Discussion

The first step is to compare the old and new source design by taking iron particles. The data cover the same charge and velocity ranges, but with the new source even higher values were found [8]. Comparing the data at the test bench (Table 2), one can conclude that the average velocities rise with an increasing pulse amplitude. A higher potential difference between the tungsten needle and the reservoir causes a higher charge to mass

Figure 3. The new dust source at the test bench (in the middle of the picture). The high voltage ports are shown on the backside of the source. At the right: Voltmeter for the needle potential. At the left side from the source one can see the collimation section of the beamline and a high vacuum valve.

Figure 3. The new dust source at the test bench (in the middle of the picture). The high voltage ports are shown on the backside of the source. At the right: Voltmeter for the needle potential. At the left side from the source one can see the collimation section of the beamline and a high vacuum valve.

ratio. Since the total surface potentials of the particles are varying between 50 and 500 V, independent of their grainsize, smaller particles reach a higher charge to mass ratio and also higher velocities. Silver and copper are showing the highest fieldstrengths for

1 pm sized particles. Therefore they are the best chargeable materials followed by iron, carbon and latex. But due to their lower density latex and carbon obtain a higher charge to mass ratio and reach even higher velocities.

Extrapolating the test bench data of the latex samples to an acceleration voltage of

2 MV, and comparing these data with results from Canterbury [10] we obtain a lower top speed, for the 1.6 and the 2.1 ^m particles (« 7 and 4.5 km s_1 at Canterbury). The top speed of the 0.5 pm sample 12km s_1 at Canterbury) is comparable to our extrapolated results. Using the maximum pulse amplitude even faster particles can be expected in Heidelberg.

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