**Figure 11.**

*The measured drop in local ionization rate as a function of distance from the cathode.*

20 m of actual travel). At 120 V the attenuation was 20% at a distance of around 1.5 m from the cathode. It should be pointed out that because of the cycloidal electron motion, the true mean free path is many times greater than this. But as a practical matter, with 100–120 V anode voltage, the non-uniformity would be about 20% for a 1500 mm broad beam, and if a cathode were placed at each end (which is easily done), the intrinsic non-uniformity would fall to well under 1% (**Figure 11**).

The spacing and profiles of the electrodes, and the suppression voltage, are modeled with OPERA/ELECTRA, and can be optimized for voltages from 2 keV to 60 keV with ease. **Figure 12** shows a 3 keV case. The beam is self space-charge neutralized, without the addition of neutralizing electrons from a gun. Great care was taken to avoid any unshielded connections to electrodes, etc., which should disrupt this neutralization. For the present we do not use a dynamically adjustable electrode gap, because the high precision required to optimize tuning along the full arc chamber length would be very mechanically challenging.

Ionizing collisions are not the only inelastic collisions that can occur. Neutral excitation and resulting subsequent light emission also occur. For this reason, viewing the glow of the ion beam can be useful but misleading, for the glow is generated by electron transitions resulting from charge-exchange and other collisions with residual gas, so the emitted light is mainly from excited neutral atoms, so some of the halo around the beam may come from excited neutrals which travel outside the beam before decaying.

Discharge uniformity can be improved by two methods. The first is simple to implement. The gas flow to the different gas ports can be adjusted in real time to adjust the uniformity of the extracted beam. Crude adjustment is particularly easy: the current on the suppression electrode is proportional to ions which strike the electrode ideally zero. The suppression current therefore measures beam de-tuning. Initially the
