Plating the Inside of Stainless Steel Waveguide To Reduce RF Losses While Retaining the Thermal Isolation of Stainless Steel

Transcription

1 Patng the nsde of Staness Stee Wavegude To Reduce RF Losses Whe Retanng the Therma soaton of Staness Stee Gerry Petenc n Hstorca Background For at east 20 years, NRAO has empoyed technques for the nteror patng of staness stee wavegude n order to reduce mcrowave sgna path osses. Staness stee s a desrabe wavegude matera ony from the standpont of therma resstance and soaton, partcuary n cryogenc appcatons at (typcay) 20K and ower. The mcrowave osses of staness wavegude are severa tmes greater than for smar copper wavegude. Patng a thn ayer of copper onto the nteror of staness wavegude w yed a composte structure wth essentay the therma propertes of staness but the eectrca propertes (ower sgna osses) of copper. Unfortunatey, the sheded nteror of wavegude can not be reaby pated n a standard patng tank, as both the eectrca current and patng chemstry fows are mted nsde the wavegude. The NRAO approach to nteror patng was deveoped by A. Kerr and J. Lchtenberger, wth eventua contrbutons from V. Summers, R. Bradey, G. Petencn, M. Saotka and others. The approach s fundamentay smpe: Suspend a patnum anode wre down the center of the wavegude and pump patng bath through the gude. Over the years a set of fttngs have been deveoped whch aow us to reaby acheve the necessary confguraton. Probems Specfc to nterna Patng One specfcaton for a commerca patng chemstres s "anode to cathode rato". Typcay the anode (source of metac ons or n our case ust the postve eectrode) w have severa tmes the surface area of the negatvey connected (cathode) pece whch s to be pated. Wth nterna patng the centray suspended wre w obvousy have ess surface area than the surroundng wavegude. n practce ths non specfed anode to cathode rato s accommodated by operatng the patng chemstry at the ower end of ts nomna current range, as specfed by the manufacturer. Recent work at NRAO, usng Enthone-Serex UBAC R "acd copper chemstry, has acheved exceent resuts at currents of about 12 Amperes per square foot of exposed wavegude surface area. Wth short sectons of wavegude ths w typcay requre currents n the tens of mamps. Other probems that have been soved wth the present generaton of fttngs ncude proper tensonng of the patnum anode wre, so as to not fex and contact the

2 surroundng wavegude. Fttngs must be desgned for proper seang to the fanged wavegude, wth 0-rng seas to the anode wre. Pumpng s done wth a ab grade tubng (perstatc) pump, usng UBAC R1 chemstry. Patng Fundamentas The process of "eectro-deposton of metas", eectropatng, has been n use for over 100 years and s a we estabshed ndustra process. As such, process for the user s generay defned by commerca manufacturers product specfcatons, NOT from frst prncpas of physcs and eectrochemstry. Essentay a commerca patng chemstres are propretary "secret formuas" and subect to the frst rue of busness: Products do the ob rght or dont get bought agan. f one foows manufacturers gudenes the resuts shoud be acceptabe. n practce, the patng process has three maor steps: 1. Ceanng (Actvaton n patng argon) 2. Patng 3. Rnsng A steps are specfed by the product manufacturer. "Patng" process w typcay be specfed n terms of bath chemstry mantenance, patng current n terms of Amperes per square foot, and the expected deposton rate of patng meta, often n terms of Ampere mnutes of current fow requred to depost a gven thckness over a gven area. For exampe, our UBAC R1 process w depost 1000 u" (.001") on a square foot of base meta wth a current fow of 1062 Amp mnutes. For easer computaton on sma areas ths may be reduced to 1 Amp mnute depostng 136 u" of copper on 1 square nch of base meta. The key pre-process step s to specfy the desred fnsh, n terms of thckness and matera. Not a base metas and processes w be compatbe, and gavanc corroson processes may dctate that certan metac combnatons be avoded. n practca terms these ssues have been ong resoved at NRAO: A few skn depths of acd copper patng w yed exceent resuts on staness stee. Recent Wavegude Patng as a Usefu Process Exampe n eary June, 2005, we were asked to copper pate the nteror of sx peces of WRO wavegude, assembed wth round "butt" (not thru) fanges. Ths was a typca ob request n terms of wavegude sze and ength, the ength beng 1.42" from face to face of the fanges. The nteror wa area for the 1.42" pece was cacuated at.43 square nches. That area requres a patng current of 40 rna to acheve a current densty of 12.5 Amperes per square foot. The UBAC R1 process mentons ASF as an acceptabe current

