- Email: [email protected]
Comparative study of photodesorption from TiZrV coated and uncoated stainless steel vacuum chambers
ARTICLE IN PRESS
Vacuum 75 (2004) 155–159
Comparative study of photodesorption from TiZrV coated and uncoated stainless steel vacuum chambers V.V. Anashina, I.R. Collinsb, R.V. Dostovalova, N.V. Fedorova, A.A. Krasnova, O.B. Malyshevc,*, V.L. Ruzinovb a
Budker Institute of Nuclear Physics, 11 Lavrentueva, 630090 Novosibirsk, Russia b CERN, CH-1211 Geneva 23, Switzerland c CCLRC Daresbury Laboratory, ASTeC-Accelerator Science and Technology Centre, Vacuum Science Group, Warrington, Cheshire WA4 4AD, UK Received 2 September 2003; received in revised form 26 January 2004; accepted 27 January 2004
Abstract A three-gauge method of measurements (Nucl. Instrum. Methods A 359 (1995) 110) is described and used to study the photodesorption yields and sticking probabilities of a stainless steel test chamber coated with approximately 3 mm of TiZrV getter material. Using the measured sticking probability for hydrogen of 0.007 for the TiZrV coated chamber and the known sticking probabilities of CH4, CO and CO2 of 0, 0.5, 0.5, respectively (Vacuum 60 (2001) 57), the photodesorption yields are deduced. After activation, the H2, CH4, CO, and CO2 desorption yields are found to be 1.5 10 5, 2 10 7, o1 10 5 and o2 10 6 molecules/photon, respectively. No saturation effect is observed even after a large photon dose. These results are compared to those from stainless steel test chamber measurements performed for the same geometry under identical conditions. After saturation with CO and during continuous CO injection, the effect of the irradiation is to reduce the pressure, contrary to the normal dynamic behaviour of a conventional vacuum system. This observation is explained in terms of photon induced pumping of the CO by the TiZrV coating. r 2004 Published by Elsevier Ltd. Keywords: UHV; Synchotron radiation; Photo-stimulated desorption; Getter; NEG; TiZrV
1. Introduction The coating of the inner surface of vacuum chambers with non-evaporable getter (NEG) film, developed at CERN [2,3], is an attractive solution for many UHV applications. One such application *Corresponding author. Tel.: +44-1925-603-948; fax: +441925-603-192. E-mail address: [email protected] (O.B. Malyshev). 0042-207X/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2004.01.080
is the vacuum systems of particle accelerators that have to be designed to provide sufficiently low pressures in a beam pipe during machine operation. The coating of all internal walls of a vacuum chamber with NEG materials results in a very low static (i.e. without induced gas load) pressure of less than 10 11 Pa inside such a chamber after activation of the getter film [4]. Although some studies have been performed with a NEG coated
ARTICLE IN PRESS 156
V.V. Anashin et al. / Vacuum 75 (2004) 155–159
vacuum chamber irradiated by synchrotron radiation [5], it has not been clear how dynamic pressure behaves inside the NEG coated vacuum chamber when exposed to synchrotron radiation. In a frame of collaboration between CERN and BINP, a new photodesorption study of a TiZrV coated stainless steel vacuum chamber has been carried out using synchrotron radiation with a critical energy of 4.5 keV from the VEPP-3 storage ring at BINP in Novosibirsk, Russia. The results of photodesorption measurements from a TiZrV coated vacuum chamber using an optimized experimental method are presented here. This method allows separation of the two main NEG parameters: the intrinsic photodesorption yield and the sticking probability. Some preliminary results of this study were reported earlier [6].
Fig. 1. Experimental set-up for photodesorption measurements. RGA1–3 are the quadrupole mass analyzers; IG’s are the Bayard-Albert gauges; LD1–LD2 are the luminescent screens; Cv and Ch are the movable collimators; D is the fixed collimator; LV1 and LV2 are the leak valves; V1, is the allmetal gate valve; SS is the safety shutter; SIP+TSP are the sputter ion pump and titanium sublimation pump, COL is the collector.
