The ESTU (Extended Stretched TransUranium)
tandem accelerator at the A.W. Wright Nuclear Structure Laboratory,
Yale University is one of the largest operating electrostatic
accelerators world-wide.
How a tandem Van de Graaff works
A tandem accelerator like the one pictured here is designed to take
advantage of two very basic principles of electromagnetism: like
charges repel, and opposite charges attract. The massive piece of
equipment is, in principle, just a souped-up version of the van de Graaff generators
you probably remember from high school physics class. Things only get
complicated when you realize that accelerating heavy ions from rest to
about a million times the speed limit on your local highway (~ 10% of
the speed of light) is not an easy task. You need careful quality
control, the budget to pay off some fairly massive electricity bills,
and a few clever tricks.
The key is to accelerate the particles in stages. The first stage is
actually outside of the accelerator, in the 300 kV ion injector. The
ion injector is designed to feed negatively charged ions (produced in
various ways, depending on the beam) into the accelerator with a
relatively small initial velocity. These negative ions are injected
into small accelerator tubes. These tubes, which are kept under vacuum,
essentially serve as the path along which all the injected particles
must travel through the accelerator.
Upon entering the accelerator itself, the negative ions are
attracted to a positively charged terminal in the center of the
accelerator. The charge on this positive voltage terminal is built up
and maintained by Pelletron chains. Picture something like a bicycle
chain, with alternating metal and nylon links. The nylon isolates each
metal link, so the chain can function almost like a water wheel for
charge collection. For more information (and some colorful animations)
on the Pelletron acceleration design, visit the Pelletron site here.
The Pelletron chains maintain a consistent positive charge on the
central voltage terminal, in order to attract (and in the process)
accelerate the negative ions to the center of the accelerator.
In their travels from injection to center, the negative ions have
gained quite a bit of energy. But physicists are greedy, and always
seem to want more beam, at higher energies. And so, we introduce a
second stage (which is where the "tandem" part starts to make sense).
We send the negative ions through a stripper foil and knock off some
electrons, thus producing a positively charged ion.The number of
electrons stripped off mainly depends on the amount of energy the
negative ions have collected enroute to that central voltage terminal.
Now, we've created a positively charged ion right next to that
positively charged high voltage terminal, and as like charges repel,
those ions get another big electrostatic kick. Thus, the ions are then
sent flying out of the accelerator and are then piped into various
experimental areas with the use of large, specialized focusing magnets
and a bit of skill.
This explanation leaves out a lot of details. For more technical information about the accelerator, please visit our Specifications page.
