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 - 16 mPa

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: 350 V, triggered 5.0 ms

The numbers of shots, used for measurement are: \(\# 25709\) to \(\# 25723\) and \(\# 25699\) to \(\# 25705\) using voltage from -115.3 to 44.7 V. Another shots used for calculating alpha at last time intervals \(\# 25981\) to \(\# 25983\). 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 possition of both probes is set to 70 mm.

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

3. Scan via \(V_{bias} = \{ -115.3, 102,4, -89.7, , -83.1-76.8, -70.4, -64.1, -57.5, \\ -51.2, -44.7, -38.4, -34.9, -25.6, -19.2, -12.8, 0, 44.7, 38.4, 31.9, 25.6, 19.2, 12.8 \} \ V\)

4. Altough the value of preassure was set to 20 mPa again, the pressure was between 14 and 16 mPa during all the shots.

5. To determine callibration coefficient during last time intervals (last 5.5 ms), we had to use difference between probes potentials with no voltage on LP, divided by electron temperature.

Data analysis

IV characteristics were using equation (1). All fits are included in .zip file. To find coefficient in last time intervals (last 5.5 ms), we had to make another shots and determine alpha, as difference of both probes potentials divided by electron temperuture, which was determined by fitting method as 15 eV, Figure 5.. Experiment setup is given in Figure 1. The figure 2 shows that coefficient alpha is eqaual to \(\alpha = 2.6\ \pm\ 0.7\).

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. There are instabilities, which might influence the measurement.

Conclusion

To sum up, in previous measurement we have found out that negative voltage of -115.3 V is enough to reach ion saturation current. The problem could be with reproducibillity, which is for some of the shots worse and instabilities can be seen.