2. Experimental method and set-up The experiments were performed on a dedicated set-up installed on a synchrotron radiation beam line in the VEPP-3 electron-positron storage ring. The average photon flux during experiments was about 4 1016 photons/(s m) along the sample tube. All experiments were performed at an incident angle of about 10 mrad. The photodesorption experiments were performed using the so-called three-gauge method [1], allowing the photodesorption yields, defined as the number of desorbed gas molecules per incident photon, to be extracted from the pressure rises measured at the centre and the ends of a test chamber during irradiation. This method allows the dynamic behaviour of the pressure inside the test chamber to be studied, even with sorbing walls. The experimental set-up is shown in Fig. 1. The ends of the test chamber are pumped with pumping units containing a 150 l/s sputter ion pump (SIP) and a 600 l/s titanium sublimation pump (TSP). The pressures in the pumping stations are measured with the ex-situ calibrated quadrupole gas analyzer (RGA1) and BayardAlbert gauge (IG1) at one end, and (RGA3) and (IG3) at the other end. The gauges IG1 and IG3
Fig. 2. The cone baffle inside the central RGA port.
were used as a reference for the RGAs. The pressure in the centre of the chamber was measured with RGA2 via a 10 mm diameter hole. The reflected photons and photoelectrons [7] could enter the hole in the central measuring port and affect the measurements significantly by increasing the local pressure inside the RGA port and, by generating additional source of gas, could saturate the NEG coating. In order to limit parasitic desorption in the central measuring port from the reflected photons and photoelectrons an additional device (Fig. 2) was mounted inside the central port (1). This device is a cone made from stainless steel (2) placed over the hole (3) in such a
ARTICLE IN PRESS V.V. Anashin et al. / Vacuum 75 (2004) 155–159
way as to cover the complete area of the hole. A filament (4) was mounted inside the cone to condition its surface by electron bombardment before the experiment. The conditioning was performed with an electron dose of approximately 1021 electrons/cm2 and an electron energy of 300 eV. Such conditioning decreases the electron stimulated desorption in the measuring port by one to two orders of magnitude, depending on the gas. The behaviour of a stainless steel chamber exposed to synchrotron radiation has been extensively studied [1,8]. In order to test the TiZrV coating, measurements on a stainless steel chamber with the same configuration were performed as a reference.
3. Test chamber preparation Two 1.5 m long 316L stainless steel test chambers of the same geometry with an inner/outer diameter of 24/28 mm were fabricated. The test chambers were cleaned with the CERN standard cleaning procedure and then vacuum fired for 2 h at 950 C. One of the test chambers was coated with 3 mm thick TiZrV getter material at CERN. The TiZrV coated chamber was mounted on the experimental set-up and pumped down to a
157
pressure less of than 10 4 Pa. The system was baked to 300 C for 24 h whilst the TiZrV test chamber was held at 80 C. For NEG activation the system was maintained at 150 C whilst the TiZrV coated test chamber was activated at 19075 C for 24 h. Measurements began 12 h after reaching room temperature. For the measurements on the uncoated stainless steel chamber, the whole system was baked in situ at 300 C for 24 h.
4. Results 4.1. Photon stimulated desorption The dynamic pressure rise in the centre of the test chamber, normalised to the synchrotron radiation flux of 4 1016 photon/(s m), versus the accumulated photon dose, is shown in Fig. 3 for the activated TiZrV coated chamber and for the stainless steel chamber. The behaviour of the stainless steel chamber is quite similar to previously measured data [7,8], the only difference being the hydrogen outgassing. The initial pressure rises (DP) at a photon dose of 2 1019 photons/m (i.e. photons per metre of tube length) are found to be about 4 10 7, 1.3 10 6,
Fig. 3. Dynamic pressure rise for the Stainless Steel (baked at 300 C for 24 h) and TiZrV coated vacuum chambers (activated at 190 C for 24 h).
