Ions are produced in the center of the 12-inch cyclotron by the simple method employing a biased hot filament. The Thorium-Tungsten filament is held in place by two connecting blocks inside a ceramic "boat." To generate ionization of hydrogen the filament is held at a few hundred volts negative with respect to ground, heated to white hot, and showered with hydrogen gas. Electrons emitted from the filamanet ionize the hydrogengas in the "boat." A thin ceramic cover with 1/8-inch apperature encapsulates the volume of the boat. Gas is fed into the boat through a small pipe. This permits a region of relatively high pressure near the filament while maintaining a good vacuum inside the dee and rest of the chamber.
The ceramic boat is mounted near the bottom of the cyclotron chamber, with it's appature located between the dee and the Dummy dee. Electrons and Ions emitted from the apperature are constrained to tightly spiral around the lines of magnetic field. This allows a column of ionization to occur in the center of the cyclotron, inside the gap between the dee and Dummy dee. Some of the ions created near the median plane are pulled into whichever dee happens to be negative and are started on there way to 1 Million Volts.
The hydrogen gas is admitted to the ion source "boat" via two possible paths. The first is a manually operated calibrated leak. The leak is backed by a high pressure regulator that takes the hydrogen from a five pound bottle at a few thousand PSI and brings it down to approximately 10 PSI. The pressure setting is empirically controlled by monitoring the vacuum ion gauge with the magnetic field off. Because the vacuum system is continually pumping on the chamber a minute flow of hydrogen occurs with the "boat" volume and the chamber volume pressure leveled off at some equilibrium.
The second possible path for the hydrogen to get into the ion source, is through a Mass Flow Controller (MFC). Essentially this is a computer controlled leak, allowing the operator to control the flow of hydrogen into the ion source from the computer controls. Or, a feedback loop can be engaged to maintain the flow rate at a desired rate unattended. The addition of the MFC has greatly increased the time available to the operator to give attention to other components of the cyclotron while running.
A safety feature of the hydrogen flow system was the addition of a solenoid valve Upstream of the leak and MFC. Since the valve is controlled by the computer, all the operator must due is actuate the solenoid valve to "turn on" and "off" the hydrogen supply. However, this valve automatically closes in any "warning status" of the machine, including power loss.
We have a bottle of Deuterium gas that can also serve as the ion supply, this will be investigated as the machine evolves, as the RF frequency will have to be retuned.
After successfully operating the new ion source for about 10 hours, it was decided to open the chamber for an inspection. During the run there was a momentary arcing as a result of too much gas. The break down occur between the dee and one of the filament leads. The arc was hot enough to melt the ceramic of the "boat" as can be seen in the following Fig.10:
Another interesting effect noticed came from the OFHC Copper block that held the filament. Apparently after running the ion source for a long while (greater than 2 hours) the copper blocks softened. One block sprayed liquid copper to the right - the beads seen below. The other block had an indentation where the copper was soften, indicating the liquid material also wanted to travel to the right. We are going to replace the OFHC blocks with the more temperature tolerant material Moly.
To avoid the problem of having the copper melt or sputter, we've replaced the copper blocks with Titanium. And to avoid other complications from pinching the filament with a small set screw, the Ti-blocks use a more sophisticated Ti-plate clamp, held in place by socket cap screws.