Theory

The aim of this work is to find calibration coefficient of ball-pen probe. To get this coefficitent, we have to use Langmuir probe to construct IV characteristics (shot to shot method) and fit them by equation 1.

\[\alpha = \frac{U_{float}^{bpp} - U_{float}^{lp}}{T_{e}},\]

Where \(U_{float}^{bpp}\), \(U_{float}^{lp}\), \(T_{e}\) are parameters via we fit. Going through all the IV characteristics can we find, how calibration coefficient depends on magnetic field.

Experiment configuration

1. Gas pressure 0.36 -19.93

2. Working gas: hydrogen

3. Pre-ionization: Upper el. Gun

4. \(C_{Bt}\) capacitors charged to: 1300 V, triggered 5.0 ms

5. \(C_{CD}\) capacitors charged to: 400 V, triggered 5.0 ms

The numbers of shots, used for measurement are: \(\# 25483\) to \(\# 25490\) and \(\# 25493\) to \(\# 25498\) using voltage from -134.3 to 25.5 V. Both probes position from the vessel centre is r = 70 mm and resistance on LP was set as R = 20 Ohm.

Minutes of the experiment

1. The radial profile was meassured before meassurement to find best position of probes.

2. Resistance on LP was set to \(20 \ \Omega\)

3. Scan via \(V_{bias} = \{ -12.8, -25.6, -38.3, -51.1, -63.9, -76.7, -89.4, -102.2, -134.3, -12.8, 0, 12.8, 25.5 \} \ V\)

Data analysis

IV characteristics were using equation (1). All fits are included in .zip file. Experiment setup is given in Figure 1. The figure 2 shows that coefficient alpha is eqaual to \(\alpha = 2.2\ \pm\ 1.2\).

Reproducibillity

To get best results is it important to have good shots reproducibillity, which is shown in Figures 3, 4

These figures show that the reproducibillity could be better so it influences the results a lot.

Conclusion

To sum up, the ion saturation current is reach, because the negative voltage goes up to - 134.3, which seems to be enoght. The problem could be with reproducibillity, which is for some of the shots worse.

Fits

At the end of the document, all fits are included.