ARTICLE IN PRESS V.V. Anashin et al. / Vacuum 75 (2004) 155–159
158
Table 1 Summary of results for stainless steel test chamber baked at 300 C for 24 h and the TiZrV coated test chamber before activation and after activation at 190 C for 24 h Gas
a
DP (Pa)
Z (molecules/photon)
a
DP (Pa)
Z; (molecules/photon)
Stainless steel vacuum chamber After 1 1021 photons/m
Initially H2 CH4 CO CO2
— — — —
4 10 7 1.1 10 1.3 10 1.3 10
7 6 6
8 10 1 10 8 10 7 10
5
— — — —
5 5 5
2 10 9 10 4 10 3 10
7
4 10 5 8 10 7 2.5 10 1.5 10
9 7 7
5 5
NEG coated vacuum chamber Before activation H2 CH4 CxHy(28) CO (28) CO2
0 0 0 0 0
After activation 6
5 10 3.3 10 — 8 10 6 7 10 6
3
6
1 10 2.5 10 — 5 10 4 3 10 4
4
1.3 10 6 and 1.1 10 8 Pa for H2, CO and CO2 and for CH4, respectively. The corresponding calculated desorption yields (Z) after a photon dose of 1 1021 photons/m are of the order 4 10 5, 8 10 7, 2.5 10 5 and 1.5 10 5 for H2, CH4, CO and CO2, respectively. The measurements on the activated TiZrV coated chamber demonstrate much lower dynamic pressure rises compared to the uncoated chamber. The initial pressure rises of 7 10 9, 2 10 9, 7 10 10 and o7 10 11 Pa for H2, CH4, CO and CO2, respectively, remain constant as a function of photon dose up to 2 1021 photons/m, indicating neither additional conditioning with photons nor a saturation of the getter coating. To compare the pressure rises before and after test chamber activation, the non-activated TiZrV coated chamber was exposed to synchrotron radiation up to a relatively small photon dose of 8 1019 photons/m. The activation was then carried out and the measurements repeated. Before activation, the pressure rises were of the order of 1 10 5 Pa and after the activation reduced by three to five orders of magnitude, depending upon the gas species. Using the measured sticking probability, a; defined as ratio of the sticking rate to the
0.007 0 0 0.5 0.5
7 10 9 2 10 9 o5 10 o3 10 o7 10
10 10 11
1.5 10 2 10 7 o3 10 o1 10 o2 10
5
8 5 6
impingement rate,1 for hydrogen of 0.007 for the TiZrV coated chamber and the known sticking probabilities of CH4, CO and CO2 of 0, 0.5, 0.5, respectively [2], the desorption yields for stainless steel and for TiZrV coated chambers before and after activation were derived and are given in Table 1. 4.2. Photon stimulated pump In a separate experiment the activated coating was first saturated by injecting CO from both ends. The pressure at the centre was equal to that at the ends due to the zero NEG pumping. The CO pressure at the ends was maintained at 2.7 10 7 Pa with a small but continuous CO injection flux during irradiation. During each exposure to synchrotron radiation, the pressure at the centre decreased as can be seen in Fig. 4. Switching off the synchrotron radiation resulted in an increase of the pressure. These observations are remarkable, since they are contrary to the normal dynamic behavior of a conventional vacuum system where the pressure would increase under 1 ISO 3529/1-1981(E/F/R), international organization for Standardization, Switzerland, 1981.
ARTICLE IN PRESS V.V. Anashin et al. / Vacuum 75 (2004) 155–159
*
*
159
chamber were found to be much lower then those of the stainless steel test chamber. Neither photon conditioning nor saturation of the getter coating was observed up to a photon dose of 2 1021 photons/m. Photon induced pumping by the getter coating has been observed for the first time. The rate of pumping was found to be around 2 10 5 molecules/photon. This result is very interesting for the application of the getter film to accelerators since it indicates that the film can continue to pump after saturation with CO when exposed to a photon flux.
Acknowledgements Fig. 4. Dynamic CO pressure in the centre and at the ends of the NEG coated test vacuum chamber saturated with CO during synchrotron radiation irradiation and constant pressure at the ends.
irradiation and recover when the radiation is stopped. It is important to note that the partial pressures of other gases were negligible in comparison to the CO partial pressure during this experiment. The synchrotron radiation seems to induce photon-induced pumping of CO. This process must be CO dissociation by photons or photoelectrons followed by the diffusion of oxygen and carbon atoms into the getter film. As a result, new adsorption sites for CO molecules appear on the getter surface. The global rate of this process can be estimated from these results (Fig. 4) using the rate of pressure decrease at the beginning of the exposure to synchrotron radiation and amounts to approximately 2 10 5 molecules/photon.
