Accelerator component development for fusion material test device

1573

JoumalofNuclearMaterials103&104(1981)1573-1576 North-Ho!JandPublishingCompany

ACCELERATOR

COMPONENT

DEVELOPMENT

FOR FUSION MATERIAL

A. Miyahara, S. Kawasaki*, Y. Kubota, N. Kobayashi** and Y. Ukai***

TEST DEVICE

Z. Kabeya, T. Kunibe,

Institute of Plasma Physics, Nagoya University, 464 Nagoya * Department of Physics, Faculty of Science, Kanazawa University, ** Toshiba Corporation, Mita Office, 108 Minatoku, Tokyo *** Toshiba Corporation, Tsurumi Works, 230 Yokohama

920 Kanazawa

Preparation of high current steady state ion source, fabrication of linac tanks by coatThe ing method, and development of high power high frequency power tube are described. first one is the application of bombarded cathode to ion source with rather high operating pressure of 150 Pa, and the second is an application of coating technique primarily Copper coating onto aluminium developed for tokamak divertor plate to linac cavity. substrate is prepared and the measurements of the change of Q-value with coating thickThe development of high power high frequency tube of ness of 10 s 30 urn is performed. 500 MHz frequency range for continuous operation is also described. 1.

INTRODUCTION

It is now widely recognized that the intense neutron source based on a nuclear stripping process of 30 % 40 MeV deuterons injected into a flowing Lithium target offers significant advantages for determining the irradiation behavior of various materials[l] of which the first wall of the future fusion reactor will be composed. The construction of the system will be a great challenge to the present day's engineering and technology, although the major component of the system namely D+ accelerator is an extension of well developed accelerator science. Many studies on the conceptual design of the whole assembly, including a deuteron accelerator able to provide an average particle beam current of O.lA, have been carried out, mainly by the LASL group. The plan, however, involves a lot of uncertain and critical factors such as a new buncher-preaccelerator combination known as RFQ which is supposed to provide the small emittance and low loss injection to the Alvarez linac, the Lithium target system which could safely manage beam power levels reaching several MW and the resulted high radioactivity, and soon. Hard efforts in R and D must be made to meet these requirements, by a much more extensive material study in fusion engineering, as well as in modern particle accelerator technologies. Recent progress in fusion material studies in relevant areas includes the treatment and fabrication of the wall surface with techniques such as low-z first wall coating, discharge cleaning of wall and electrode materials based on the knowledge of plasma-wall interaction mainly for the case of hydrogen isotopes impinging on the surface of the ceramics and metals. The data will be also very useful to promote improvements in the operation of the accelerator which must push the technological limit as is the case for FMIT. Re-examination of the FMIT concept from these points of view could provide insights for

0022-3115/81/0000-0000/$02.75 0198 1 North-Holland

achieving good reliability, optimizing the device feasibility and reducing the estimated cost in the project. This paper describes a brief review of the design studies and the component developments in progress at IPP of Nagoya University, with the primary emphasis on accelerator related topics. 2.

BRIEF DESCRIPTION

OF ACCELERATOR

DESIGN

The chaise of the main accelerator structure is the Alvarez linac. The key problem in selecting the accelerator system involves the prebuncherpreaccelerator and the ion source sections. After careful checks of the various requirements for these components, a tentative reference design was developed with the features given in Fig. 1 and Table 1, and is apparently very similar to those of FMIT in LASL[l], except for the higher operating frequencies for the linacs. There is a basic problem of how to evaluate the role which the RFQ can take in the concept. A preliminary assumption made is that the RFQ can be evaluated as long as it works as a buncher, supplying a well bunched beam with low emittance to the next Alvarez 1 accelerator section to avoid a large particle loss at the transit due to the mismatch of the emittance-acceptance, which otherwise might cause hazardous, intense radiation. At this point we are close to the design policy taken by LASL, but with less attention to the prescribed output particle energy of RFQ. Choice of the operation frequency is still an open question. Higher frequency operation could be expected to have some advantages such as the smaller structures possible, less difficulties in their construction, lower costs, and more flexible choice of high power rf tubes. Also it will match the requirement of smaller energy gain we assume. The actual configuration of the RFQ will be a product of the compromise among many design elements such as the accelerated beam qualities, the power dissipations and the heat transfer

A. Miyahara

1574

et al. I Accelerator

component

dcwlopnwnt

for fusion

material

P(MW)

1

test device

problem, the machining of the complicated electrodes, the availability of the higher power rf source and finally the cost of the fabrication. A three-dimensional computer code of RFQ is now being developed to calculate self-consistently the beam behavior in the accelerator structure. The beam is radially focused and longitudinally bunched by RFQ electric fields. However, there are defocusing and debunching electric fields due to self and image fields because of nonrelativistic particle velocity and non-neutralizations of the space charge effect. These fields have a significant effect on the well bunched beam in the RFQ. The image fields of the beams were considered only at the top of the RFQ vane. The beam cross section is approximately elliptical and it is expected that the major axis and the minor axis will exchange each other every half cycle.

