Anatomy of an MST shot
- The five capacitor banks behind the wall are charged from 2000 to 5000 Volts by the main power supply.
- A puff of deuterium gas is squirted into the chamber by the puff valves.
- A handful of loose electrons are emitted into the chamber to provide some electrical conduction.
- Current is driven into the coils around the large iron core that threads through the center of MST. This fills the core with a magnetic flux that forces the iron in the core to the bottom of its hysteresis curve. This 'cocks' the iron core to maximize the amount of flux the primary power supply can later inject before saturating the core.
- One of the capacitor banks fires driving a current through the shell of MST, creating a toroidal magnetic field inside the vessel. The rapidly rising toroidal magnetic field generates a poloidal electric field via Faradays Law driving current in the poloidal direction. The plasma starts to form.
- The other four capacitor banks fire into the coils around the large iron core, driving up to half a million amps of current in the toroidal direction in the plasma. By comparison, a 100 Watt light bulb draws 1 amp. The gas is fully ionized and we have a plasma.
- The bank driving the toroidal field poops out. A supplementary supply, called the crowbar bank, kicks in aiding field reversal at the plasma edge.
- The puff valves continue to squirt deuterium gas into the edge of the plasma throughout the shot.
- The plasma is quenched when the large iron core saturates and the toroidal current drive decays.
The entire process takes about 100 msec.
The MST Power Supplies
The main component of the power supply that feeds MST is the four floors of suitcase sized capacitors that are slowly charged to as much as 5000 Volts by the main supply. Each one of these capacitors holds about 4000 Joules of energy when fully charged. There are nearly a thousand of them behind that wall.
They are all dumped at once in a 20 MegaWatt pulse. Thats enough juice to light up a small town (very briefly.)
They and the large coils you see attached to the walls form a pulse forming network. The PFN generates a pulse that ramps up quickly, supplies a steady current for about 40 msec, then drops quickly.
The loud 'WOMP' you hear during each shot comes from the coils of the PFN reacting to thousands of amps of current being dumped into them so quickly. The current in the windings crosses the magnetic flux in the coil pulling the windings tighter. Each acts like an enormous stereo speaker, scaring the bejeezus out of unsuspecting students as they walk by.
The deuterium gas that becomes the plasma is injected into the vessel by an array of puff valves. The valves can turn on and off in milliseconds giving the operator a great deal of control over the density of the plasma throughout the shot.
In order to apply the needed voltages at the edge of the plasma there are gaps in the vacuum vessel that holds the plasma. These gaps spoil the symmetry of MST, causing the magnetic field to bulge outward. This distortion allows plasma particles streaming along the magnetic field lines to find their way to the wall, causing wall damage and poor confinement. The feedback coil system pushes the bulging magnetic field lines back where they belong.
An array of 32 sense coils is mounted inside MST to monitor the magnetic field bulging into the gap. This information is sent to a set of 40 amplifiers; each amplifier can produce about 250,000 watts. Each amplifier is connected to a drive coil. The amplifier sends just the right current to the drive coil to cancel out the field bulge at that location. Each unit is a stereo amplifier rated at a quarter of a million watts.
Error correction is a recent addition to MST and has been a rousing success, reducing field errors to immeasurable levels.