5. Conclusions *
*
Photon stimulated gas desorption from a TiZrV coated stainless steel chamber and from an uncoated stainless steel chamber of the same geometry were measured. The pressure rises of H2, CH4, CO and CO2 in the centre of the activated TiZrV coated
This work was made possible thanks to support of many people: the LHC vacuum group led in the . past by O. Grobner and presently by P Strubin, EST/SM group led by C. Benvenuti (CERN), the VEPP-3 operation group and especially the Vacuum Laboratory assistant N. Pimonov (BINP). It is a pleasure for us to thank R. Reid and K. Middleman (ASTeC, DL) for helpful suggestions and discussions.
References [1] Anashin VV, Evstigneev AV, Lysenko AP, Malyshev OB, Osipov VN, Maslennikov IL, Turner WC. Nucl Instrum Methods A 1995;359:110. [2] Benvenuti C, Chiggiato P, Costa Pinto P, Escudeiro Santana A, Hedley T, Mongelluzzo A, Ruzinov V, Wevers I. Vacuum 2001;60:57. [3] Benvenuti C, Chiggiato P, Mongelluzzo A, Prodromides A, Ruzinov V, Scheuerlein C, Taborelli M, L!evy F. J Vac Sci Technol A 2001;19(6):2925. [4] Benvenuti C, Escudeiro Santana A, Ruzinov V. Vacuum 2001;60:279. [5] Chiggiato P, Kersevan R. Vacuum 2001;60:67. [6] Anashin V, Collins IR, Dostovalov R, Fedorov N, Krasnov A, Malyshev OB, Ruzinov VL. Proceedings of EPAC Paris, France, 2002. p. 2550. [7] Anashin VV, Malyshev OB, Fedorov NV, Nazmov VP, . Goldenberg BG, Collins IR, Grobner O. Nucl Instrum Methods A 2000;448:76. [8] Halama HJ, Foerster CL. Vacuum 1991;42:185.
Vacuum 75 (2004) 155–159
Comparative study of photodesorption from TiZrV coated and uncoated stainless steel vacuum chambers V.V. Anashina, I.R. Collinsb, R.V. Dostovalova, N.V. Fedorova, A.A. Krasnova, O.B. Malyshevc,*, V.L. Ruzinovb a
Budker Institute of Nuclear Physics, 11 Lavrentueva, 630090 Novosibirsk, Russia b CERN, CH-1211 Geneva 23, Switzerland c CCLRC Daresbury Laboratory, ASTeC-Accelerator Science and Technology Centre, Vacuum Science Group, Warrington, Cheshire WA4 4AD, UK Received 2 September 2003; received in revised form 26 January 2004; accepted 27 January 2004
Abstract A three-gauge method of measurements (Nucl. Instrum. Methods A 359 (1995) 110) is described and used to study the photodesorption yields and sticking probabilities of a stainless steel test chamber coated with approximately 3 mm of TiZrV getter material. Using the measured sticking probability for hydrogen of 0.007 for the TiZrV coated chamber and the known sticking probabilities of CH4, CO and CO2 of 0, 0.5, 0.5, respectively (Vacuum 60 (2001) 57), the photodesorption yields are deduced. After activation, the H2, CH4, CO, and CO2 desorption yields are found to be 1.5 10 5, 2 10 7, o1 10 5 and o2 10 6 molecules/photon, respectively. No saturation effect is observed even after a large photon dose. These results are compared to those from stainless steel test chamber measurements performed for the same geometry under identical conditions. After saturation with CO and during continuous CO injection, the effect of the irradiation is to reduce the pressure, contrary to the normal dynamic behaviour of a conventional vacuum system. This observation is explained in terms of photon induced pumping of the CO by the TiZrV coating. r 2004 Published by Elsevier Ltd. Keywords: UHV; Synchotron radiation; Photo-stimulated desorption; Getter; NEG; TiZrV
1. Introduction The coating of the inner surface of vacuum chambers with non-evaporable getter (NEG) film, developed at CERN [2,3], is an attractive solution for many UHV applications. One such application *Corresponding author. Tel.: +44-1925-603-948; fax: +441925-603-192. E-mail address: [email protected] (O.B. Malyshev). 0042-207X/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2004.01.080
is the vacuum systems of particle accelerators that have to be designed to provide sufficiently low pressures in a beam pipe during machine operation. The coating of all internal walls of a vacuum chamber with NEG materials results in a very low static (i.e. without induced gas load) pressure of less than 10 11 Pa inside such a chamber after activation of the getter film [4]. Although some studies have been performed with a NEG coated
