4. Vacuum Skimmers etc.
1. Vacuum pumping requirements:
The vacuum pump size required for proper operation depends on several parameters:
1. The pressures used in the valve (scales linearly with the pressure).
2. The gas used in the valve (Helium because of it low viscosity will flow more than Ar, expect a factor of 5).
3. The Temperature at which you operate the valve (Gas density in the valve is higher at low temperature).
4. The current you use to drive the valve (adjustable).
5. The repetition rate of the valve operation.
6. The nozzle diameter used.
The load on the pump is given here for a specific combination of these parameters: Scaling to other parameter values is obvious.
Gas used is Helium at 50 Bars and at room temperature (300K).
Our standard conical nozzle has a diameter of 0.15 mm.
Output in each gas pulse is 10^16 atoms or (3*10^(-4) Liter*mbar).
At a rate of 10 pulses per second this gas load is 3*10(-3) Liter*mbar.
A pump of 300 liter/sec will maintain a pressure of 10^(-5) mbar. At background pressures higher than that, beam attenuation will be noticed as the beam propagates on its way to the skimmer.
Operating the valve at high repetition rate (say 1000Hz) requires a pumping capacity of 30,000L/S!!
of course we can lower the required pumping capacity by reducing the nozzle opening, the operating pressure or the nature of the carrier gas (say Neon instead of He).
2. Beam propagation to the skimmer:
The beam produced by the nozzle can be attenuated by the background gas remaining in the source chamber between pulses. We found that a pressure of 10^(-4) mbar will attenuate the beam to half it intensity over a distance of 100mm. This requirement dictates the pumping speed required when skimmed beams are used. If working near the nozzle exit (as is the case when experimenting with High Harmonic Generation) the pumping requirement can be relaxed.
3. Beam spread from a conical nozzle.
Extensive simulations and measurements went into the nozzle cone design. the following are some results:
This is a simulation of a gas expanding through a 40 degree nozzle. The model allows us to simulate a starting pressure of 0.1 Bar only. The angular spread of the beam is 20 degrees. We expect a narrower beam spread when the pressure is 100 Bars and higher beam number densities..
Glowing Neon beam emerging from the 40 degrees conical nozzle. The Neon is made glowing by an electrical discharge inside the nozzle. Beam spread is only 10 degrees (Full angle at half maximum).
4. Skimmer shape, placement and entrance hole.
The high on-axis beam intensity produce by our valves requires changes in skimmer design:
I. Skimmer entrance hole has to be large.
The transmission of a simple conical skimmer as a function of it entrance hole diameter and various gas densities (i.e. distance from nozzle). A skimmer opening of 3 0r 4 mm is required to pass the beam through the skimmer. The transmission of high density beam through a small skimmer can be dismally small (less than 1%).
II. Skimmers have to be placed at a distance of 1000 nozzle diameters from the valve (100-200 mm.) The gas density at the skimmer produced by various nozzle cones is shown here. The maximum on axis density is achieved for our chosen 40 degrees nozzles.
Simulation of cold He gas flowing through a skimmer.
This density plot shows the shock wave generated near the skimmer outer surface. The skimmer does not act a scalpel slicing the beam at its entrance. The beam is expanding after going through the skimmer entrance, indicating a heating of the gas in its passage. If the skimmer is placed near the nozzle, or the skimmer hole is too small, this spread will be enhanced and cause a significant loss of the on axis beam intensity.
Temperature map (on a logarithmic scale) of a cold Helium flow over a narrow (25 degree) conical skimmer. A warm gas plug is formed near the entrance of the skimmer. This heated gas can cause the beam to spread after its passage through the skimmer.