**8.3 The CusPIG confinement trap**

One of our first thoughts was to use a multicusp arrangement, the most obvious simple arrangement being a quadrupole magnetic field, oriented transverse to the beam breadth dimension, and extending uniformly down the arc chamber. However, we knew from prior work with multicusp trapping that it was far less efficient than a Penning trap, and fast electron densities in multicusp confinement were significantly lower than in a Penning trap, because of significant electron losses occurring at the cusps. So we looked for a way to block the four magnetic cusps with electrodes at cathode potential. An initial success with this arrangement led to a simplification: make the chamber walls at cathode potential, and place the anodes in a location where the magnetic shielding would be most efficient:

The arc chamber was an almost square section extruded in our first prototype to a length of 350 mm, but this could have been longer without any practical limit. At one end is an indirectly heated cathode, and at the other end is a wall, all the walls being at cathode potential. Magnetic poles run along the length of the arc chamber. The version shown in **Figure 9** uses permanent magnets, but during the development, we used electromagnets. In principle there are four poles, but one pole is cut away for the beam exit slit. This producs a distorted quadrupole magnetic field extending along the length of the arc chamber, with the field components entirely in the plane of the cross *DC Parallel Ribbon Ion Beams for High-Dose Processes DOI: http://dx.doi.org/10.5772/intechopen.111487*
