ELECTROPLATING OF PROTON LINEAR ACCELERATOR TANKS

Particle Accelerators, 1971, Vol. 2, pp. 235-241 GORDON AND BREACH, SLIE~CE PUBLISHERS LTD. Printed in Glasgow, Scotland ELECTROPLATING OF PROTON LINEAR ACCELERATOR TANKS J. TANAKA, T. NISHIKAWA AND H. BABA Institute.for Nuclear Study, University of Tokyo, Tanashi-shi, Tokyo, Japan An electroplating method was developed for fabricating proton linear accelerator tanks. A uniform, smooth and thick copper layer was obtained by using a copper bath with an organic brightening agent and improving the plating techniques. The mechanical, electrical and vacuum properties of accelerator tanks are much improved by the newly developed method. Measured Q values as large as 90 per cent of the calculated values are obtained. 1. INTRODUCTION As already reported, (l,2) we have planned to fabricate the accelerator tanks of the injector linac for the Japanese Proton Synchrotron Project by means of an electroplating method. The tanks are made by copper-plating the inner surface of steel cylinders. Thus, difficulties such as hard soldering and machining of the inner copper layers of large-size copper-clad cylinders can be eliminated. At the beginning of this study, an ordinary acid copper bath with a rubbing method was used for the process of copper deposition, but later, a new copper bath was developed. In this, an addition agent was used. It was found that the use of the addition agent (U.B.A.C.R-I) realized fine crystalline and smooth surface copper deposits without rubbing. In this plating process, stem holes, rf and vacuum ports, end flanges and the other parts are welded and machined before plating. After copper-plating, any final machining or finishing is not necessary. Consequently, the new process is superior to the previous one with respect to the fabrication technique and the machining costs. 2. COPPER-PLATING IN AN ORDINARY ACID COPPER BATH The copper layers of test cavities which were plated in an ordinary acid copper bath with a rubbing method, were proved to be electrically good, vacuum tight and of low outgassing.(l) However, there remained some difficulties. They were jncrea~e of the surface roughness, growth of irregular deposits, excessive deposition on corner edges and lack of deposits in deep grooves. Accordingly, the base metal surfaces to be plated had to be as smooth as possible, and the holes for the stems, the rf-coupling holes and the vacuum ports had to be machined after copper-plating. Also, dummy cylinders which were used in order to deposit copper uniformly over the end regions of the tank, were required. Moreover, to produce the uniform, smooth and thick copper deposits, it was necessary to rotate the accelerator tank around its axis and to rub its inner surface by ceramic edges during the electrodeposition process. 3. ACID COPPER BATH WITH BRIGHTENING AGENT The results of further work showed that an acid copper bath containing some organic addition agents deposits uniform and smooth copper layers. The process of the plating and the composition of the solutions are as follows: (1) Degreasing by organic solvent. (2) Water rinsing. (3) Electrolytic degreasing. Solution: NaOH Current density: Temperature: Time: 50 g/liter 0.05 A/cm 2 50 C 3 min. (4) Water rinsing. (5) Sulfuric acid bath. Solution: H 2 S04 (10 per cent) Temperature: 20 C Time: 30 sec

