Particle Accelerators, 1971, Vol. 2, pp. 235-241 © GORDON AND BREACH, SLIE~CE PUBLISHERS LTD. Printed in Glasgow, Scotland

ELECTROPLATING OF PROTON LINEAR ACCELERATOR TANKSJ. 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 andthick copper layer was obtained by using a copper bath with an organic brightening agent and improving the platingtechniques. The mechanical, electrical and vacuum properties of accelerator tanks are much improved by thenewly developed method. Measured Q values as large as 90 per cent of the calculated values are obtained.

50 g/liter0.05 A/cm2

50°C3 min.

(1) Degreasing by organic solvent.

(2) Water rinsing.

(3) Electrolytic degreasing.Solution: NaOHCurrent density:Temperature:Time:

(4) Water rinsing.

(5) Sulfuric acid bath.Solution: H2S04 (10 per cent)Temperature: 20°CTime: 30 sec

3. ACID COPPER BATH WITHBRIGHTENING AGENT

The results of further work showed that an acidcopper bath containing some organic additionagents deposits uniform and smooth copperlayers.

The process of the plating and the compositionof the solutions are as follows:

Accordingly, the base metal surfaces to be platedhad to be as smooth as possible, and the holes forthe stems, the rf-coupling holes and the vacuumports had to be machined after copper-plating.Also, dummy cylinders which were used in orderto deposit copper uniformly over the end regionsof the tank, were required. Moreover, to producethe uniform, smooth and thick copper deposits, itwas necessary to rotate the accelerator tank aroundits axis and to rub its inner surface by ceramicedges during the electrodeposition process.

2. COPPER-PLATING IN AN ORDINARYACID COPPER BATH

The copper layers of test cavItIes which wereplated in an ordinary acid copper bath with arubbing method, were proved to be electricallygood, vacuum tight and of low outgassing.(l) How­ever, there remained some difficulties. They werejncrea~e of the surface roughness, growth ofirregular deposits, excessive deposition on corneredges and lack of deposits in deep grooves.

1. INTRODUCTION

As already reported, (l ,2) we have planned tofabricate the accelerator tanks of the injector linacfor the Japanese Proton Synchrotron Project bymeans of an electroplating method.

The tanks are made by copper-plating the innersurface of steel cylinders. Thus, difficulties such ashard soldering and machining of the inner copperlayers of large-size copper-clad cylinders can beeliminated. At the beginning of this study, anordinary acid copper bath with a rubbing methodwas used for the process of copper deposition, butlater, a new copper bath was developed. In this,an addition agent was used. It was found that theuse of the addition agent (U.B.A.C.R-I) realizedfine crystalline and smooth surface copper depositswithout rubbing.

In this plating process, stem holes, rf and vacuumports, end flanges and the other parts are weldedand machined before plating. After copper-plating,any final machining or finishing is not necessary.Consequently, the new process is superior to theprevious one with respect to the fabrication tech­nique and the machining costs.

236 J. TANAKA, T. NISHIKAWA AND H. BABA

t Made by Ebara-Udylite Co. U.B.A.C.R-l should beadded every 24 h during the plating.

(14) Water rinsing.

(15) Hot water rinsing.

Generally, the plating of a cylindrical surface isnot difficult. In fact, the copper-plating of theinner surface of a steel cylinder of 1 m in diameterand 1 m in length was sufficiently good to a thick­ness of 0.1 mm. However, the deposition rate ofthis plating is influenced by air agitation. For alonger cylinder (~2 m), it becomes difficult toobtain uniform air agitation along the verticalsurface. So, if the cylinder is placed vertically inthe bath, the thickness of deposits at the upper andlower side of the cylinder is slightly different.

Consequently, it may be necessary to reverse thecylinder during the deposition process, and toaverage the difference of thickness. For thispurpose, the cylinder must be exposed to air forsome time. The larger the size of the cylinder, thelonger the duration of the reversing process isrequired. In order to investigate the effect ofexposure to air on the adhesion of copper, steel testplates 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 pieceswas examined by a 90° bending method, but noflaking and no chipping of the deposited copperwere found. In addition, these plated pieces wereheated to 450°C, but no blister was observed.These results show that the adhesion of the depositsby this process is satisfactory.

