Postmortem—An autopsy of an NEC column postD. C. Weisser, T. A. Brinkley, G. P. Clarkson, M. D. Malev, T. R. Ophel, and R. B. Turkentine Citation: Review of Scientific Instruments 57, 723 (1986); doi: 10.1063/1.1138897 View online: http://dx.doi.org/10.1063/1.1138897 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/57/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Sonar-induced pressure fields in a post-mortem common dolphin J. Acoust. Soc. Am. 131, 1595 (2012); 10.1121/1.3675005 WEA201B06: Quantification of Breast Density Using Dual Energy Mammography: A Post Mortem Study Med. Phys. 37, 3413 (2010); 10.1118/1.3469338 SUFFI135: Breast Density Measurement with ConeBeam CT and MRI: A Post Mortem Study Med. Phys. 36, 2466 (2009); 10.1118/1.3181256 Autopsy Am. J. Phys. 53, 949 (1985); 10.1119/1.14009 PostMortem Acoustic Impedance of Human Ears J. Acoust. Soc. Am. 35, 104 (1963); 10.1121/1.1918421

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Post-morteM-An autopsy of an NEe column post D. C. Weisser, T. A. Brinkley, G. P. Clarkson, M. D. Malev, T. R. Ophel, and R. B. Turkentine

Nuclear Physics Department. Australian National University, Canberra, Australia

(Presented on 21 October 1985)

Cracks in ceramic components ofNEC column posts pose a serious threat to the 14UD in Canberra. The failed posts have been examined with a view toward identifying the probable cause of puncture and deciding a strategy to contain the problem.

!NTRODUCTlON

During normal general inspection, cracks were discovered in post ceramics, which are crucial components in the column structure of the 14UD. Ceramics failures could be a danger­ous and expensive phenomenon. In order to predict the fu­ture of the as yet undamaged components, one must ask why ceramics fail. Is the failure due to unavoidable "old age" or are there peculiar avoidable circumstances which predis­posed the Canberra 14UD to this disease? Preliminary inves­tigations of post failures and other "old age" phenomena appear in Ref. 1.

I. THE ANATOMY OF AN NEC COLUMN POST

Figure 1 shows a post with its end flange and first two spark-gap electrodes removed and Fig. 2 shows the detail of the ceramic to titanium bond. There are four points to note.

FIG. 1. NEC column post with the end flange and electrode removed from ceramic A. The spark gap electrode has been removed from ceramic C. The puncture of ceramic B caused cracks in A and C.

( 1) Electric field is experienced by ceramics marked Band D. Ceramic C is enclosed within an electrode. (2) The elec­trode material covers only the outer 7 -mm edge of the ce­ramic leaving a O.5-mm-thick void. This contains the two aluminum foils which are involved in the diffusion bond only in the edge region. (3) The aluminum foil is not in intimate contact with the ceramic other than at the bonded edge; this leaves a gap between the ceramic and the foil. (4) The foil often has a wrinkled surface and protuberances.

II. POST MORTEM DISSECTION

It is desirable to confirm that a suspected mark on a ceramic is indeed a crack while the post is still in the accel­erator, let alone before it's sawn in two. The use of a penetrat­ing dye seems a reliable technique since cracks show up while dirt marks don't absorb stain preserving their voltage holding ability.

Failed ceramics were sawn through revealing that the externally observed cracks were the results of punctures wen in from the edges, as seen in Fig. 3. A diamond cylindrical saw was used to trepan the ceramic from the undamaged section toward the failed ceramic. This confirmed that the puncture always occurred in the ceramic which was experi­encing voltage gradient. The cracks in the ungraded ceramic radiated from the puncture in the graded one.

The aluminum foil at the puncture point generally showed signs of electrical activity and was destroyed in the region of the puncture. One could not, therefore, examine the foil for a protuberance which might have contributed to the original puncture.

L DIFFUSION BOND REGION J SCALE I mm. '-----'

FIG. 2. Detail of ceramic to titanium bond.

723 Rev. Sci.lnstrum. 57 (5), May 1986 0034-6748/86/050723-03$01.30 @ 1986 American Institute of Physics 723

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FIG. 3. Sawn ceramic showing the puncture radiating cracks.

Trepanning undamaged ceramic/foil interfaces re­vealed several protuberances on each 25-mm-diam sample (Fig. 4). Many of these features are benign, that is ofrela­tively large radius of curvature, or pointing away from the adjacent live ceramic. Figure 5 is an electron micrograph of a feature displaying a sharp point. It can also be seen that the aluminum seems to have crystallized in this region. Presu-

FIG. 4. Aluminum foil removed from an unfailed ceramic/foil interface. Features A, B, and C are protuberances, which if convex toward a ceramic will cause field enhancement. •

724 Rev. Sci.lnstrum., Vol. 57, No.5, May 1986

FIG. 5. An electron micrograph of a feature on an aluminum foil showing a complex sharp shape.

mably, this is an artifact of the bonding process wherein the aluminum approaches the melting temperature. In Fig. 4 one can see six features: three convex and three concave to­ward the ceramic. The concave ones would be benign except for the fact that they raised corresponding points on the adja­cent foil, which pointed towards its ceramic.

