Difference between revisions of "Ion Source"

Revision as of 12:38, 6 September 2010

Background

The ion source we will be using is a General Ionex 860A Inverted Sputter negative ion source. We use negative ions so that we can benefit from (ie. accelerate with) the voltage of the Tandem Van de Graaff twice. Sources of this type have been commercially available for about 25 years and are still sold by National Electrostatics Corporation (the SNICS), High Voltage Engineering Europa (the 860A and C and Peabody Scientific (the PS-120. You should be familiar with all of the components of the ion source, understand what they can do and how you can use them to help you. In addition, all of these components are controlled with [LabVIEW], so you should develop a basic understanding of how LabVIEW works and how to use it.

The Cesium Boiler

Warning: Cesium is a metal in group 1 of the periodic table. This means that like the rest of the group 1 metals (Li, Na, K), it reacts violently with water. So the cesium metal is stored in an airtight glass vial to protect it from oxidizing with water vapor in the air. To load the cesium boiler, the vial must be broken in a dry box, and care must be taken in washing unreacted cesium out of the boiler's storage boat.

The cesium boiler is simply an oven that stores and vaporizes cesium metal. Its temperature is controlled through the current on the boiler power supply.

The Ionizer

After the cesium has been evaporated, it condenses in a thin film on the ionizer. This component's function is twofold; it is heated to give Cs atoms enough energy to throw off an electron and ionize to Cs⁺, and it is held at a potential to ensure that these positive particles are focused onto the cathode and sample cone.

The Cathode

The cathode is held at about -15 kV, attracting the Cs⁺ ions and impacting them into the sample cone (which contains titanium infused with hydrogen). Negative H^-ions "splash" out of the sample cone; due to the shapes of the potential surfaces defined by the cathode and ionizer potentials, these ions are focused into a beam, while other extraneous ions (including the Cs⁺ ions) are scattered.

The Extraction Electrode

The extraction electrode is held at about 15 kV and takes the beam of H^- ions and accelerates them through a series of optical components (Y-Steerer, einzel lens, GS Triplet, LE Steerer). The effectiveness of these beam optics can be judged by measuring the beam current at the LE Faraday cup.

Operation

Preparation

Before one operates the ion source, there are checks to perform. Only then can one switch the AC power for the ion source table from "wall power" to the motor-generator set. Typically students will do these steps under the direct supervision of the accelerator staff.

It is wise to check that the LabView control system is running before taking further steps. Use the Measurement and Automation Explorer (MAX) to see that all of the satellite computers are on and responding. If a machine does not respond, try a soft re-boot using MAX. If this fails, it may be necessary to do a hard re-boot at the machine.

• Checks: DI water, Vacuum, Gates
• Isolating vacuum and preparing Turbo to be off
• Switching AC Power source
• Restoring vacuum
• Turning on power supplies at Table, Magnet and Source stations
• Closing the Cage
• Opening LE Tube Valve
• Turning on AC power for the Cathode, Extraction and Glassman (table/injection/pre-accel) power supplies

Start-up

• Ensure that the ion source drive shaft is on, vacuum is good, power supplies are on, gate is closed, LE Tube valve is open
• Ensure that the control system is running. On MasterRT.vi front panel do the rest.
• Ensure that the LE Faraday Cup is in (Green, as of 8/21/2010) and that the selector switch sends that current to the electrometer
• Ensure that the Extraction Voltage is ~15 kV, the Inflection Magnet is near a useful ion mass (1, 12 or 16 are good choices).
• Raise the ionizer voltage to 8V gradually over a few minutes. Do not exceed 22 Amps. You will observe the heating of the Tantalum by an increase in the resistance of the ionizer. OK, at the very end you may exceed 22 amps if required to reach 8V.
• Set other parameters to likely values:
  • Einzel Lens to 3 kV
  • Variable Steerer to same as Fixed Steerer
  • Ground Station Triplet X to 1.1KV and Y to 1.2KV
  • LE Steerer H to -700V and V to +1100 V (note, these units are bogus as of 8/2010)
• Once the ionizer has 8V (~175 Watts) raise the Cathode Voltage slowly. If you see rapid changes to the Extraction Voltage/current or cathode current or a decrease in the ionizer current there may be sparking. In this case, lower the cathode voltage below the sparking point and wait. Increase slowly. A good first value will be 2 kV.

Tuning

• Observe the LE Faraday Cup current on the electrometer. Note that it should be negative. Recently we have found that one must wait 15 minutes for current to appear: Don't Panic. Once current appears you can tune all of the above mentioned parameters (not the ionizer, however!).
  • If there is no current, note your starting value and scan the Inflection magnet. Hint: mass 16 will be present for any sample unless there is something wrong with the set-up.

Troubleshooting

If the beam cannot be found or has a low beam current:

• The cesium boiler can be "pushed"; more current means a higher temperature, which means more vaporized Cs, which means more ionized Cs⁺, which means more of a "splash" from the target.
• The ionizer may not be working as expected; if Cs is not ionized it will not be focused effectively, and the sample will be relatively untouched.

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