Tandem

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

There are many descriptions of the operation of a Van de Graaff available online, in textbooks and in the literature. An excellent reference recently added to the SBU Math/Physics Library is Electrostatic Accelerators: Fundamentals and Applications, edited by Hellborg.

In short, a Van de Graaff produces a high, dc voltage via having a constant current source deliver charge into an enormous resistance.

The constant current source is the charging belt, which mechanically transports charge from ground against the potential and delivers it to the high voltage terminal. In our FN-8, this belt is a chain of aluminum, stainless steel and plastic called a Laddertron. It is ~12 meters long, revolves at 12 meters per second and can deliver a total charge of 250 microAmps. Charge is induced on the links of the belt by exposure to a high voltage (up to 50 kV) power supply while the links are in contact with a grounded, conductive pulley. Each charged link is then moved away from the pulley and is now isolated, except for a very high resistance path back down the chain through the plastic insulators. The charged link is moved to the terminal where another conductive pulley receives the charge.

There are three paths for this charge to return to ground: with the beam, into the corona circuit (used for control, see below) and down the VdG support columns. These columns (locally referred to as High Energy and Low Energy) are split electrically into 200 planes. These planes are connected by 800 MegOhm resistor assemblies, creating a total column resistance of ~80 GigaOhms. If the total beam current is small (less than 1 microAmp) and the control circuit takes 20 microAmps, then 70 microAmps of Laddretron current will result in 80 x 10⁹ x 50 x 10^-6 = 4 Million Volts.

Voltage control is achieved with the Corona Circuit. Sharp needles are placed on a motor-driven rod and moved close enough to the terminal to draw ~20 microAmps of current. This current is allowed to flow to ground only through a vacuum triode; control of the grid voltage of that triode regulates the Corona Current and so regulates the terminal voltage. The grid voltage is controlled either by the Generating Voltmeter circuit, which compares the terminal voltage as measured by a GVM to a reference value, or by Beam (sometimes called slit) regulation. In this method the Tandem output beam is bent 90^o by the Analyzing magnet and allowed to hit a pair of slits. Most of the beam passes through but the tails of the current are intercepted, amplified and compared; a circuit then controls the triode grid to keep the slit currents equal.

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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