Difference between revisions of "Tandem"

Revision as of 12:33, 5 October 2010

Background

The particle accelerator we will be using is FN-8, the eighth ""King Tandem"" built by the High Voltage Engineering Corporation. High Voltage was founded by Robert Van de Graaff and partners as the first commercial manufacturer of Van de Graaff generators. Our FN was built in 1966 and first brought into operation in 1968. The original specification was for 7.5 Million Volts with beam; our FN attained over 10 MV without beam (or accelerator tubes) and has run for experiments at 9.8 MV. It will still reach the original spec. today but the maintenance staff will limit operation to below 8.5MV out of simple fear.

Van de Graaffs are still being produced today by two manufacturers: National Electrostatic Corporation and High Voltage Engineering Europa. The primary uses for VdG's are Accelerator Mass Spectrometry, materials characterization and modification and nuclear physics. HVEE is a descendant of the original HVEC, along with Vivirad. Lists of current installations can be found on the manufacturers websites.

Theory of Operation

Operation

Preparation

The Tandem drive motor is interlocked to several systems to prevent danger to operators and equipment. These are all satisfied under normal conditions; one usually finds a problem when the interlock acts by preventing the Drive Motor from starting. They include

• a flow switch on the Cooling Water to the coils inside the High Energy end;
• switches on each of the 4 access ports;
• relays in the Low and High Energy Vacuum gauges; these are set to ~5 x 10^-6 Torr

Start-up

• Go to the High Energy end of the tandem
• Ensure that the High Energy vacuum is ~1 x 10(-7) Torr and that the Laddertron tension is ~900 lbs
• Plug in the start switch. "Jog" the belt by pressing Start and then quickly pressing Stop. Ensure that the AC Monitor responds, the Vibration Monitor responds, the Tension Monitor briefly shows lower tension and that the sound of the belt is ... right.
• Start the Drive Motor again. The sound should go up in pitch, stabilize and have a 1 Hz hum. The AC Monitor should show ~118 Vac on all three phases. The Vibration Monitor should show ~1.5 units (the units are actually mm/sec). The tension should fall to ~600lbs.
• If any of the above values are far from nominal, please seek assistance.

Taking the Tandem to Voltage

At present, raising and setting the Tandem voltage involves a mixture of analog controls and the Master Control program in LabView. Proceed to the Control Room and you will learn the latest version. As of October 2010 the procedure is:

• Raise the Base Charging potentiometer to 2.2 turns. This will raise the base charging supply to voltage and result in positive charge travelling up the chain to the terminal. The Laddertron charging ammeter should show ~35 microAmps.
• Raise the Terminal Charging potentiometer to 2.0 turns. This will raise the terminal charging supply to voltage and result in negative charge travelling down the chain from the terminal. The Laddertron charging ammeter should show ~70 microAmps (35+35).
• Select the Tandem tab in on the front panel of MasterRT.vi. Observe that the displayed voltage will be approximately 3 MV. Use the control to set a voltage near 3MV. You will observe the control signal change on the console oscilloscope and the grid and control currents respond. The Tandem should reach the new reference voltage in 1-2 seconds.
• The desired voltage can be found through calculation from the desired energy. An online utility can be found at CASE Experiment Manager

Troubleshooting

• Seek expert assistance!

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