4. COPPER DEPOSITION ON THE INNERSURFACE OF A STEEL CYLINDER

5. INFLUENCE OF HOLES

In plating, the shape of the base metal to beplated is very important. Since there are a numberof holes on the accelerator tank cylinder, it isrequired to examine the influence of holes on thecopper deposits.

As regards the ordinary acid copper bath, exces­sive copper is apt to be deposited on the edge ofholes as shown in Fig. l(a). However, the defect isimproved when the bath with U.B.A.C.R-I is used.The effect of corner radius of the hole on theexcessive deposits was investigated. On the base­plate to be plated, several holes were drilled whoseradii were 20 mm in diameter and the edges were

50 gjliter5 gjliter

53 gjliter103 gjliter

0.02 A/cm2

50°C15 min.

(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.

ICuS04 5H 20: 225 gjliter

SI' jH2S04 : 75gjliter

o utIon I U B A C R 1 5 /1' ..!., .. . . - cc. Iter f

lCI ion: 20~ 50 mg/literCurrent density: 0.05 A/cm 2

Temperature: 30°CStrong air agitation.

(12) Water rinsing.

(13) Chromate bath.

S I . {cr03o utlon H

2S0

4

Use of this addition agent increases the depositionrate of copper and improves smoothness and uni­formity of the deposits. The copper deposits on asmooth base metal have a surface roughness of lessthan 0.3 J1 and good uniformity with a variation ofless than 10 per cent in thickness. The other testsshowed that the roughness of deposits is in­dependent of the roughness of the base metalsurfaces. The excessive deposition on the edgesand the lack of the deposits in the deep grooves arealso improved.

In this process, the optimum current density isabout 0.05 Ajcm2

• The rotation of the basecylinder during the plating may be unnecessary~ butto produce smooth surface of deposits, strongand uniform air agitation along the surface isrequired.

ELECTROPLATING LINAC TANKS 237

ANODE SIDE

EXCESSIVE DEPOSITS

relatively low, because of the low current densityat these places. However, the effect is improvedby suitably shaped shields and appropriate airagitation.

ANODE SIDE

BASE METAL

(0 )

The electrical characteristics of the plated copperwere tested by measuring 'Q' value of a test cavity.For this purpose, a small-size test cavity wasemployed. A cylindrical cavity with the dimensionsof II cm in diameter and 7.78 cm in length wastested at the resonant frequency of the TMolomode (2.09 GHz). It consists of a cylinder and twoend plates (see Fig. 3). They were made of steel,and after machining, copper-plated to the thicknessof 0.5 mm. The cylinder and the end plates werejoined by means of 0.2 mm thick copper contactorsand rubber 'Q' rings. (1) The surface roughness ofthe deposited copper was less than 0.3 Ii. Since the

7. -Q' VALUE OF COPPER-PLATED TESTCAVITYBASE METAL

EXCESSIVE DEPOSITS

~r• LACK OF DEPOSITS

(b)

FIG. 1. (a) Excessive deposition on the edge ofhole; (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, nonoticeable difference was found. Accordingly,edge rounding of stem holes may be sufficient withI mm radius. However, if the base plate was placedvertically in the bath, then the depositions on theupper side and the lower side of the hole weredifferent, because the existence ofthe hole influencesthe stream of air bubbles along the vertical surfaces(see Fig. 2). This nonuniformity of copper can beaveraged by reversing the base plate.

o0) ANODE SIDE

/0DEPOS I TE 0 //

CO PPER

UPPERSIDE

6. PLATING OF FLANGES WITHCONTACTOR GROOVES

As the thickness of deposited layer increases(~ 0.5 mm), the deposits on the edges of bothcylinder sides increase excessively. However, theexcessive deposits can be prevented by means ofnonconducting shields.

For the contactor grooves on the flanges, thegrowth of the deposits is shown in Fig. 1(b). At theinner corners of a groove, the deposition rate is

b) ANODE SI DE

STREAMOF BUBBLES

LOWERSI DE

FIG. 2. Influence of hole. (a) Horizontal crosssection; (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 is1.46 fl, the ~Q' value of the cavity may not beaffected by the surface roughness. So, themeasured ~Q' is considered to be influenced pre­dominantly by the deposited copper and thecontactors.