In undamaged ceramic/aluminum interfaces, no evi­dence of electrical activity was seen. In summary, protuber­ances on the foils pointing toward the ceramic must cause additional electric field enhancements as suggested by Rath­me1J.2 This enhancement is in addition to the factor of 10 increase of the electric field in the gap due to the delectric constant of the ceramic.

m. DIAGNOSIS OF DiSEASE

It is important to know whether punctured post ceram­ics are an unavoidable consequence of "old age." The age of an accelerator cannot be measured in mere years. Surely a stressful life accelerates aging. The Canberra 14UD has op­erated at and above its design gradient for the last eight years. During that time, it experienced on the order of sever­al hundred sparks per year. Spark frequency is susceptible to the treatment of more modest operation or of eliminating identifiable causes of sparks. We have consistently observed that when there are free particulates in the accelerator's SF 6

space, sparks occur. There are at least two removable sources of particulates. The first is spark-erosion particu­lates from the post end flange to titanium electrode. Several cubic millimeters of material from such locations was found while investigating ceramic punctures. Evidence of this gray powder has been seen in the 14UD for at least the last 8 years. The presence of this material in the machine can be demon­strated by inserting a compressed gas jet into the split in a post flange. This will result in a cloud of dust in about 15% of the cases. During normal operation, this dust migrates into the SF 6 and deposits on all electrostatic surfaces. The move­ment of such particulates detonates sparks which cause further spark erosion of the aluminum-titanium interface increasing its electrical resistance, enlarging the gap, and

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predisposing the gap to further damage. It is our observation that the dust problem is worsening rapidly, consistent with this source of material. Poorly spot-welded titanium-titan­iumjoints on column electrodes contribute particulates to a lesser degree.

Another intrinsic source of spark detonating particu­lates is the solid SF6 breakdown products built up on the planes opposite corona points. Sparks liberate these solids which then cause subsequent sparks. Only replacing the cor­ona system with resistor grading will eliminate this source of particulates.

Through an attack on particulates, one can reduce the number of sparks. In our case, there were two avoidable cir­cumstances which were probably crucial in causing the ce­ramic punctures. The first was the long-term operation at excessive SF 6 pressure, i.e., 110 psia (7.5 atm). This situa­tion arose in response to severe attack of the nylon links of the chains by corona produced SF6 breakdown products. The high SF6 pressure reduced the corona currents and al­lowed two-chain operation. Unfortunately, the high pres­sure increased the breakdown voltage threshold on the spark gaps protecting the ceramics, exposing the ceramics to high­er than desirable electric fields. We persisted with high-pres­sure operation because we interpreted the lack of spark-in­duced deconditioning of the accelerator tube as evidence that no damage was being done. The accelerator tube in a 14UD might be more spark resistant than the column.

Perhaps ceramics would not have been punctured even by sparks at high SF6 pressure and high gradient for, in fact, the cracked ceramics were first found immediately following an occasion in which the accelerator was left sparking from negative potential for 7 h. When the chains were left running with the charging supplies off, negative, frictional self­charge caused the terminal to go to about - 4 MY. Since corona points operate satisfactorily only when the point is negative with respect to the plane, the voltage distribution system is inoperative in this back bias mode. Thus the ma­chine would column spark when the field over spark gaps became large enough. There is no obvious mechanism wher­eby negative, or ungraded sparking would overstress ceram­ics beyond what they might experience in a positive 14.8-MY spark. Confirmation that ungraded sparking is especially

725 Rev. Sci.lnstrum., Vol. 57, No.5, May 1986

dangerous, is obtained from a similar experience at NEC, where ceramics were punctured by successive sparking of an ungraded small accelerator. 2

A further hint to the cause of punctures can be found in the epidemiology of the disease. Why, out of 2000 ceramics, did only a dozen fail? Were these ceramics intrinsically weaker? Were their voltage environments especially malig­nant because of foil protuberances? Given the variety in sharpness and orientation of foil protuberances, perhaps only the few really severe ones have now resulted in punc­tures. Both these hypotheses imply that the remaining ce­ramics are intrinsically more damage resistant. They would also be consistent with the disappearance of the disease as soon as those susceptible have succumbed and been re­placed. Indeed this appears to be the situation. No new cases have been observed since the original outbreak.

iV. PRESCRIPTION FOR THE fUTURE

Sensible steps have already been taken. The SF6 gas pressure is now 80 psia (5.4 atm) to help protect the ceram­ics and reverse voltage operation is now more consistently avoided. Unfortunately, we have no reliable negative termi­nal voltage interlock. Perhaps future ceramic punctures will be avoided by these measures plus the possibility that the remaining ceramics may be stronger than those that failed.

Indeed, the noninvasive medicine of low SF6 pressure and no reverse sparking, may be the best cure. Nevertheless, we will start a spark reduction program in 1986 by refurbish­ing posts to reduce spark erosion of end flanges. Given our choice, we would also replace the corona point distribution system with resistors. The most radical treatment would be for NEC to modify their post-manufacture technique to eliminate the ceramic-gap-foil region by evaporating a layer of aluminum directly onto the ceramic. At present, this un­tried expensive treatment may be excessive.

IT. A. Brinkley, G. P. Clarkson, M. D. Malev, T. R. Ophel, R. B. Turken­tine, and D. C. Weisser, Nucl. Instrum. Methods Phys. Res. A244, 89 (1986).

2R. Rathme1l (private communication).

Electrostatic status reports 725

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