ARTICLE IN PRESS 156
V.V. Anashin et al. / Vacuum 75 (2004) 155–159
vacuum chamber irradiated by synchrotron radiation [5], it has not been clear how dynamic pressure behaves inside the NEG coated vacuum chamber when exposed to synchrotron radiation. In a frame of collaboration between CERN and BINP, a new photodesorption study of a TiZrV coated stainless steel vacuum chamber has been carried out using synchrotron radiation with a critical energy of 4.5 keV from the VEPP-3 storage ring at BINP in Novosibirsk, Russia. The results of photodesorption measurements from a TiZrV coated vacuum chamber using an optimized experimental method are presented here. This method allows separation of the two main NEG parameters: the intrinsic photodesorption yield and the sticking probability. Some preliminary results of this study were reported earlier [6].
Fig. 1. Experimental set-up for photodesorption measurements. RGA1–3 are the quadrupole mass analyzers; IG’s are the Bayard-Albert gauges; LD1–LD2 are the luminescent screens; Cv and Ch are the movable collimators; D is the fixed collimator; LV1 and LV2 are the leak valves; V1, is the allmetal gate valve; SS is the safety shutter; SIP+TSP are the sputter ion pump and titanium sublimation pump, COL is the collector.
2. Experimental method and set-up The experiments were performed on a dedicated set-up installed on a synchrotron radiation beam line in the VEPP-3 electron-positron storage ring. The average photon flux during experiments was about 4 1016 photons/(s m) along the sample tube. All experiments were performed at an incident angle of about 10 mrad. The photodesorption experiments were performed using the so-called three-gauge method [1], allowing the photodesorption yields, defined as the number of desorbed gas molecules per incident photon, to be extracted from the pressure rises measured at the centre and the ends of a test chamber during irradiation. This method allows the dynamic behaviour of the pressure inside the test chamber to be studied, even with sorbing walls. The experimental set-up is shown in Fig. 1. The ends of the test chamber are pumped with pumping units containing a 150 l/s sputter ion pump (SIP) and a 600 l/s titanium sublimation pump (TSP). The pressures in the pumping stations are measured with the ex-situ calibrated quadrupole gas analyzer (RGA1) and BayardAlbert gauge (IG1) at one end, and (RGA3) and (IG3) at the other end. The gauges IG1 and IG3
Fig. 2. The cone baffle inside the central RGA port.
were used as a reference for the RGAs. The pressure in the centre of the chamber was measured with RGA2 via a 10 mm diameter hole. The reflected photons and photoelectrons [7] could enter the hole in the central measuring port and affect the measurements significantly by increasing the local pressure inside the RGA port and, by generating additional source of gas, could saturate the NEG coating. In order to limit parasitic desorption in the central measuring port from the reflected photons and photoelectrons an additional device (Fig. 2) was mounted inside the central port (1). This device is a cone made from stainless steel (2) placed over the hole (3) in such a
ARTICLE IN PRESS V.V. Anashin et al. / Vacuum 75 (2004) 155–159
way as to cover the complete area of the hole. A filament (4) was mounted inside the cone to condition its surface by electron bombardment before the experiment. The conditioning was performed with an electron dose of approximately 1021 electrons/cm2 and an electron energy of 300 eV. Such conditioning decreases the electron stimulated desorption in the measuring port by one to two orders of magnitude, depending on the gas. The behaviour of a stainless steel chamber exposed to synchrotron radiation has been extensively studied [1,8]. In order to test the TiZrV coating, measurements on a stainless steel chamber with the same configuration were performed as a reference.