236 J. TANAKA, T. NISHIKAWA AND H. BABA (6) Water rinsing. (7) Copper strike. S I. {CUCN: o ution KCN: Current density: Temperature: Time: (8) Water rinsing. (9) Sulfuric acid bath. (10) Water rinsing. (11) Copper plating. (14) Water rinsing. (15) Hot water rinsing. 53 gjliter 103 gjliter 0.02 A/cm 2 50 C 15 min. ICuS0 4 5H 2 0: 225 gjliter ' jh SI 2 S0 4 : 75gjliter o ution I U B A C R 1 5 /1'..!.,.... - cc. Iter f l CI ion: 20 ~ 50 mg/liter Current density: 0.05 A/cm 2 Temperature: 30 C Strong air agitation. (12) Water rinsing. (13) Chromate bath. S I. o utlon {cr03 H 2 S0 4 50 gjliter 5 gjliter Use ofthis addition agent increases the deposition rate of copper and improves smoothness and uniformity of the deposits. The copper deposits on a smooth base metal have a surface roughness of less than 0.3 J1 and good uniformity with a variation of less than 10 per cent in thickness. The other tests showed that the roughness of deposits is independent of the roughness of the base metal surfaces. The excessive deposition on the edges and the lack of the deposits in the deep grooves are also improved. In this process, the optimum current density is about 0.05 Ajcm 2 The rotation of the base cylinder during the plating may be unnecessary~ but to produce smooth surface of deposits, strong and uniform air agitation along the surface is required. t Made by Ebara-Udylite Co. U.B.A.C.R-l should be added every 24 h during the plating. 4. COPPER DEPOSITION ON THE INNER SURFACE OF A STEEL CYLINDER Generally, the plating of a cylindrical surface is not difficult. In fact, the copper-plating of the inner surface of a steel cylinder of 1 m in diameter and 1 m in length was sufficiently good to a thickness of 0.1 mm. However, the deposition rate of this plating is influenced by air agitation. For a longer cylinder (~2 m), it becomes difficult to obtain uniform air agitation along the vertical surface. So, if the cylinder is placed vertically in the bath, the thickness of deposits at the upper and lower side of the cylinder is slightly different. Consequently, it may be necessary to reverse the cylinder during the deposition process, and to average the difference of thickness. For this purpose, the cylinder must be exposed to air for some time. The larger the size of the cylinder, the longer the duration of the reversing process is required. In order to investigate the effect of exposure to air on the adhesion of copper, steel test plates were plated by the following process. Copper-plating ~ exposure to air for 3 h ~ degreasing ~ water rinsing ~ dipping in NaCN solution ~ water rinsing ~ copper plating ~. The adhesion of copper plated on the test pieces was examined by a 90 bending method, but no flaking and no chipping of the deposited copper were found. In addition, these plated pieces were heated to 450 C, but no blister was observed. These results show that the adhesion of the deposits by this process is satisfactory. 5. INFLUENCE OF HOLES In plating, the shape of the base metal to be plated is very important. Since there are a number of holes on the accelerator tank cylinder, it is required to examine the influence of holes on the copper deposits. As regards the ordinary acid copper bath, excessive copper is apt to be deposited on the edge of holes as shown in Fig. l(a). However, the defect is improved when the bath with U.B.A.C.R-I is used. The effect of corner radius of the hole on the excessive deposits was investigated. On the baseplate to be plated, several holes were drilled whose radii were 20 mm in diameter and the edges were

ELECTROPLATING LINAC TANKS 237 ANODE SIDE EXCESSIVE DEPOSITS relatively low, because of the low current density at these places. However, the effect is improved by suitably shaped shields and appropriate air agitation. ANODE EXCESSIVE BASE (0 ) SIDE DEPOSITS ~r METAL BASE METAL LACK OF DEPOSITS (b) FIG. 1. (a) Excessive deposition on the edge of hole; (b) Lack of deposits in a groove. rounded with the radii of I, 2, 3 and 5 mm respectively. As to the deposits on these holes, no noticeable difference was found. Accordingly, edge rounding of stem holes may be sufficient with I mm radius. However, ifthe base plate was placed vertically in the bath, then the depositions on the upper side and the lower side of the hole were different, because the existence ofthe hole influences the stream of air bubbles along the vertical surfaces (see Fig. 2). This nonuniformity of copper can be averaged by reversing the base plate. 6. PLATING OF FLANGES WITH CONTACTOR GROOVES As the thickness of deposited layer increases (~ 0.5 mm), the deposits on the edges of both cylinder sides increase excessively. However, the excessive deposits can be prevented by means of nonconducting shields. For the contactor grooves on the flanges, the growth ofthe deposits is shown in Fig. 1(b). At the inner corners of a groove, the deposition rate is 7. -Q' VALUE OF COPPER-PLATED TEST CAVITY The electrical characteristics of the plated copper were tested by measuring 'Q' value of a test cavity. For this purpose, a small-size test cavity was employed. A cylindrical cavity with the dimensions of II cm in diameter and 7.78 cm in length was tested at the resonant frequency of the TM olo mode (2.09 GHz). It consists of a cylinder and two end plates (see Fig. 3). They were made of steel, and after machining, copper-plated to the thickness of 0.5 mm. The cylinder and the end plates were joined by means of 0.2 mm thick copper contactors and rubber 'Q' rings. (1) The surface roughness of the deposited copper was less than 0.3 Ii. Since the o 0) ANODE SIDE DEPOS I TE 0 ///0 CO PPER b) ANODE SI DE STREAM OF BUBBLES UPPER SIDE LOWER SI DE FIG. 2. Influence of hole. (a) Horizontal cross section; (b) Vertical cross section.