1 4x1$

3x104

Qo

f... operating

7 x104

D... linac diameter

frequency,

L... linac length,

3

Ef... final beam energy

Pw/S... density of rf power loss P... exitation

rf power

Q,... unloaded Q factor As pointed out in the research at LASL[3], however, no computer code can have sufficient precision to predict the beam spill even to the order of 10T3; it should have a relatively limited role in the design study. Consequently, the check of the theoretical prediction with the test stand including a prototype of the RFQ and the first stage of Alvarez structure is crucial for estimating the residual activation. The main acceleration is performed in Alvarez 2. In the design, we try to arrange the accelerator line up with the higher operating frequency as adopted in PIGMI at LASL[3] and our previous From the viewpoint of lowering linac design[4]. cost, the careful choice of operating frequency is necessary and many factors must be considered, including shielding and building designs. At present, the operating frequency of 500 MHz was chosen for Alvarez 2 because of availability of klystron tubes and reasonable thermal load of 7.4 kW/m2 on the cavity wall.

Accelerated

0.1 A

Normalized Emittance

0. lxcm-rad

RFQ

HARDWARE

DEVELOPMENTS

Ion Source

A source of 50 keV O.lA with continuous operaCharacteristion was constructed and tested. tic features of this ion source are high operating pressure of 150Pa and wide area cold cathode type without filament assembly to obtain long life time. As the candidate cathode materials, LaB, cathode for high power NBI[5], Molybdenum At first a Tantaand Tantalum were considered. lum cathode was adopted because of the availThe key parameter of this ability of material. type of ion source is the thickness of the cathode; if it is too thick, start up of ion source is difficult or conversely, if it is too

1 cm

Radius

Alvarez 1

3.1

ACCELERATOR

35 MeV

Average Beam Current

Beam

3.

Design parameters of each accelerator sections

D’

Particle

FinaNAvaiiable) Energy

Operating Frequency

Table 1

125-250-500

MHz

Alvarez 2

I 0

10m

Fig. 1. Line up of accelerator for fusion material irradiation test

Fig. 2.

Cross sectional

view of ion source

A. Miyahara et al. /Accelerator

component

thin, cathode life will be short and ultimately be determined by erosion due to sputtering. In this case, because incident deuteron energy is around 50 eV, sputtering yields are negligible under typical operating conditions. Applications of electron bombarding or direct heating in addition to the ion bombarding heating of the cathode at the initial phase of operation were considered in order to achieve easier start up of the ion source and output beam current.

IOnsource F&m~t

diffractometry analyses have been done to measure the internal stress field between the coated surThe measured lattice conface and substrate. stants for each direction and corresponding internal stresses are as described in Table 2, where do is the lattice constant of copper powder, d is the lattice constant of coated copper, F[=E/w X (d- d,)/d,] is internal stress, E is Young's modulus of 1.45~10'~dyn/cm~, and v is Poisson's ratio of 0.324, respectively.

333n

-3kV

d(i)

(2, 2, 0)

1.2795

1.2787

2.5

(3, 1, 1)

1.0909

1.0903

2.5

(2, 2. 2)

1.0443

1.0439

1.7

(4.0, 0)

0.9044

0.9039

2.5

(3, 3. 1)

0.8295

0.8294

0.54

10 PF

l$=Jf’ 10.25kV 46kV

:

2n

==

Table 2. The measured Power supply for pulse conditioning

Usually before we impose full operating voltages on the extracting electrode, we must first use a lower voltage to avoid arcing and then slowly It takes increase the electrode potential. several days to arrive at final operating voltIn contrast a newly developed method of ages. conditioning is to apply a full voltage from the beginning of this process and change the pulse length from microsecond to several seconds. By means of this procedure, we can carry out the conditioning process successfully. A cross-sectional view of this ion source and the power supply for pulse conditioning in the several microsecond regions are shown in Fig. 2 and 3 respectively. 3.2

Development

F( 10 ‘dynlcm’)

d,(i)

(h, k. 1)

-___==z=

DW?S

Fig. 3.

1575

-_-

I

PS.

