mechanical structure calculations with theABAQUS code have been done to determine the cavity shapeparameters as well as electromagnetic ones with theSUPERFISH code[3]. To adjust the changes of proton velocity, energy range of 100 MeV to 1500 MeV, eight different
cavity lengths are proposed. In the preliminary design of theSC system, one cavity consists five cells and one cryomodulecomposes two cavities. The schematic drawing of the JAERISC accelerator cryomodule is shown in Fig. 2. The designconcept for the major part of the cryomodule is followed fromthe experience of the KEK-TRISTAN .

l ~~tTooll
Mounting.

.

I

Cryostat InstallalionID

1~=~1II

D

Dala Acquisition

I

D'Fig. 3 The major process flow of the cavity examinationrlPoworFig. 2 Schematic drawing of the JAERI SC cryomodule

Test facility for SC cavities
A test stand for the vertical tests of 600MHz cavities
has been installed at the JAERITokai site to examine the SCcavity performances for variable enetgy range. The SC cavity requires a pure material of niobium, ultra clean and smoothcavity surface and clean handling area[4]. The major processof the cavity examination is shown in Fig. 3.

-241-Fig. 4 The layout of the JAERI SC cavity test standThe lith Symposium on Accelerator Science and Technology,Harima Science Garden City, 1997

Table 2 Characteristics of the JAERI SC cavity test stand
Cryostat:
Test Area Dimension:
Diameter:
800 mmHight:
1,500 mm
Residual Mag. Field:< 10 mGauss
Thermal Permiation:
2.2 W
Cooling Speed(RT to 4.2K): 2 h
Ultimated Temperature:
1.90 K
LHe Feed Speed :2.5 LPM

Cavity Baking Fumace :Effective Baking Area :Baking Temperature:
Vacuum Pressure :830W x 830H x 1,800l
900°C
1 x 10-4 Pa (at 9000C)

cavity surface was made by 90 kgflcm.2 HPR with ultra purewater of 1.2 m3• To prevent a cavity collapse at the vacuum
loading on the flatter shape cavity, the support rods were
mounted between the beam pipe flanges. The measurements
were done for the Ep-Qo characteristics at 4.2 K and 2.1 K.
The peak surface electrical field gradient (Ep(max.)) =30 MV/m and quality factor (Qo) =2 x 1010 at 2.1 K were measured.
" Field emission " was observed at the high field measurement The rusult of the measurement is shown in Fig. 6. The
residual resistivity of 5.5 nQ of the test cavity was obtained.

Clean Room:< 10 pltt3 (> 0.3 J.i.m)

Clean Levei(Ciass 10):High Pressure Rinsing :UPW Pressure:
UPW Resistivity:
TOC Level:

90 kgf/cm2> 17.6Mn.cm
70to 300 ppb

Cavity Evacuation System :Ultimated Vaccum Pressure:5 x 10-11 TorrFirst SC cavity test
Bulk niobium superconducting test cavity of single cellfor the region of 150MeV is fabricated according to the preliminary cavity design. The major dimension of the test cavity is shown in Fig. 5. The pure niobium sheets of 3 mm
thick, RRR > 200, were pressed and trimmed to make the halfcells and the beam pipes. Two half cells and beam pipes wereconnected by EBW at the equator area and the iris area, respectively. After the forming, the cavity surface was polishedby mechanical polishing (BP) and electropolishing (EP). The
average removal thicknesses are51 J.lm by BP and 23 J.lm(lst)+ 33 J.lm(2nd) by EP, respectively. To eliminate the adsorbedhydrogen gas caused by EP process, the baking of the cavity
was done for 750 °C and 3 hours. The fmal cleaning of theFig. 6 Performance of 600 MHz single cell cavity 43 =0.5)

4.Summary
JAERI has started the R&D of the SC cavity to beappied in the NSP proton linac. A test stand was constructed ·to examine the cavity characteristics. First vertical test of asingle cell cavity was performed for bulk niobium cavity of
13= 0.5. Many of the cavity characteristics show in good
agreement with the design values at the frrst SC cavity test.

125

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References

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Iris- ---- . ... - - - - · C e l l - - - - - - - - - - ! II..
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Thickness of Nb Sheet

[2]

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=3 mm

[3]II~:)[4]

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Fig. 5 The dimension of 600MHz single cell cavity (jj =0.5)-242-M. Mizumoto et al., "A High Intensity Proton Linac
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