Ion Source

We use a ​Peabody Scientific Duoplasmatron ion source to produce our H^- beam. The Source is a three element arc discharge device with high efficiency ionization. An arc discharge is focused by a somewhat conical intermediate electrode between a cathode and anode. An axial magnetic field constricts electrons from the cathode to spiral a longer path through the discharge, increasing the ionization efficiency. For negative ion beam production the intermediate electrode is offset from the anode aperture because the periphery of the arc discharge is rich in negative ions.

Wien Filter

Right after the extraction gap of the source is a Wien filter, or ExB filter, or charge/mass (q/m) filter, where crossed electrical and magnetic fields let only ions with a selectable ratio q/m pass. The ions are first focused by an Einzel lens (Einzel 1) into the Wien Filter, then after the Filter they are focused again by Einzel 2.

Source Setup

Source Maintenance

Filament Coating

Inside the source is a platinum mesh electron gun which starts a plasma. The mesh has a special coating which wears out after about 2-3 months of running (depending on amount of use). As time goes on and the coating wears out the source will need more arc current and source gas (H2) to maintain beam magnitude. When the coating is really worn out you will not be able to start a plasma. Renewing the filament coating is usually referred to as "re-dipping the filament". The source is vented up to room air pressure (make sure to turn off the hot ion guage attached to the source turbo pump before!). Then the Faraday Cage is removed and the Filament element is unscrewed from the back. When this is removed the filament should have its resistance tested to make sure that it is not corroded. If there is significant resistance, then it is time to make a new filament. The filment re-dipping should be done in a well ventilated area due to the fumes from the dip. The filament will simply be briefly put into the coating dip, then heated with a heat gun to dry. This should be repeated about 3 times. After the filament has dried it can be put back into the source. The source will take some time (about 12 hours) until it is able to get back to typical base pressures of order of 10^-7 Torr.

Water Filter Change

Another paramater to keep on eye on is the current drawn by the primary electrode. If the power source of the primary electrode (labeled Beam Energy) is over 1.2 mA, then this could be a sign that the cooling water has too many impurites and has a decreased electrical resistance. The water must be changed and possibly the de-magnetizing filter. The filter can be obtained by contacting Peabody Scientific (Frank Schmurra: 978.535.0444). The filter is a bag of beads in a sealed cloth bag. DO NOT OPEN THE BAG!!! The bag must be intact when it is put into the system. Go to the blue-grey box under the bender and open it. Be careful, there is electric equipment everyhwere, shut off all close by electic equipment, the turbo and rough pump are ok to leave on.Inside is the cooling system for the source. Using a pan, drain the water out of the system from the bottom of the PVC tube. Make sure that you disconnect the water circuit that feeds water through the ion source and not the water coming from the chiller in the other room. There is a clear plastic cylindrical filter holder, unscrew the outside cap carefully so to not spill the water in it on top of the motor that pumps the water through the cooling lines. The old filter bag is sitting in this piece in a mesh basket. Simply take out the old one, and place the new filter inside the mesh. The old one can be thrown out with the regular trash. Make sure that the rubber gasket is in place and screw the filter holder back on. Re-connect the water pipes and add distilled water in the PVC pipe top , it should only take 0.5 to 0.75 gallons (2-3 Liters) at most. There is a flow meter interlock in this cooling system, the source power will shut off if it does not detect fluid flowing. When turning the cooling system on, hold down the power button for a minute or so, this makes sure all the air pockets get pushed out of the system which could have shut down the power.