The measured ~Q' value was 2.0(±0.I)x 104~ itcorresponds to 90 per cent of the theoretical value(2.2 x 104 calculated by using dc conductivity ofcopper). The result sho\\"5 that the plated copper isexcellent in electrical properties.

8. I-m-LONG MODEL ACCELERATORTANK

A prototype accelerator model tank with 14 drifttubes, an rf port, a vacuum port, tuners and coolingchannels was fabricated for the study of tankfabrication 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 theactual tank. The drift tube body consists of twostainless-steel shells. The outer surfaces of theshells were copper-plated to a thickness of 1 mm. tAfter a Q-magnet was installed, the body of thedrift tube was seamed by an electron beam welder.If the copper exists on the joint surfaces to bewelded, it causes cracks in the welded joints. Toprevent this difficulty, the copper on the. j~intregions 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 of0.02 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) PretreatIl1entsSolution: H 2 S04 (20 -- 30 per cent)Tetnperature: roon1 ten1peratureCurrent 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

SolutIonl HCl : 80 -- 85 g/literTel11perature: r00l11 tel11peratureCurrent density: 0.02-0.03 A/cln 2

Til11e: 3 -- 4 n1in(4) Cu strike

CLlCN 60 -- 70 g,'literNaCN 70 -- 80 g/literNaOH 10--20g/liter

Solution K' CN . S 12 -- 17 g/literexcessive NaOH: 10-- 15 g/literpH 12.2--12.6

TeIl1perature: 60 --70 DCCurrent 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

FIG.6. 2.S-nl-Iong Inodel tank.

239

240 J. TANAKA, T. NISHIKAWA AND H. BABA

FIG. 7. PI:lting plant.

dimensions were within 0.02 mm. The tank wasmade of mild steel. End flanges, rf and vacuumport flanges, drift tube support rails and coolingchannels were welded to the cylinder, and aftermachining, the tank was copper-plated to a thick­ness of 0.1 mm.t

The dimensions and mechanical tolerances of thetank were as follows:

Diameter 940.00 mm ±0.1 mmLength I,036.00 mm ±0.1 mmPosition of the stem hole ±0.1 mm.

Measured unloaded "Q' of the model tank was3.9 x 104

• The value corresponds to 84 per cent ofthe calculated value (4.6 x 104

). In this calculation,losses due to the coupling loop, tuners and a

t Although the thickness of 0.1 n1m is sufficient for theskin depth (4.65 fL at 200 MHz), the thickness of 0.5 ~ 1 mmwill be favorable for the actual accelerator tank to improvethe mechanical toughness and thermal conductivity.

number of contactors are not included and dcconductivity of the copper is used. Consequently,it may be concluded that excellent results have beenobtained from the copper-plating method.

9. 2.5-m-LONG ACTUAL SIZE MODELTANK

In order to perform the thick copper-plating(I mm in thickness) in actual size, a 94 em diameterand 2.5-m-Iong model tank unit was fabricated(Fig. 6). It has vertical and horizontal stem holesfor 28 drift tubes, 3 ports for tuners, and rf couplingholes, a vacuum port and both end flanges withcontactor grooves.

The cylinder and the flanges were made of steelboiler plate. After the welding and machining, thetank 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 ofcopper was controlled by an integrating electricpower meter. The excessive deposition at thecorners of both end flanges were suppressed byP.V.C. shields. The thickness of copper layer onthe inner surface was 1.0 ±0.1 mm, and the surfaceroughness was less than 0.3ft. Furthermore, in thisplating method, it was shown that correction ofmachining errors in the inner diameter of the tankis possible to a fraction of the copper thickness byadjusting the deposition of copper.

ACKNOWLEDGEMENT

The authors express their thanks to Mr. S. Ikeda,

T. Mori, Y. Fujita, A. Ino and J. Kisaki ofMitsubishi Heavy Industries and Mr. H. Nomuraof 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 TankDesign and on Tank Fabrication', Proc. 1968 ProtonLinear Accelerator Conference, Brookhaven NationalLaboratory, May 20-24, 1968 (BNL 50120), Part 2,p.543.

Received 13 January 1971