Kosuke Okamoto

Ionization of Nitrogen Cluster Beam

A nitrogen cluster beam (neutral particle intensity of 28.6 mAeq) is ionized by electron collisions in a Bayard-Alpert gauge type ionizer. The extraction efficiency of about 65% is obtained at an electron current of 10 mA with an energy of 50 eV. The mean cluster size produced at a pressure of 663 Torr and temperature of 77.3 K is 2×105 molecules per cluster. By the Coulomb repulsion force, multiply ionized cluster ions are broken up into smaller fragments and the cluster ion size reduces to one-fourth at an electron current of 15 mA. Mean neutral cluster sizes depend strongly on the initial degree of saturation Φ0 and are 2×105, 7×104 and 3×104 molecules per cluster at Φ0s of 0.87, 0.66 and 0.39, respectively.

Necessary Conditions for Generating Intense Cluster Beam

There is a critical value for the pressure in the nozzle skimmer region above which no cluster beam can be generated. This result is in general agreement with that calculated from the flow accompanied with condensation. There also is a critical distance between the nozzle and the skimmer beyond which clusters disintegrate into molecules. This phenomenon is due to Mach disk formation. The location of the Mach disk is obtained from Pitot tube readings PT on the beam axis and its validity is confirmed by measuring radial distributions of PT.

Some Considerations on Large-Sized Catalytic Pumps for Nuclear Fusion Apparatus

Studies have been made of the problems encountered in the development of a large-sized catalytic pump for the pumping of hydrogen in nuclear fusion apparatus. An experimental pump having a speed of about 5000 liter/sec is assumed to use at partial pressures between 10-7 and 10-5 Torr. The influence of water vapor on the atomization of hydrogen at the rhenium filament is one of the problems, and is discussed from the structural point of view. The lifetime of the cuprous oxide layer is another important problem. A convenient method of oxidation of copper plates by atomic oxygen produced at an incandescent rhenium filament is proposed for the generation and regeneration of the oxide. The optimum temperatures of the cuprous oxide layer are also determined for the rapid desorption of water vapor from the oxide and for the effective reaction with hydrogen atoms.

A Multichannel System for Determination of Time Varying Spectral Line Profiles

An instrument has been constructed which can directly display on an oscilloscope time-resolved profiles of a spectral line emitted from a transient plasma. It has closely spaced 17 optical channels, and the wavelength coverage between the outermost channels can be varied from 10.4 to about 0.8A by the proper choice of a grating and a magnifying cylindrical lens. Profile measurements can automatically be repeated with time resolution from 0.1 to 2 µs. The display of these time-resolved line profiles has been accomplished by means of electronic circuit. An example of application to a transient plasma is shown.

CONTROL OF CLUSTER ION SIZES FOR EFFICIENT INJECTION HEATING

For heating of plasmas by injection of hydrogen cluster ions, the specific size (N/Z)≈102 molecules/charge is believed to be most desirable. A fundamental research to develop a practical method for tailoring large cluster ions into small suitable sizes has been carried out by using nitrogen cluster ions of the initial mean specific size (N/Z¯)o≃105. The beam of neutral large clusters of total intensity 20 mAeq was led to an ionizer and then the large cluster ions are accelerated to 8.9 keV before entering the divider which disintegrates them into small fragments by multiple ionization. The mean specific size of disintegrated cluster ions (N/Z¯)′ becomes smaller with increase in ionizing electron current of the divider. (N/Z¯)′ becomes 103~104 at an electron current of 140 mA and an accelerating voltage of 680 V of the divider with its efficiency of 20~60%. Thus, the original large cluster ions are divided into small fragments of which the mean specific size is 1/20~1/100 of the initial value without much decrease in total intensity of the cluster ion beam.