Effect of surface polishing and vacuum firing on electron stimulated ...

Effect of surface polishing and vacuum firing on electron stimulated desorption from 316LN stainless steel Oleg B. Malyshev,a) Benjamin T. Hogan, and Mark Pendleton ASTeC, STFC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD Cheshire, United Kingdom

(Received 27 February 2014; accepted 24 June 2014; published 8 July 2014) The reduction of thermal outgassing from stainless steel by surface polishing or vacuum firing is well-known in vacuum technology, and the consequent use of both techniques allows an even further reduction of outgassing. The aim of this study was to identify the effectiveness of surface polishing and vacuum firing for reducing electron-stimulated desorption (ESD) from 316LN stainless steel, which is a frequently used material for particle accelerator vacuum chambers and components. It was found that, unlike for thermal outgassing, surface polishing does not reduce the ESD yield and may even increase it, while vacuum firing of nonpolished sample reduces only the H2 ESD yield by a C 2014 Author(s). All article content, except where otherwise noted, is licensed under a factor 2. V Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1116/1.4887035] I. INTRODUCTION The process of thermal desorption (thermal outgassing) has been intensively studied in the past because it is a significant source of gas in many ultrahigh vacuum and extreme high vacuum systems. Most interest has been concentrated on the materials used to manufacture vacuum chambers and components, such as stainless steel, copper, and aluminum. The data available from the literature demonstrate that the outgassing rate [gt; molecules/(sm2)] depends upon many factors such as material selection, surface roughness, cleaning procedures, the temperature and duration of bakeout and the pumping time, etc.1–8 In the design of a large vacuum system, stainless steel is the most common material selected for the vacuum chambers because it can routinely achieve a value of gt ¼ 1012 mbarl/(scm2) for hydrogen (and much lower for other species) after a 24-h bakeout at 300 C.1 Further reduction can be achieved using the methods of an air bake, vacuum firing, and surface polishing. The best outgassing results of gt ¼ 1015 mbarl/(scm2) have been obtained after performing either an air bake or a very fine polish and subsequent vacuum firing at a pressure better than 106 mbar.6–9 In high energy particle accelerators where synchrotron radiation, electrons, ions, or other particles bombard the vacuum chamber walls, the main source of outgassing may be the induced desorption caused by these particles. The aim of the present study was to determine the effectiveness of fine surface polishing and vacuum firing for reducing the electron-stimulated desorption (ESD) yields from 316LN stainless steel.

filament ran along the axis of the tube and was connected to a high voltage floating power supply that allowed bombardment of a tube with electrons possessing an energy up to 6.5 keV and a current up to 200 mA over a length of 480 mm. A section of the tube 10 mm from either side was not bombarded to avoid electron bombardment of the edge welds, flanges, and gaskets. The total and partial pressures were measured with an extractor gauge operating with an IONIVAC IM 540 controller (Oerlikon-Leybold Vacuum, Cologne, Germany) and a residual gas analyzer (RGA) (MKS Instruments, Andover, MA, USA) connected to the test chamber where the tubular sample was mounted. The test chamber effective pumping speed (Se) was defined by the vacuum conductance of the tubes and valve, leading to a sputter ion pump and was measured prior to ESD measurements via gas injection with known gas flows for different gases of interest, such as N2, H2, CO, CO2, CH4, and Ar. In this way, the effective pumping speed for this system was found to be Se(N2) ¼ 5 l/s. B. Sample preparation and experimental procedure

The four samples studied were made of 316LN stainless steel and are shown in Table I. The samples S1, S2, and S3 were ordered from Allectra GmbH (Berlin, Germany), and the sample S4 was provided by Henniker Scientific Ltd (Warrington, Cheshire, UK). Sample S1 was not polished,

TABLE I. Samples and surface treatments applied to the samples. Tube No.

Supplier

Reference

Polishing

Vacuum firing

RA

S1 S1VF S2 S2VF S3 S3VF S4 S4VF

No No Yes Yes Yes Yes Yes Yes

No Yes No Yes No Yes No Yes

— — 0.150.2 0.150.2 0.150.2 0.150.2 0.10.15 0.10.15

II. EXPERIMENT A. Experimental facility

The ESD yield measurements were performed on an existing facility10–13 that was designed for studying tubular samples with a length of 0.5 m and an inner diameter of 36–42 mm with CF40 flanges at either end. An iridium a)

1

Allectra GmbH

2

Allectra GmbH

3

Allectra GmbH

4

Henniker Scientific Ltd.

Electronic mail: [email protected]

051601-1 J. Vac. Sci. Technol. A 32(5), Sep/Oct 2014

0734-2101/2014/32(5)/051601/5

C Author(s) 2014 V

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051601-2 Malyshev, Hogan, and Pendleton: Effect of surface polishing and vacuum firing on ESD from 316LN

FIG. 1. (Color online) ESD yields for H2, CH4, CO, CO2, O2, Ar, and H2O as a function of electron dose. J. Vac. Sci. Technol. A, Vol. 32, No. 5, Sep/Oct 2014

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FIG. 2. (Color online) ESD yields for H2, CH4, CO, CO2, O2, Ar, and H2O as a function of the amount of desorbed gas. JVST A - Vacuum, Surfaces, and Films

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while the inner surfaces of samples S2, S3, and S4 were polished by the manufacturer using different techniques. The sample S2 was mechanically polished with abrasive “flap” wheels, sample S3 was honed to remove material, and S4 was treated with an undisclosed technique by the manufacturer. In this way, we were able to compare the effect of different polishing techniques, different vendors, and the effect of vacuum firing on all samples. The samples were all treated using the same procedure. After the installation of each sample tube, the vacuum chamber and sample tube were evacuated followed by a bakeout of the system to a temperature of 250 C. When baked for a period of 24 h, the system was allowed to cool. After cooling down to 150 C, all filaments, such as those found in the RGA, extractor gauge, and tube, were outgassed at this temperature. Once the system reached room temperature and was pumped for approximately 16 h, the background pressure reached a range below 108 mbar. All ESD measurements were performed with fixed electron energy of 500 eV. The electron current was not fixed, but increased with time within the range from 10 to 75 mA. The sample tube temperature was maintained at around 30 C using a cooling jacket connected to a chiller/heater unit. After a seven-day duration bombardment, each sample was removed to be vacuum fired to 950 C for 2 h at a pressure of 105 mbar. When the firing process was completed, the samples were vented with nitrogen, exposed to air for several hours, and then connected to a simple pumping system, which maintained a pressure of