3 densty range and we operate at the ow end to accommodate the non optma anode/cathode rato. Skn depth n copper s approxmatey 8 u" at 100 GHz and "rue of thumb" suggests that fve skn depths w acheve the same RF oss as that of a pure copper wavegude. Therefore we determned the patng thckness shoud be 40 u". Referrng to the manufacturers data sheet ndcatng that 1062 Amp mnutes w depost 1000 u" of copper on 144 square nches of area t was cacuated that 3 mnutes of UBAC R1 patng at 40 ma w yed the desred fve skn depths of copper. (Pease refer to the usefu Mcrowaves10.com webste for a dscusson of skn depth and ts cacuaton.) Step Ceanng Recent ab tradton had been to ony eectrocean the wavegude nteror wth 30% H2S04 and a norma (wavegude cathodc, negatve termna) current of about 100 ASF. The H2S04 s pumped through the same fttngs as used for the eventua patng. Such ceanng has proven adequate, but we may wsh to use the abs standard utrasonc and acd dp ceaners to further assure exceent resuts. Current was set at 300 rna wth crcuaton for about four mnutes. These vaues are standard but non crtca, onger tmes shoud be avoded as there w be some eroson of the brass fanges. Step Patng Pease refer to the attached photo for a vew of the patng apparatus. The patnum anode wre s.010" n dameter and weghs about.15 gm. The wre w ast for 5-6 pated peces and s the maor matera cost for ths operaton, wth wre at about $50 per gram. Patng s ntated by movng the bath net (sucton) tube from the H2S04 ceaner to the beaker of UBAC R1 copper bath. The bath s bue n coor and as t moves toward the work pece we reduce the current from 300 ma (ceanng) to the nomna patng current of 40 ma. Tmng of the three mnute patng perod begns when the bue copper souton enters the pece. As three mnutes approaches the net tube s moved to a beaker of rnse water and after about 10 seconds the power suppy votage w be seen to ncrease (constant current suppy) as rnse water dspaces patng bath from the wavegude. When that votage rse s noted the power suppy s turned off and patng ceases. Step Rnsng As soon as the power s turned off the pump speed may be ncreased to ncrease the rnse water fow. After a few seconds the pump may be turned off and the nes removed from the fttngs and tap water can be run through the assemby for a mnute or so. At ths pont the wavegude may be removed and rnsed wth warm tap water for mnutes before beng bown dry wth ab ntrogen.

4 Patng s a Vsua Art Good patng s pretty patng. nspecton of the rnsed and dred wavegude s done by hodng one end up to the abs sunt wndow and observng the quaty of the nteror patng. The desred resut s brght, shny copper, wth cose to a mrror fnsh. Some copper w have pated out on the fanges and ths can be removed wth a Scotchbrte abrasve pad. The fanges w eventuay be fnshed wth BDT-200 "brght god", typcay to 100 u". But the Proof s n the Network Anayzer Measurement (HP8510) A sx peces of wavegude, when pated to the same thckness of copper, were essentay dentca n RF oss. Resuts stated here were measured on pece "WG-2", as abeed on the removabe fange caps. Stated vaues are at 100 GHz, for 1.42" of WR10, wth fanges. Data pots for GHz are attached. Loss of orgna wavegude, staness stee: Loss of wavegude, 40 u" copper pated: Reducton of oss wth copper patng: db db db An Opton for the T hermay Chaenged As an experment, one pece of wavegude (WG-5) was pated to ony one skn depth of copper, about 8 u". Wth that thckness of copper the oss at 100 GHz was reduced by.382 db as compared to the orgna staness. Patng to fve skn depths (40 u") yeds a oss reducton of.419 db as compared to.382 db for a snge skn depth. f maxmzng therma resstance s of crtca mportance, very thn patng w resut n a oss penaty of ony a few hundredths of a db. Patng to 10 skn depths dd not yed a further reducton of osses, compared to the 40 u" peces. Concuson We can reaby copper pate the nteror of staness stee wavegude usng standard ndustra process and NRAO deveoped fttngs. Wth copper patng a oss reducton factor of four or better can be expected. Ths process s consdered reabe and robust, havng been used for over 20 years at NRAO. Very thn (snge skn depth) patng can be consdered f therma management s crtca. The ong term reabty and possbe agng of such thn patng has not been evauated. Fve skn depths shoud be the nomna patng thckness for maxmum oss reducton wth an estabshed hstory of ong term reabty. Anecdota accounts (A. Kerr) menton such copper pated wavegude as dspayng no deteroraton n performance over perods years.

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