A simple faraday collector was incorporated to initially detect the accelerated ions. The principle of operation is simple: as a high energy proton hits a brass target it gives up its energy in the form a X-rays and thermal heating. The proton lodges itself into the brass, thereby implanting positive charge. If available the proton will collect an electron and neutralize again. If the brass target is connected to a source of electrons, such as ground, it is possible to count the number of incident protons by measuring the electron current that arises. An electrometer is placed in between the collector and ground making it possible to resolve picoamps of electron current. A consideration in beam current measurements is the creation of secondary electrons at the target from the incident high energy protons. ‘Removal’ of negative charge looks the same as addition of positive charge, potentially producing erroneously high current measurements. A small positive bias can be placed on the collector to suppress secondary electrons, but care must be taken not to deflect incident protons by too high of a positive bias.
Plate 4. Ion Collector – at minimal insertion
This faraday collector was constructed from a 3/8 inch brass slug and is suspended as well as isolated in a coaxial arrangement by a Teflon spacer inside a ˝ inch hollow copper cylinder. The copper cylinder forms an RF shielded housing for the brass slug. The copper cylinder has a 0.185 inch slit diametrically traversing one half of the hollow portion near the tip. This slit exposes the brass slug centered inside and is positioned such that it is only exposed to positively accelerated ions, while any negative ions hit the grounded RF housing. The current signal leaves the faraday collector by a short length of RG-174 coaxial wire. The RG-174 then connects to a coaxial vacuum feed through. Outside of the chamber a 15 foot length of RG-174 completes the connection to a Keithly 610CR electrometer. The recorder output of the 610CR was measured by a five digit Keithly DVM whose data is read out via HPIB. It was noted with a bias supply inserted in the electrometer line that there were minimal secondary electrons escaping.
The faraday collector assembly is mounted on a vacuum tight linear motion feed through. It is mounted such that the collector can be inserted radialy with a two inch travel, effectively determining the maximum ion radius. The minimum measurable ion radius, maximum insertion of the collector is 2.50 inches while the maximum ion radius, minimum collector insertion is 4.50 inches. A plot of beam current against radius, Fig.10, shows that the beam current linearly drops off as the radius grows.
Fig.10 Radial Beam Current Profile./center>
The second ion collector, in the form of a phosphorescent screen also located at the end of a linear positioner was inserted into the beam region to locate the beam. Indeed, brilliant illumination occurred where the beam was incident. However, after a short period of ion bombardment the luminescence ceased. This is due to the charging of the screen, the strong electric field that developed deflected the incident proton beam off target. The screen charging issue was resolved by sputtering approximately 50 Angstroms of gold over all of it’s surfaces and ensuring a connection to ground. Such a thin layer of metal is almost completely transparent yet conductive. After metallization the beam indeed re-appeared and remained on the screen without any deflection, Plate5.
Plate5. Beam visual on flag.