3. Test chamber preparation Two 1.5 m long 316L stainless steel test chambers of the same geometry with an inner/outer diameter of 24/28 mm were fabricated. The test chambers were cleaned with the CERN standard cleaning procedure and then vacuum fired for 2 h at 950 C. One of the test chambers was coated with 3 mm thick TiZrV getter material at CERN. The TiZrV coated chamber was mounted on the experimental set-up and pumped down to a
157
pressure less of than 10 4 Pa. The system was baked to 300 C for 24 h whilst the TiZrV test chamber was held at 80 C. For NEG activation the system was maintained at 150 C whilst the TiZrV coated test chamber was activated at 19075 C for 24 h. Measurements began 12 h after reaching room temperature. For the measurements on the uncoated stainless steel chamber, the whole system was baked in situ at 300 C for 24 h.
4. Results 4.1. Photon stimulated desorption The dynamic pressure rise in the centre of the test chamber, normalised to the synchrotron radiation flux of 4 1016 photon/(s m), versus the accumulated photon dose, is shown in Fig. 3 for the activated TiZrV coated chamber and for the stainless steel chamber. The behaviour of the stainless steel chamber is quite similar to previously measured data [7,8], the only difference being the hydrogen outgassing. The initial pressure rises (DP) at a photon dose of 2 1019 photons/m (i.e. photons per metre of tube length) are found to be about 4 10 7, 1.3 10 6,
Fig. 3. Dynamic pressure rise for the Stainless Steel (baked at 300 C for 24 h) and TiZrV coated vacuum chambers (activated at 190 C for 24 h).
ARTICLE IN PRESS V.V. Anashin et al. / Vacuum 75 (2004) 155–159
158
Table 1 Summary of results for stainless steel test chamber baked at 300 C for 24 h and the TiZrV coated test chamber before activation and after activation at 190 C for 24 h Gas
a
DP (Pa)
Z (molecules/photon)
a
DP (Pa)
Z; (molecules/photon)
Stainless steel vacuum chamber After 1 1021 photons/m
Initially H2 CH4 CO CO2
— — — —
4 10 7 1.1 10 1.3 10 1.3 10
7 6 6
8 10 1 10 8 10 7 10
5
— — — —
5 5 5
2 10 9 10 4 10 3 10
7
4 10 5 8 10 7 2.5 10 1.5 10
9 7 7
5 5
NEG coated vacuum chamber Before activation H2 CH4 CxHy(28) CO (28) CO2
0 0 0 0 0
After activation 6
5 10 3.3 10 — 8 10 6 7 10 6
3
6
1 10 2.5 10 — 5 10 4 3 10 4
4
1.3 10 6 and 1.1 10 8 Pa for H2, CO and CO2 and for CH4, respectively. The corresponding calculated desorption yields (Z) after a photon dose of 1 1021 photons/m are of the order 4 10 5, 8 10 7, 2.5 10 5 and 1.5 10 5 for H2, CH4, CO and CO2, respectively. The measurements on the activated TiZrV coated chamber demonstrate much lower dynamic pressure rises compared to the uncoated chamber. The initial pressure rises of 7 10 9, 2 10 9, 7 10 10 and o7 10 11 Pa for H2, CH4, CO and CO2, respectively, remain constant as a function of photon dose up to 2 1021 photons/m, indicating neither additional conditioning with photons nor a saturation of the getter coating. To compare the pressure rises before and after test chamber activation, the non-activated TiZrV coated chamber was exposed to synchrotron radiation up to a relatively small photon dose of 8 1019 photons/m. The activation was then carried out and the measurements repeated. Before activation, the pressure rises were of the order of 1 10 5 Pa and after the activation reduced by three to five orders of magnitude, depending upon the gas species. Using the measured sticking probability, a; defined as ratio of the sticking rate to the
0.007 0 0 0.5 0.5
7 10 9 2 10 9 o5 10 o3 10 o7 10
10 10 11
1.5 10 2 10 7 o3 10 o1 10 o2 10
5
8 5 6
impingement rate,1 for hydrogen of 0.007 for the TiZrV coated chamber and the known sticking probabilities of CH4, CO and CO2 of 0, 0.5, 0.5, respectively [2], the desorption yields for stainless steel and for TiZrV coated chambers before and after activation were derived and are given in Table 1. 4.2. Photon stimulated pump In a separate experiment the activated coating was first saturated by injecting CO from both ends. The pressure at the centre was equal to that at the ends due to the zero NEG pumping. The CO pressure at the ends was maintained at 2.7 10 7 Pa with a small but continuous CO injection flux during irradiation. During each exposure to synchrotron radiation, the pressure at the centre decreased as can be seen in Fig. 4. Switching off the synchrotron radiation resulted in an increase of the pressure. These observations are remarkable, since they are contrary to the normal dynamic behavior of a conventional vacuum system where the pressure would increase under 1 ISO 3529/1-1981(E/F/R), international organization for Standardization, Switzerland, 1981.
