Thin Solid Films, 118 (1984) 15-21

METALLURGICAL AND PROTECTIVE COATINGS 15

SURFACE PREPARATION OF THE S-1 SPHEROMAK FLUX CORE LINER*

R. MOORE¢ AND C. MACEY~

Physical Electronics Division, Perkin-Elmer Corporation, 5 Progress Street, Edison, NJ 08820 (U.S.A.)

S. COHEN AND A. JANOS

Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08540 (U.S.A.)

(Received March 23, 1984; accepted April 9, 1984)

Different methods of preparing the S-1 Spheromak flux core liner for exposure to plasma (i.e. cleaning and polishing techniques) were studied with the goal of reducing the net impurity resources available for contaminating the plasma during Spheromak formation. The S-1 Inconel 601 liner is described together with an analysis of topography and surface and/or near-surface composition of various Inconel samples subjected to mechanical, electrochemical and chemical preparation techniques. The general conclusion based on the analysis of the samples is that the different techniques are roughly competitive on the basis of surface composition, while less preparation tends to give better results in terms of the criteria mentioned above. This has helped to simplify the liner preparation.

1. INTRODUCTION

A major concern in designing controlled thermonuclear fusion reactor prototypes is the composition of the first-wall components, i.e. the composition of the components that come into actual contact with the ionized gas (plasma). The process of controlling impurities in plasmas becomes better defined once the origin of the impurities and their transport into the plasma is understood. One prototype configuration currently being studied is the Spheromak.

Spheromak plasmas have now been successfully generated by three formation schemes. However, plasma temperatures (less than about 50 eV) and lifetimes (less than 1 ms) are presently 1 limited by impurity radiation, the dominant plasma energy loss mechanism. It is essential to identify and eliminate major impurity sources. Significant improvements in plasma parameters have been made in recent years by the proper choice and conditioning of surfaces exposed to the plasma 2.

Substantial amounts of impurities of low atomic number Z and lesser amounts

* Paper presented at the International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 9-13, 1984. 5" Present address: David Sarnoff Researcl:i Center, RCA, Princeton, NJ 08540, U.S.A. :~ Present address: Department of Ceramics, College of Engineering, Rutgers University, P.O. Box 909, Piscataway, NJ 08854, U.S.A.

0040-6090/84/$3.00 ~) Elsevier Sequoia/Printed in The Netherlands

16 R. MOORE el a[.

of high Z impurities are created in all formation schemes. The coaxial plasma gun (Los Alamos Scientific Laboratory) 3 and the field-reversed O-pinch with center- column Z-discharge (University of Maryland)'* techniques both utilize current- carrying electrodes. Spheromak plasmas have recently been formed using the slow scheme proposed and developed at Princeton Plasma Physics Laboratory 5. This technique utilizes a toroidal "flux core" containing poloidal and toroidal field coils.

Magnetic flux is inductively transferred from the core to a plasma forming around the core. This plasma is "pinched off" of the core to produce a Spheromak configuration. This technique is in principle electrodeless, but the plasma is nevertheless in intimate contact with the core early in the discharge. Thus, the core may serve as the major resource for impurities, while the vacuum vessel walls are relatively far removed from the plasma.

The Spheromak S-I flux core liner is constructed with lnconel 601 for reasons given in the following section. The present study was conducted to investigate the effect(s) of various chemical and mechanical pretreatment steps on the surface impurity concentrations of Inconel 601.

2. FLUX CORE I,INER

The flux core is covered with a metal liner which serves as the high vacuum surface. It is (10 15)× 10 3in thick so as to be sufficiently resistive to prevent magnetic field penetration. The liner material is lnconel 601 (nominal composition: 60.5"~,i Ni, 23!~,, Cr, 14.0,~.~i Fe, 1.35,~i AI, 0.5!',0 Mn, 0.2'~, Si, 0.20~, Cu) and was chosen for its relatively high resistivity (compared with stainless steel) and for its machinability and availability (large blanks of material more than 4 m 2 in area were required). The liner fabrication began with a pair of three 90 × 10 3 in sheets of Inconel welded together. These were spun into half-toruses on a Meehanite mandrel. Spinning required several (approximately six) iterations of annealing and spinning. Spinning grooves on the outer side (the side exposed to the plasma) were then mechanically ground to bring the liner to a more uniform thickness. Successively finer mechanical polishings were performed to remove scratches and sharp small- scale protrusions, to reduce the surface area and to lessen the chance for foreign matter to be trapped on the surface. The inside of the liner halves were then chemically milled to bring the thickness to (10-151 × 10 3 in, except for selected areas left thick for welding purposes. The subsequent liner had a thickness of one to three times the typical grain size. The grain size was increased by a factor of 2 3 as a result of the spinning annealing process, as expected 6.

