Frequently Asked Questions

1. Why use a CO2 laser?
   Most measurements found in literate suggest high charge state ions can be made with lasers producing pulses of a few nano-seconds in length and MegaWatts powers. The wavelength range seems to be restricted from about 400nm to 10 000nm. This would allow us to use neodymium, CO2 and iodine lasers. The pulse energy and rep-rate is best for CO2 and perhaps neodymium. We are concentrating on CO2 lasers.

2. Why do we need a new laser?
   Firstly, the present laser has too low a repetition rate (1 shot per 30 seconds) and upgrading it to 1Hz would not be possible. Secondly the pulse form of the laser is not very favourable, in particular the long tail, for which reason an Master Oscillator - Power Amplifier arrangemnt is chosen. Lastly, we need higher charge states for the final source, which means higher power density on target, which causes faster ions, which in turn need a longer extration distance and therefore we take a lower fraction of these ions and hence need more energy on target, hence the 100 Joules.

3. Why use Ta and Pb as targets?
   We are studying a heavy ion source for LHC. The current injector provides Lead ions, and this can be considered our principle goal. But we use Ta for most measurements as our present laser causes too much damage with to lead (due to its low melting point, and the low power tail of the laser pulse). The new laser has shown less damage to the target so should be useable with lead.

4. What about liquid and gas targets?
   The laser has to hit something at least 0.2mm in diameter. A liquid pool would be difficult as the pool would normally be parallel to the earth and hence the ions would travel upwards. Either bending the beam or building the accelerator vertically doesn't look very interesting. liquid drops are possible also, but technically rather challenging to make very reliable. Gas jets could be tried. For heavier ions for which we want high charge states, the recombination with the gas would be strong, and also the density of the gas must be at least the critical density for the laser (10¹⁹cm^-3 for CO2).

5. Why 80 or 110 kV for the source?
   This is a trade-off. We want the source potential as high as possible to reduce the space-charge effect in the ion beam. We want the source potential as low as possible to make running the source easy, to reduce the strength required in the focusing lenses and because the pre-accelerator becomes very long at higher energy. At the moment we cannot find the "correct" answer except we know that we must go higher than 60kV or the space-charge is too high for us to handle (known from experiments since 1996).

6. Why use an electrostatic focusing system?
   A magnetic focusing system (e.g. solenoids) has been tried and showed very high coupling of the the space-charge of different ion types, which lead to very high emittance growth. A benefit was the that not all the current was focused to the same point, making the dynamics of the accelerator easier. An electrostatic system keeps all the charge-states together and hence the space-charge field is more linear, but on the negative side all the current must be focused to the same point (we have ~100mA/cm² at the end of the focusing system).

7. Why not use space-charge compensation of the ion beam?
   Space-charge compensation is widely employed on ion source beam transport to limit the beam expansion due to space-charge. For positive ion beams, the ions impact on rest gas molecules and ionise them. The electrons are trapped in the ion beam potential well, while the molecular ions are ejected (relatively slowly). The result is a reduction in the beam charge.
   However it takes time to create enough electrons to compensate the beam, typically 50 us are required, while our beam length is 5 us. Secondly, the resulting high electron density and the high charge-states of the ions in the beam, lead to a strong recombination rate, which can result in up to half the ion losing the correct charge-state.