238 J. TANAKA, T. NISHIKAWA AND H. BABA FIG. 3. 2.09 GHz test cavity. calculated skin depth of copper at this frequency is 1.46 fl, the ~Q' value of the cavity may not be affected by the surface roughness. So, the measured ~Q' is considered to be influenced predominantly by the deposited copper and the contactors. The measured ~Q' value was 2.0(±0.I)x 104~ it corresponds to 90 per cent of the theoretical value (2.2 x 10 4 calculated by using dc conductivity of copper). The result sho\\"5 that the plated copper is excellent in electrical properties. 8. I-m-LONG MODEL ACCELERATOR TANK A prototype accelerator model tank with 14 drift tubes, an rf port, a vacuum port, tuners and cooling channels was fabricated for the study of tank fabrication techniques (Fig. 4). The ITIodel tank FIG. 4. I-Ill-long model tank. FIG. 5. Drift tubes. has the same dimensions as the injection end of the actual tank. The drift tube body consists of two stainless-steel shells. The outer surfaces of the shells were copper-plated to a thickness of 1 mm.t After a Q-magnet was installed, the body of the drift tube was seamed by an electron beam welder. If the copper exists on the joint surfaces to be welded, it causes cracks in the welded joints. To prevent this difficulty, the copper on the. j~int regions of the shell was stripped by machining. After welding the stainless-steel shell and the stem, the drift tube was copper-flashed to a thickness of 0.02 mm (Fig. 5). The tolerances of the drift tube t Copper-plating process of stainless steel. ( 1) Degreasing (a) organic solvent (b) weak alkaline solution (2) PretreatIl1ents Solution: H 2 S0 4 (20 -- 30 per cent) Tetnperature: roon1 ten1perature Current density: 0.005-0.015 A/cn1 2 (a) Cathodic: 2 -- 3 111il1 (b) Anodic: 30 sec (3) Ni strike,. rniclz6h 2 0: 230 -- 250 g'liter SolutIon l HCl : 80 -- 85 g/liter Tel11perature: r00l11 tel11perature Current density: 0.02-0.03 A/cln 2 Til11e: 3 -- 4 n1in (4) Cu strike CLlCN 60 -- 70 g,'liter NaCN 70 -- 80 g/liter NaOH 10--20g/liter Solution K' CN. S 12 -- 17 g/liter excessive NaOH: 10-- 15 g/liter ph 12.2--12.6 TeIl1perature: 60 --70 DC Current density: 0.01 A/cn1 2 Tin1e: 1 h (thickness -- 20 fl) (5) Copper-plating with U.S.A.C.R -1

ELECTROPLATING LINAC TANKS 239 FIG.6. 2.S-nl-Iong Inodel tank.

240 J. TANAKA, T. NISHIKAWA AND H. BABA FIG. 7. PI:lting plant. dimensions were within 0.02 mm. The tank was made of mild steel. End flanges, rf and vacuum port flanges, drift tube support rails and cooling channels were welded to the cylinder, and after machining, the tank was copper-plated to a thickness of 0.1 mm.t The dimensions and mechanical tolerances of the tank were as follows: Diameter Length Position of the stem hole 940.00 mm ± 0.1 mm I,036.00 mm± 0.1 mm ± 0.1 mm. Measured unloaded "Q' of the model tank was 3.9 x 10 4 The value corresponds to 84 per cent of the calculated value (4.6 x 10 4 ). In this calculation, losses due to the coupling loop, tuners and a t Although the thickness of 0.1 n1m is sufficient for the skin depth (4.65 fl at 200 MHz), the thickness of 0.5 ~ 1 mm will be favorable for the actual accelerator tank to improve the mechanical toughness and thermal conductivity. number of contactors are not included and dc conductivity of the copper is used. Consequently, it may be concluded that excellent results have been obtained from the copper-plating method. 9. 2.5-m-LONG ACTUAL SIZE MODEL TANK In order to perform the thick copper-plating (I mm in thickness) in actual size, a 94 em diameter and 2.5-m-Iong model tank unit was fabricated (Fig. 6). It has vertical and horizontal stem holes for 28 drift tubes, 3 ports for tuners, and rfcoupling holes, a vacuum port and both end flanges with contactor grooves. The cylinder and the flanges were made of steel boiler plate. After the welding and machining, the tank was placed vertically in plating bath (Fig. 7) and to obtain more uniform deposits, it was rotated

ELECTROPLATING LINAC TANKS 241 around its axis during the plating. The thickness of copper was controlled by an integrating electric power meter. The excessive deposition at the corners of both end flanges were suppressed by P.V.C. shields. The thickness of copper layer on the inner surface was 1.0 ± 0.1 mm, and the surface roughness was less than 0.3ft. Furthermore, in this plating method, it was shown that correction of machining errors in the inner diameter of the tank is possible to a fraction of the copper thickness by adjusting the deposition of copper. ACKNOWLEDGEMENT The authors express their thanks to Mr. S. Ikeda, T. Mori, Y. Fujita, A. Ino and J. Kisaki of Mitsubishi Heavy Industries and Mr. H. Nomura of Nomura Plating Co. for their collaboration. REFERENCES ]. Progress Report of Injector Working Group 1 (1966), University of Tokyo, Institute for Nuclear Study, Report SJC-A-67-2 (November 1967). 2. T. Nishikawa et at., 'Model Cavity Studies for Tank Design and on Tank Fabrication', Proc. 1968 Proton Linear Accelerator Conference, Brookhaven National Laboratory, May 20-24, 1968 (BNL 50120), Part 2, p.543. Received 13 January 1971