In the hot cathode ionization gauge, the most disturbing noise current is the photo current induced by the X-rays generated at the anode. In the ionization gauges measuring Extreme high vacuum, the ion collectors (IC) are shielded from the X-ray sources.
In the Counter Flow Gauge (CFG) proposed in this paper, the electrons used for ionization are decelerated before they are absorbed by the electron collector (EC). Therefore, the effect of X-rays is negligible.
The CFG has eleven ring electrodes (24.8 mm dia) consisting of a grid, the potential of each ring being determined independently. The total length of the grid is about 50 mm. On one end of the grid, a ring filament (17 mm dia) is coaxially installed. The emitted electrons are accelerated in the grid and generate ions. At the other end, the electrons are decelerated by a strong electric field before they are absorbed by the EC. The field also suppresses the so called "electron stimulated ion desorption". Thus, in the CFG, the two most disturbing noises in the ionization gauge are both avoided.
The ions generated in the grid flow in the opposite direction to the electron beam because the electric field accelerating the electrons simultaneously accelerates the ions in the opposite direction. The ion beam converges and passes through the center of the ring filament. A needle electrode of the IC is installed on the axis behind the filament, and a small shielding cylinder (10 mm dia) is placed between the IC and the filament. This cylinder shields not only the heating rays from the filament but also the stray X-rays generated by the stray electrons.
In order to determine the geometries and potentials of the electrodes, a simulation program (SIMION ver.7) was used. A preliminary experiment was made in a vacuum of about 1x10e-7 Pa. The transmission ratio of the electrons, I(EC)/I(emission), was 91% when the potential difference between the filament and the EC was about 8 V. The sensitivity factor was estimated to be higher than 0.1/Pa.
The latest experimental data will be presented at the congress.
|