ARTICLE IN PRESS V.V. Anashin et al. / Vacuum 75 (2004) 155–159
*
*
159
chamber were found to be much lower then those of the stainless steel test chamber. Neither photon conditioning nor saturation of the getter coating was observed up to a photon dose of 2 1021 photons/m. Photon induced pumping by the getter coating has been observed for the first time. The rate of pumping was found to be around 2 10 5 molecules/photon. This result is very interesting for the application of the getter film to accelerators since it indicates that the film can continue to pump after saturation with CO when exposed to a photon flux.
Acknowledgements Fig. 4. Dynamic CO pressure in the centre and at the ends of the NEG coated test vacuum chamber saturated with CO during synchrotron radiation irradiation and constant pressure at the ends.
irradiation and recover when the radiation is stopped. It is important to note that the partial pressures of other gases were negligible in comparison to the CO partial pressure during this experiment. The synchrotron radiation seems to induce photon-induced pumping of CO. This process must be CO dissociation by photons or photoelectrons followed by the diffusion of oxygen and carbon atoms into the getter film. As a result, new adsorption sites for CO molecules appear on the getter surface. The global rate of this process can be estimated from these results (Fig. 4) using the rate of pressure decrease at the beginning of the exposure to synchrotron radiation and amounts to approximately 2 10 5 molecules/photon.
5. Conclusions *
*
Photon stimulated gas desorption from a TiZrV coated stainless steel chamber and from an uncoated stainless steel chamber of the same geometry were measured. The pressure rises of H2, CH4, CO and CO2 in the centre of the activated TiZrV coated
This work was made possible thanks to support of many people: the LHC vacuum group led in the . past by O. Grobner and presently by P Strubin, EST/SM group led by C. Benvenuti (CERN), the VEPP-3 operation group and especially the Vacuum Laboratory assistant N. Pimonov (BINP). It is a pleasure for us to thank R. Reid and K. Middleman (ASTeC, DL) for helpful suggestions and discussions.
References [1] Anashin VV, Evstigneev AV, Lysenko AP, Malyshev OB, Osipov VN, Maslennikov IL, Turner WC. Nucl Instrum Methods A 1995;359:110. [2] Benvenuti C, Chiggiato P, Costa Pinto P, Escudeiro Santana A, Hedley T, Mongelluzzo A, Ruzinov V, Wevers I. Vacuum 2001;60:57. [3] Benvenuti C, Chiggiato P, Mongelluzzo A, Prodromides A, Ruzinov V, Scheuerlein C, Taborelli M, L!evy F. J Vac Sci Technol A 2001;19(6):2925. [4] Benvenuti C, Escudeiro Santana A, Ruzinov V. Vacuum 2001;60:279. [5] Chiggiato P, Kersevan R. Vacuum 2001;60:67. [6] Anashin V, Collins IR, Dostovalov R, Fedorov N, Krasnov A, Malyshev OB, Ruzinov VL. Proceedings of EPAC Paris, France, 2002. p. 2550. [7] Anashin VV, Malyshev OB, Fedorov NV, Nazmov VP, . Goldenberg BG, Collins IR, Grobner O. Nucl Instrum Methods A 2000;448:76. [8] Halama HJ, Foerster CL. Vacuum 1991;42:185.