Due to problems encountered during our first run it seemed desirable to revisit the focusing tube design. There are two primary design constraints, minimize the field gradient at all points on the proton paddle (hopefully reducing field emission), and optimize the cell geometry for proton acceptance. To easily address both goals I have used an electrostatic tracking program (Simion) and realistic beam protons (5000 each run from Alejandro's Monte Carlo) to compare a variety of designs. The figures of merit used were the percentage of protons to hit the detector and the field gradient at the edge of the focusing tube. The plot to the right shows the equipotential surfaces and a few typical trajectories for the current design. What you see is a slice through the center of one of the focusing cells. This plot makes it very clear where the highest gradient regions are.
The result of these simulations is that the new design will use thicker walled tube (allowing for an increased radius), and the length of the foucusing tube will be slightly decreased
Raised 1 mm
Lowered 1 mm
Lowered 2 mm
82.2% at -25 kV
76.7% at -25 kV
82.8% at -25 kV
82.4% at -25 kV
83.0% at -25 kV
82.7% at -25 kV
78.5% at -25 kV
MAX FIELD GRADIENT
One should note that the differences between the results I show here and Eric's are primarily due to acceleration voltage. I have run at a variety of voltages and my results match Eric's fairly closely (e.g 88.4% versus ~91% at -35 kV). Note that my results are based on a realistic proton energy distribution which is peaked at the higher energy end of the spectrum. I would attribute the remaining difference to slight changes in geometry. Anyway, my feeling is that with the proposed redesign we will be able to completely eliminate the field emission. Therefore the extra 5% is worth going for. Particularly if we can achieve it without raising the acceleration voltage which is constrained by field emission from the edges of the Quansit Hut. This part of the old design we have little control over, and thus I'd like to run at as low a voltage as possible. If it turns out that this design doesn't solve the problem (We should know in a few weeks) it is trivial to make a plate with no tubes.
Mark has produced a list of efficiencies as a function of misalignment.
|And here is a bit of additional information which you may find interesting.|
The following two plots show the angle at which the proton impacts the surface
of the detector or Q-hut. Zero degrees and negative ninety are perpendicular to the detector surface
for the elevation and azimuthal angles respectively. The three designs are compared.
The next two plots show the same information as above but for just the
'No tube' design and at three acceleration voltages.
The following plots show histograms of the number of proton hits relative to the
current design. Red is more hits while blue is fewer hits. The viewer is above the
groundplane looking down at the focusing tube and detector. In both plots one can see
partial rings where the protons hit the focusing tube. These plots demonstrate that the primary advantage
to the wider design is that it simply moves some of the metal out of the proton trajectory.
|Plot Showing the result of decreasing the inner diameter||Plot Showing the result of increasing the outer diameter|