Further surface preparation was considered, and testing for an opt imum preparation method is described below. Optimization of plasma conditions would include any liner preparation which would (1) minimize the amount of impurities on the flux core liner surface, i.e. surface contaminations from normal handling and exposure and adsorbed gases in the first several atomic layers, (2) decrease the effective surface area by smoothing the topography, (3) stabilize the surface chemistry to retard native oxide growth or (4) remove sharp protrusions which would tend to catch contamination during routine wiping or would promote arcing. Tests were done on samples (coupons) of Inconel 601 cut from the 90 × 10 3 in

SURFACE PREPARATION OF FLUX CORE LINER 17

blanks used to spin the actual liner pieces. They were mechanically polished (roughened) to simulate the preparation of the actual liner's outer surface.

3. TREATMENT OF TEST SAMPLES

The coupons were given the following treatments: sample A, mechanical polish with abrasive grinder, solvent dip, rinse in distilled water; sample B, mechanical polish with abrasive grinder, 3 min wipe with Aluma-brite; sample C, mechanical polish with abrasive grinder, 2 min electropolish in Summa Processing (proprietary electropolishing solution commonly used as a brightener) solution, rinse in distilled water; sample D, mechanical polish with abrasive grinder, 20 min electropolish in Summa Processing solution, rinse in distilled water.

For all cases of electropolishing the current density j was 0.72 A cm- 2. It was observed during electropolishing that 2 min of immersion at this current density was required to change the topography noticeably. For this reason, the 2 and 20 min coupons were used for analysis.

Both the Summa Processing and Aluma-brite solutions were at ambient temperature during polishing. The Summa Processing solution temperature did increase from 23 to 30 °C during the longer immersions.

After the samples were treated, they were cut with a clean jeweler's saw to a size that would allow them to be mounted on a sample stage of the analytical instrumentation.

4. ANALYSIS OF TEST SAMPLES

Evaluation of the treatments was done with two major concerns: topography and surface and/or near-surface composition.

All the treated samples were observed and photographed using an optical microscope (magnification, 200 x). This not only characterized the relative effects of the treatments on the topography of the samples, but indicated any anomalies on the surfaces which would be eventually analyzed.

From the optical micrographs we drew the following conclusions. (1) The mechanically polished surfaces had a microscopic roughness that

might be problematic with respect to gas adsorption and arcing. (2) The electropolishing, while smoothing the surface considerably, preferenti-

ally etched grain boundaries. (3) The wipe-etch with Aluma-brite left a visible pattern in the direction of the

wipe. The samples were then mounted in an analysis chamber for scanning electron

microscopy, Auger electron spectroscopy and ion sputter depth profiling. The instrumentation used was a Perkin-Elmer Physical Electronics model 590 scanning Auger microprobe with a minimum beam size of less than 1 rtm. An ion pump maintained the base pressure in the analysis chamber at less than 1 x 10- 8 Torr. The major residual gas constituents in the analysis chamber were COz and CH4. Sample recontamination during analysis was found to be unimportant in these analyses.

Secondary electron micrographs were taken of all samples at 200 x (Fig. 1). The most interesting sample proved to be the coupon electropolished for 2 rain (sample

]8 R. MOORE •[LI].

C). The surface had a large number of inclusions typically 0.5 mm in diameter. The physical appearance of these structures suggested grains of some interstitial material in the matrix of the Inconel or perhaps Inconel with the phase modified by the spinning-annealing process.

Ial (hi

(c) Fig. 1. Secondary electron micrographs of samples A, C and D of lnconel 601 illustrating typical topography r,s. treatment (see Section 3 for sample treatments). The topography of sample B approximated by that of the mechanical polish onty surface (sample A).

Auger electron spectroscopy was then used to ascertain the composition of the samples' surfaces. Typical spectra of the surfaces of the samples are illustrated in Fig. 2. These spectra indicate the presence of a number of elements on the surface of the electropolished and wipe-etched samples that are not constituents of the alloy. In addition to the constituents of the alloy, the surfaces of the electropolished treated samples contained varying amounts of phosphorus, potassium, zinc, magnesium and chlorine. It must be assumed that the major portions of these impurities are residues from the polishing treatments. Multiple-point analysis on samples B and C indicated gross surface inhomogeneity, particularly on the electropolished samples. The silicon and aluminum peak shapes and kinetic energies in the spectra were indicative of oxide species: perhaps imbedded grit from the abrasive polishing. Ion

SURFACE PREPARATION OF FLUX CORE LINER 19

- ~ Z

- - tO

\.

