MAGIC simulation of ion extraction - ~500 mA

Below is shown a simulation of ion extraction performed by the MAGIC particle-in-cell code.

Details

5.0 A of both Pb25+ and electrons is emitted from the surface at the left of the simulation, which is biased at +80 kV. This plasma (which has a high directional velocity) drifts to the source outlet aperture in the centre. The next electrode is biased at -10 kV. The elctrons are held back by the fields and the ions are extracted.

As the extraction hole is 1.5 cm in radius and the emitter electrode is 5.0 cm in radius, around 500 mA should be extracted.

Results

Due to the high space charge field from the extracted ions, the electrons penetrate much further into the area between the two electrodes than would normally be the case. The plasma surface then changes the shape of the electric field distribution in the extraction gap, reducing the normal focusing field arrangement, to a strongly defocusing one.

Each images shown corresponds to about 50 ns of beam time.

We firstly see the the final ion beam extracted as a reasonably low divergence beam. This is due to the low space-charge that is it present at early times.

As the time continues the space-charge builds up and the plasma pushes further into the extraction region, producing a field distrribution which is far from a linear lens approximation. At this point the aberrations become high and the field distrubution defocus the ion beam. A highly divergent highly aberrated beam is then seen.

Many problems exist in this simulation. Apart from the uncertainty on many input parameters, we see some streaming of the ion flow in the plasma. Also the ions are being deflected in the plasma, which should be free of fields.

Below the animation, the phase-space diagram for after the extraction region is shown. The beam is clearly very highly aberrated and produces a very large emittance. The hole in the beam at the source exit seen at the end of the animation is shown on this phase-space diagram.

Animation of extraction
Go to 130 mA and 25 mA animations.

Phase space diagram
Go to 130 mA and 25 mA phase-space.

back to home... prepared by Richard Scrivens 21st January 1998