Outgassing properties of the spallation neutron source ring vacuum chambers coated with titanium nitride

Abstract

The stainless steel vacuum chambers of the 248 m accumulator ring of the Spallation Neutron Source are to be coated with ∼100 nm of titanium nitride (TiN). This is to minimize the secondary electron yield from the chamber wall, and thus avoid the so-called e–p instability caused by electron multipacting as observed in a few high-intensity proton storage rings. Reports in the literature suggest that a TiN coating, by acting as a hydrogen permeation barrier, may also reduce the ultimate outgassing rate. The outgassing rate of TiN coated chambers deposited at various sputtering pressures was measured and compared to uncoated chambers, both with and without 250 °C in situ bake. Some coated chambers were subjected to glow discharge treatment (GDT). It was found that the surface roughness, analyzed with a scanning electron microscope, depends on the deposition pressure and is also influenced by GDT. The outgassing rate varies as a function of the surface roughness of the TiN layer, with rougher coatings more hydroscopic in nature. The in situ postbake outgassing rate was reduced ∼30% for a chamber coated with TiN at low pressure and subsequently subjected to GDT, thus giving evidence that the TiN layer acts as a permeation barrier to hydrogen diffusion. It was also found that a 450 °C vacuum degas reduced the hydrogen outgassing rate one order of magnitude, although the amount of reduction does not agree with the value predicted by standard diffusion equations.

Demonstration of electron clearing effect by means of a clearing electrode in high-intensity positron ring

“…Thus far, various types of methods have been proposed and studied both theoretically and experimentally in order to provide a solution to this problem . Some of the proposed methods that have been extensively investigated and verified are the use of solenoid fields [9,10] and the application of a coating [9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24] or attaching a grooved structure [9,25,26] on the inner surface of the beam pipe of the storage ring in order to reduce the secondary electron yield (SEY). Another direct and efficient method involves the installation of a clearing electrode in the beam pipe.…”

Thin film studies toward improving the performance of accelerator electron sources

“…There are numerous reports of successful reduction of the outgassing rate via moderate [51][52][53][54][55][56][57] and high temperature [56][57][58][59][60][61][62][63] heat treatments, which serve to reduce hydrogen outgassing. There are also numerous reports of coatings such as titanium nitride (TiN) [64][65][66][67][68][69], boron nitride [69,70], silicon [71,72], or silicon oxide [73] that may reduce the outgassing rate by acting as diffusion barriers.…”

Thermal outgassing rates of low-carbon steels

“…37 Surface roughness can also affect water outgassing because an increase in effective surface area can increase the number of potential water adsorption sites. 9,34,38 The measured surface roughnesses R a of the pipes were 2-2.5 lm and those of machined steel were 0.5 lm after machining and honing. Outgassing of the chambers made from pipes were higher than that of the machined steel chamber (Fig.…”

“…9,10,14,15 These differences may be due to difference in the steelproduction processes, in particular, the secondary metallurgy during which impurities are removed. 16,17 The reference values (Table II) for mild steels were published in the early 1970s.…”