**8.1 Background**

During and after the 1990s, almost all companies used ion sources based on the Bernas source with enhancements by White of a passive anticathode electrode at the opposite end of the arc chamber from the hot filament cathode, both being identically biased at about 60 to 120 V. This was later improved by Horsky [18], who replaced the hot filament with an indirectly heated cathode, (IHC) a block of tungsten with a hollow interior, heated by electrons from an internal hot filament, biased several hundred volts negative. This has a much greater service life, typically 500 hours, than a White source (150 hours) or a Freeman source (60 hours). I refer to all these variants as HC PIG (Penning Ionization Gauge) discharge sources. Early reports attributed little benefit to this HC PIG arrangement, but White and Westner found that when running BF3 gas, the fast electron density could be significantly higher, and since B<sup>+</sup> ions are generated from BF2 <sup>+</sup> ionic molecules, which in turn are produced from BF3 gas, the higher electron density results in a much higher fraction of B<sup>+</sup> .

In an HC PIG source, the locally applied magnetic field confines the fast primary electrons from the hot cathode radially, so they cannot reach the chamber walls at anode potential, and the cathode/anticathode electrodes repel the electrons from the ends. The result is the efficient creation of Penning Trap, in which fast electrons with energies between about 75 and 140 eV, depending on the cathode voltage, are confined. These efficiently ionize gas or vapor molecules within this zone.

The cross section for elastic scattering rises dramatically as the energy of electrons is reduced; as a result, those electrons which excite or ionize the gas, losing significant kinetic energy, then interact more strongly with each other through elastic collisions, and form a relatively cool plasma. The cooler electrons can diffuse quite readily across the magnetic field—it is only effective at confining the faster electrons, and so a cool plasma forms in thermal equilibrium at a temperature of 4–8 eV, and following classical plasma laws reaches an equilibrium voltage positive with respect to the potential of the anode walls (**Figure 8**).

Now consider the problems of developing much larger ribbon beams, with sizes from 450 mm to say 2 m. With a much longer ion source, much higher beam currents could be obtained. The linear current density at the ion source which HC PIG sources produce is over 10 mA per cm, but they cannot be scaled up in their present form. The 400 mm Calutron sources were immersed in a gigantic uniform magnetic field; this approach is possible but completely uneconomic.
