Particles sent to the tandem accelerator from the ion source are accelerated at a high voltage of 5MV to 14% of the speed of light. This is equal to the speed traveling around the earth in a second. In Irradiation Room 1, accelerated particles are used for analyzing the elemental composition of solids such as metals. In Irradiation Room 2, they are used for research works such as ion-injection into metal experiments. The tandem accelerator also serves as the ion-injector for the synchrotron accelerator, so that accelerated protons are sent to the synchrotron accelerator.

Tandem accelerator is a type of accelerators to accelerate ions by applying an electrostatic field with a feature of supplying high voltage in the middle of accelerating tube. It can accelerate ions in two steps: first, accelerating negative ions and second, further accelerating positive ions after transforming negative ions into positive in the high-voltage terminal.

Positive ions transformed from injected monovalent negative ions to the charge q are accelerated by the terminal voltage V [MV] to the energy level given by the following formula.

　　　　　　　　　　　　E=(q+1)V [MeV]　

This means that particles can be accelerated to the higher energy by charge conversion during the acceleration.

Tandem accelerator achieves a two-step acceleration with a single voltage by converting the electric charge of a negative ion source on the high-voltage terminal. Applying a positive ion source means placing an ion source on the high-voltage terminal, which makes the maintenance work very difficult. A negative ion is generally more difficult to produce than a positive ion, but the advantages outweigh the difficulties.

As for the high voltage supplying equipment to the high-voltage terminal, Van de Graaff, Cockcroft, etc. are available. The tandem accelerator of W-MAST has adopted Schenkel circuit which makes it possible to accelerate high-current beams.