T-

development for fusion material test device

of Cavity Wall Materials

Selection of the cavity wall materials is a key problem for the linac engineering because the wall must withstand a high thermal load and have high electric surface conductivity. Moreover, the wall activation due to irradiation must be small for easy repair and maintenance. From these reasons, among the conventional wall materials, we chose an aluminum alloy with a surface coating of copper. From the point of view of energy conservation, a part of the input energy will go to the particles (beams) but another part of it is imparted to and heats up the walls. The dissipated energy on the walls results in large thermal loading as illustrated in Table 1. X-ray

lattice constants

and

corresponding

internal stresses

The results show existence internal stresses.

of very small positive

The particle loading is usually a negligible effect for conventional low duty linacs, but for a continuously operated machine, it cannot be neglected. The most important part of particle loading is activation of the wall materials. If a portion of the accelerated particles are spilled out from orbit, interactions with wall materials will occur. From this standpoint, aluminum as a bulk material is preferable because its induced activity has a rather short life. Also the thickness of copper coating must remain as thin as possible. Surface coating was done by the PVD method and the thickness of copper was 20 % 40pm, which is thick enough compared to the skin depth of copper at 5OOMHz. Skin electrical conductivity measurements were done by means of Q-value estimation. High Q-values above lo4 have been obtained with sufficiently thick coatings as shown in Fig. 4[4]. The diameter and the length of the copper coated test cavity is 295 mm and 200 mm respectively, and is excited in TM010 mode with the frequency of about 780 MHz. Also we must mention that a thin coating does not violate the mechanical tolerances of the accelerator and machining of substrate can keep the mechanical accuracy of the structures. It is very preferable to reduce the fabrication cost. Of course it we adopt copper coated Al for the wall material, vacuum component such as

A. Mi.vahara et al. / Accelerator componerzt development for fusion muterial test device

1576

flanges, gaskets, valves, etc. which were developed at KEK Japan for the TRISTAN project, must be adapted[4]. 3.3

Klystrons

and High Frequency

Components

As the high frequency power sources for Alvarez 2 accelerator, we will adopt 5OOMHz 1OOOkW CW klystron now being developed for the lower hybrid heating of tokamaks, rf acceleration of The storage rings and other conventional uses. character of this tube is as shown in Table 3. Because of low efficiency, replacement of this tube by high efficiency solid state elements is This improvement is essential bedesirable. cause the cost of electricity will exceed 5M$ per year for linac operation alone. E 3706 E 3774 (mder preparatbn) (avaWable)

operathe Fmwew Output Power

f (MHZ) Psat(M

500

500

1.16

0.19

Beam Voltage

Eb (kV)

90

46

Beam Current

I (A)

20

6.9 1

Efficiency

11(%)

Perveance Anode Voltage

61

0.74

0.7

Ea(kV)

65

33

1

0.3

250

300

1b(A)

Body Current Axial Magnetic Fields

Table 3.

64.4

P&P)

B(gauss)

The authors express their sincere thanks to Prof. K. Kamada for his valuable help. REFERENCES [II

I

I

20 +

40

T(pm)

Fig. 4. Effect of coating thickness on Q-value CONCLUSION

In this paper, we describe several developmental efforts of accelerator components, but still

Holmes, J.J. and Straalsund, J.L., Irradiation Sources for Fusion Materials Development, J. Nucl. Mater., 85&86 (1979) 447-451.

121 Jameson, R.A., High-Intensity Deuteron Linear Accelerator (FMIT), IEEE Trans. on Nucl. Sci., NS-26, 2 (1979) 2986-2991. [31

Boyd, T.J. et al., The PIGMI Technology, Los Alamos Scientific Laboratory Report, LA-UR80-3561.

[41

Kobayashi, N., Miyahara, A. and Kawasaki, S., Higher Frequency Operation of Linear Accelerator, IEEE Trans. on Nucl. Sci., NS-28, 3 (1981) 3476-3478.

[51

Uromoto, J., An Ion Source Plasma Starting from a Cold LaBs Cathode I, Research Report of Institute of Plasma Physics, Nagoya Univ., Nagoya,Japan, IPPJ-406 (1979). Kubota, Y., C'ramoto, J. and Miyahara, A., Long Pulse Ion Source Using Filamentless LaBs Multicathodes, Research Report of Institute of Plasma Physics, Nagoya Univ., Nagoya, Japan, IPPJ-513 (1981).

[61

Ishimaru, H., Horikoshi, G., Minoda, K. and Irisawa, T., Ultrahigh Vacuum Systems with Aluminum, Journal of the Vacuum Society of Japan, 22 (1979) 373-388.

Specifications of 5OOMHr Klystron

0

4.

there are a lot of problems to be solved that are inherent in continuous wave operation with high reliability. As the guideline for this design, we have chosen higher frequency operation of linac over a more conventional value because we feel this direction introduces the advantage of reduction of radial dimensions. The penalty is of course a high thermal load on the wall, but we hope this difficulty will be overcome during a future collaboration with the fusion engineering group.