Toshiaki Kobari

Photodesorption from stainless steel, aluminum alloy and oxygen free copper test chambers

Abstract Photodesorption yields due to synchrotron radiation (SR) from type 316 stainless steel, type A6063 aluminum alloy and high purity oxygen free copper (ASTM Class-1 OFC) test chambers were measured. SR with critical energy of 4 KeV from the 2.5 GeV storage ring of the Photon Factory at KEK was used. The photon beam size is 0.385 mrad in both vertical and horizontal directions. Seven test chambers which have the same shape were tested. The yields of OFC finished with machining is reduced to 9 × 10 −6 molecules photon −1 (N 2 equivalent) which is the lowest value throughout the experiments. Stainless steel treated with electrolytic polishing and 48h prebaking at 450°C shows the second lowest yield. The yield of aluminum alloy is somewhat higher than the others. The yields for other gas species were also measured.

Photodesorption and photoelectron yields at normal and grazing incidence

Abstract The photodesorption and photoelectron yields in a test chamber exposed to synchrotron radiation were examined at both normal and grazing incidence with some different photon energy distributions. The results were discussed and compared with experimental and theoretical calculations of the photoelectron yield. It is concluded in this study that photoelectron emission is the dominant process of the photon stimulated desorption (PSD), and that the PSD yield agrees qualitatively well with the calculated photoelectron yield.

Photoelectron effect of photodesorption in a chamber irradiated by synchrotron radiation

Photodesorption in vacuum chambers exposed to synchrotron radiation were measured at the PF storage ring of KEK. Two types of wire electrodes, one is a cylindrical grid in an axial direction near the inner surface of the test chamber, and the other is a semispherical grid around a surface irradiated by synchrotron radiation, were installed in the test chambers. Desorption and photoelectron currents were measured under conditions of bias voltages applied to the electrodes. Photodesorption caused by synchrotron radiation is greatly influenced by bias voltage. Therefore, electron stimulated desorption due to photoelectron contributes considerably to the photodesorption. Photodesorption can be decreased by a negative electric field around the primary incident surface of the chamber.

NEG performance under synchrotron radiation

We investigated the performance of non‐evaporable getter (NEG) under synchrotron radiation (SR). A 10mm wide and 100mm long NEG strip was installed in a stainless steel chamber and exposed to SR from the PF (Photon Factory) storage ring at KEK (the National Laboratory for High Energy Physics). The residual gas species and the pressure rise were measured under various conditions: e.g. 1) in the presence both of reflected radiation and secondary electron emission from the inner surface of the chamber, and 2) under direct SR exposure. In addition, we measured the desorption and the photocurrent by applying a bias voltage to the NEG strip. It was found that, even after the NEG strip had pumped desorbed gases and accumulated them, exposure to reflected SR or direct SR did not cause a drastic pressure rise. It was found, in addition, that there was no significant release of CH4 or production of CH4 on the NEG strip.

Fabrication of a copper chamber for a storage ring with broached inner surface

Abstract There have been several reports that photodesorption due to synchroton radiation from OFC copper chambers with machined surfaces is far lower than in stainless steel or aluminum alloy chambers. However, it is difficult to process machined surfaces for the inner surfaces of beam chambers. A surface treatment using broaching has been developed to cut the inner surface of the chamber. Tests of the photodesorption caused by synchrotron radiation in a small, broached, cylindrical cross-section chamber with a length of 30 cm have been reported. To adapt the broaching method to long chambers, prototype copper vacuum chambers with lengths of 1.8 m and 2 m have been fabricated. A chamber consists of a beam channel, a cooling channel, and a NEG pump channel. The beam channel, which has a hexagonal cross section, is broached. An ultimate pressure of 1 × 10 −8 Pa was obtained after bakeout at 200 °C for 24 h and NEG activation.