Motoi Mushiaki

LaB6 coating to reduce the outgassing rate of a vacuum wall

Abstract Use of LaB6 is proposed to develop a low outgassing wall for vacuum chambers in which baking is not possible. Sputter deposition of LaB6 was performed in a device equipped with a d.c. plasma source, and the pumping characteristics of the chamber were investigated before and after depositioning the LaB6 coating onto the wall. The chamber with the LaB6 coating reached a lower ultimate pressure than that without the coating, and the residual water vapor pressure was remarkably reduced with the coating. The pumping speed on the coated surface was estimated to be 0.45 1 s−1 cm−2.

Pumping experiment of water on B and LaB6 films with an electron beam evaporator

Abstract The pumping characteristic of water vapor on boron and lanthanum hexaboride films formed with an electron beam evaporator have been investigated in high vacuum between 10 −4 and 10 −3 Pa. The measured initial maximum pumping speeds of water for the fresh B or LaB 6 films with a deposition amount from 2.3 × 10 21 to 6.7× 10 21 molecules/m 2 separately formed on a substrate are 3.2–4.9 m 3 /sm 2 , and the saturation values of adsorbed water on these films are 2.1 ×10 20 −1.3 × 10 21 H 2 O molecules/m 2 .

Deuterated Impurities Monitored by Microwave Spectroscopy during Glow Discharge Cleaning

Microwave spectroscopy was applied to monitoring of deuterated impurities produced by glow discharge cleaning (GDC). This method estimated the change in the quantity of the isotopic impurities by observing a rotational spectrum of water, HDO. For verifying the validity of the method, mass spectral behaviors of several light impurities (HDO, D2O, H2O, HD, D2 and H2) were studied simultaneously. The deuterated species decreased with time at their own diminishing rates, while normal water, H2O, showed no decrease during GDC. The rotational spectrum of HDO stood at the frequency of 22.3077 GHz. It was distant enough from the nearest neighbor (about 70 MHz) so that the observed line was identified definitely. The lowest partial pressure detected with a microwave spectrometer was 7×10-5 Pa.

On the effect of bakeout for the pumping speed dependence of outgassing rate in a stainless steel chamber

In order to investigate the effect of bakeout on the dependence of outgassing rate on pumping speed in a vacuum system, the outgassing rates of a test chamber at room temperature without baking and after bakeout at 100 and 250 °C were measured as a function of pumping speed. The measuresd outgassing rate q(Torr · [ls · cm2) obeys power law q = C(S/A)m where C and m (0 < m < 1) are constants, S/A is the pumping parameter defined as the ratio of the pumping speed S(l/s) and the surface area A(cm2) of the chamber, and the variable range of S/A is from 2.46 × 10−5 to 1.26 × 10−2 (ls · cm2). The obtained power laws at RT and 100 °C have two different exponents but that at 250 ° has only one exponent. The experiment shows that the bakeout weakens the pumping speed dependence on outgassing rate. In addition the theoretical consideration for the effect suggests that the sticking probability of the wall plays an important role on the pumping speed dependence.

A monitor for discharge cleaning using microwave spectroscopy

Abstract The measurement of the partial pressure of water is essential to monitor the conditioning during glow or ECR discharge cleaning. Usually QMF is adopted for this purpose, however, it has to be operated in better vacuum conditions of lower than 10 −2 Pa and with a hot filament for the electron source. The method described in this article is an application of the microwave spectroscopy so that measurement can be carried out with relatively high pressures of up to 100 Pa and needs no hot filament. The absorption rotational spectral line of water molecules at 22.235 GHz is used for this purpose. The spectrometer consists of a waveguide absorption cell with Stark electrode, a frequency-sweep microwave source and a phase-sensitive detector. The change in the intensity of the absorption spectrum of water could be detected during hydrogen glow discharge.

Monitoring of discharge cleaning for fusion devices by microwave spectroscopy of water molecules

Abstract The monitoring method of discharge cleaning which was based on microwave spectroscopy was developed in order to apply it to fusion devices and availability of this method was experimentally investigated. Water molecules are one of the major residual molecules in a vacuum vessel, and hence the partial pressure of water is a good index of progress in conditioning. Temporal changes in the partial pressure were measured by using this method during discharge cleaning consequently. Three subjects were studied with a Stark modulated microwave spectrometer, namely, proportionality between the spectrum intensity and the partial pressure, resolving power of the spectrometer for isotopic water molecules and applicability of this method to the measurement of radioactive water molecules. Rotational spectra of the light water H2O (22.235 GHz) and the hydrogen isotopic waters HDO (22.307 GHz) and HTO (16.563 GHz) were observed in several plasma devices for these purposes.

Pumping down characteristics of LaB6 and TiN coated vacuum vessels prepared by magnetron sputtering

Abstract Lanthanum hexaboride was coated on the wall surface of the vacuum vessel by magnetron sputtering, and the characteristics of pumping down of the coated vessel were investigated by comparing with those of the titanium nitride coated vacuum vessel. Right after the sputter deposition, both vacuum vessels were evacuated in the same way and the effective pumping speed of those surfaces was estimated to be about 4 m 3 /(m 2 s) for water vapor. These thin films also formed the low outgassing surface, and consequently, after the exposure of the coated vessel to air, the pressure of the vessel decreased by less than 1 3 of that before coating. The outgassing rate of lanthanum hexaboride coated surface was about five times larger than that of the titanium nitride coated surface at a time after 100 min from the start of evacuation. The XPS measurement showed that much oxygen was used in both films.

Measurements of tritiated water by microwave spectroscopy for the monitoring of discharge cleaning

The rotational spectrum of monotritiated water (THO) was measured with a Stark modulation spectrometer at 16.625 GHz. It was verified that the spectrum intensity was proportional to the pressure of THO. This result showed that the method was applicable to the monitoring of discharge cleaning under a tritium environment. Radioactivity of exhaust gases was kept below 0.56 Bq/cm3 during the experiment in compliance with statutory requirements.

A microwave spectrometer to monitor discharge cleaning

Abstract A new type of gas monitor using microwave spectroscopy is applied for studying glow discharge cleaning in a toroidal plasma device. The amount of H 2 O partial pressure during the cleaning procedure is determined by the absoprtion of the microwave power with corresponding rotational spectra of 22.235 GHz for water molecules. By applying this method, it is possible to measure the relatively high gas pressure (up to 100 Pa) encountered during the cleaning. The rate of H 2 O production and the time history of H 2 O during the cleaning are monitored and the efficiency of the reduction of H 2 O is discussed.