_~ - = - - t 3

u~

t~

t~

o w

o

o

E

t ~

, z o

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sputter depth profiling was then used to determine the oxide thickness, as well as the base metal stoichiometry beyond the oxide-metal interface,

An Ar + ion beam of 4 keV was used to etch the surface while the composition v e r s u s depth was monitored. Etching rates of about 70/~ rain ~ were used, calibrated with Ta2Os. The depth profiles for the four samples are shown in Fig. 3. The vertical lines in the profiles indicate a depth of 300/~ (relative to TazOs sputter rates).

e

[a)

i

Ni

~ 0 i Cr S Fe

O 300 Estimated Sputter Depth (Angstroms)

>,

7,

(b)

' Ni

o i i Cr

i Fe

O 3 0 0 Estimated Sputter Depth (Angstroms)

m

"2

.-2

.o

N i

0 3 0 0 Estimated Sputter Depth (Angstroms)

Ic) (d)

Ni

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Estimated Sputter Depth (Angstroms)

Fig. 3. Argon-sputtered depth profiles (depths calibrated with Ta,O 5) of the t\>llo~*ing samples (see Section 3 for sample treatments): (a) sample A: (b) sample B; (c) sample C" (dl sample D.

TABLE 1

SUMMARY OF DEPTH PROI:ILE STUI)Y

Sam[i f ( , a

A B

C D

O x i d e t h i c k n e s s ~ o vvgen hal l: D e p t h at which .vteaetv .state

m a x i m u m ) s t o i c h i o m e t r y , i.e. d v / d x --+ O, a c h i e l w d

200 8(X) 1 O0 500 1 O0 200 80 20O

For sample treatments see Section 3.

Using the half-maximum of the oxygen signal as the oxide-metal interface, the relative oxide thicknesses of the samples can be compared (care was taken to minimize topographical effects in these profiles, but as with any ion sputtering data these are only first-order comparisons). The electropolished samples, particularly the 20 rain sample indicated distinct changes in the slope of the oxygen signal v e r s u s

SURFACE PREPARATION OF FLUX CORE LINER 21

depth. This may be due to either compositional change through the oxide layer, affecting the sputter rate, or topography effects. Scrutiny of the profiles would give evidence to support compositional change. The relative stoichiometry of the oxide film changes as a function of electropolishing time. One explanation for this might be the preferential etching of iron and nickel during electropolishing. Table I summarizes the depth profile study.

5. SUMMARIZING REMARKS

In conclusion, the electropolished samples have oxides thinner than the unpolished sample by a factor of 2-3, yet have a residue that is not easily removed with normal solvent or distilled water cleaning. The wipe-etched sample, while having a thinner oxide, is also thoroughly contaminated with residual components of the Aluma-brite. These residues complicate the evaluation of the treatments. If a clean metal oxide is preferable to a thinner contaminated oxide, then the least treatment is the best treatment. In situ glow discharge cleaning is also a possibility that has not been evaluated, but should be considered.

As a consequence of this study, the liners were spared the electropolishing process, a technically difficult task if it were to be implemented, considering the awkwardness of handling these large liner halves. Also, the relatively large grain sizes and the tendency for electropolishing preferentially to etch grain boundaries eliminated electropolishing as a safe method for the presently fabricated thin liners.

A simple and proven cleaning scheme of using a degreaser and alcohol wipe in situ were tentatively chosen instead of the above-described techniques.

REFERENCES

1 S. Paul, Proc. 5th Compact Toroid Meet., Seattle, WA, 1982. 2 K. Bol et al., Radiation, impurity effects, instability characteristics and transport in ohmically heated

plasma in the PLT tokamak, IAEA Rep. CN-37-A-I, 1978 (International Atomic Energy Authority, Vienna).

3 T.R. Jarboe, I. Henins, H. W. Hoida, R. K. Linford, J. Marshall, D. A. Platts and A. R. Sherwood, Phys. Rev. Lett., 45 (1980) 1264.

4 G. Goldenbaum, J. H. Irby, Y. P. Chong and G. W. Hart, Phys. Rev. Lett., 44 (1980) 393. 5 M. Yamada, H. P. Furth, W. Hsu, A. Janos, S. Jardin, M. Okabayashi, J. Sinnis, T. H. Stix and K.

Yamazaki, Phys. Rev. Lett., 46 (1981) 188. 6 A. Guy and J. J. Hren, Elements of Physical Metallurgy, Addison-Wesley, Reading, MA, 3rd edn.,

1974.