Since neutral hydrogen gas supplies the protons that will be accelerated, the hydrogen must undergo a process that will remove the electron and leave the positively charged proton behind. This is accomplished fairly easily by bombarding the hydrogen gas with low energy electrons. Located inside, close to the center of the top of the vacuum chamber the ion source is mounted on the face of the dummy DEE, see Plate 2. A W-Th-Ir filament is suspended between two electrical feed-throughs with spring loaded clamps at the tip. When approximately 7 amps flow through the filament it is heated to glow white hot. Since the filament is completely isolated from the chamber ground it can be negatively biased with respect to chamber ground causing thermionic emission to occur. A portion of the emitted electrons will pass vertically downward to the bottom of the chamber ionizing the hydrogen atoms present in their path. The electric fields of the ion source and accelerating RF sweep away the freed hydrogen electrons, leaving the massive protons behind. Further more, because of the very strong magnetic field parallel to the desired electron path, strong focusing occurs. Any electron that attempts to stray off of a vertical ascent or decent is immediately steered back towards the central axis of motion. Due to this corrective focusing, the electrons tend to oscillate back and forth in both X and Y while traveling downward in Z. Instead of following a linear path, the traversal then becomes a helical path with a very tight radius. The exposed filament is roughly 1 inch long, thereby producing a very thin sheet of electrons with a similar width of 1 inch. It was found that an optimum bias voltage of the filament was –320 Volts D.C.
Plate 2. Ion source filament arrangement
The thermionic emission rate is proportional to the heating of the filament. The heating of the filament is proportional to the square of the
Fig.9 Emission of test filament, bias -300VDC
applied current. Fig.9 shows the exponential emission of electrons in a test of the W-Th-Ir material. Hence slight changes in the filament current can produce great changes in thermionic emission. Ultimately changing the number of thermionic electrons available to ionize the hydrogen. In this way the cyclotron proton beam current can be controlled. As of yet the limiting factor in the maximum achievable beam current at the periphery is due to filament heating limitations.
Hydrogen gas is emitted to the chamber via a calibrated leak. The leak is backed by a high pressure regulator that takes the hydrogen from a lecture 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 chamber pressure leveled off at some equilibrium.
For pressures greater than 5E-5 Torr, and filament emission currents greater than 1 mA, a dramatic cathode ray appears. Plate 3 was taken through the view-port that looks down the accelerating gap. Electrons travel down from the filament along the magnetic field lines to the bottom of the chamber.
Plate 3. Ion Source Electron Beam.
It was found that the optimum hydrogen pressure was 5.5E-5 Torr. Pressures higher would decrease the collected beam due to the protons decreased mean free path, while pressures lower than optimum decreased the available hydrogen of which to create ions from.