## 5 Process of Optimal Foil Width Determination and Results
### TP2 Results  
The calculation for TP2 was carried out to confirm the correctness of the selected function \(U_{\text{Smax}}=f(\bar{f}w)\), and thus the correctness of the shown approach to determine  
Figure 19: TPI, \(fw=6\) m: (**a**) potential above the protective foil, (**b**) potential under the protective foil, (**c**) potential difference. _fu_optimal. Both the geometry and the structure of the earth were different from those of TP1, as seen in Sections 3.1 and 3.2. Calculations of \(U_{\text{s}}\) for _fu_ values of 2, 3, 4, 5, 6, 7, and 8 m were made in order to determine function \(U_{\text{Smax}}=f(fu)\). Figure 20 shows the \(U_{\text{s}}\) for _fu_ values of 2, 4, 6, and 8 m.  
Similar to TP1, it can be seen for TP2 that the largest \(U_{\text{s}}\) is in the area next to the protective foil. The dependence of \(U_{\text{Smax}}\) on _fu_ is written in Table 17 and shown in Figure 21.  
As described in Section 5.1.2, parameters \(a\), \(b\), and \(c\) were calculated for functions F1 to F5 for TP2. Tables 18-22 show the results of parameter calculations of functions from F1 to F5. For each of the functions, three combinations of foil widths were used, namely the following: \(U_{\text{Smax}}\) for 2, 4, and 8 m; \(U_{\text{Smax}}\) for 2, 5, and 8 m; and \(U_{\text{Smax}}\) for 2, 6, and 8 m. For each combination, OF7 was calculated, and the functions were va \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**TP2** & **2** & **3** & **4** & **5** & **6** & **7** & **8** \\ \hline \(U_{\text{Smax}}\) (V) & 1737.7 & 1329.8 & 1108.4 & 954 & 849.1 & 760.8 & 702.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 17: Dependence of \(U_{\text{Smax}}\) on _fu_ for TP2. OF7 all = OF7248m + OF7258m + OF7268m. OF3 is also shown in the tables, which we expected to be zero or very close to zero. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Function F3**} & \multicolumn{3}{c}{**Used Foil Width**} \\ \cline{2-4}  & **2 m, 4 m, 8 m** & **2 m, 5 m, 8 m** & **2 m, 6 m, 8 m** \\ \hline OF three foil widths & 0 & 0 & 0 \\ \(a\) & 2546.86 & 2462.06 & 2374.51 \\ \(b\) & 1.4973 & 1.4606 & 1.4185 \\ \(c\) & 601.69 & 583.65 & 557.67 \\ \hline OF all seven foil widths & \(1.4916\times 10^{-3}\) & \(1.5242\times 10^{-3}\) & \(3.3845\times 10^{-3}\) & \(\sum\)6.4003 \(\times\) 10\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 20: Calculated parameters and OF for three foil widths and for all foil widths based on three foil widths for F3 using TP2 data. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Function F1**} & \multicolumn{3}{c}{**Used Foil Width**} \\ \cline{2-4}  & **2 m, 4 m, 8 m** & **2 m, 5 m, 8 m** & **2 m, 6 m, 8 m** \\ \hline OF three foil widths & 0 & 0 & 0 \\ \(a\) & 2651.2 & 2555.8 & 2479.7 \\ \(b\) & \(-527.8\) & \(-468.2\) & \(-420.6\) \\ \(c\) & 35.53 & 29.57 & 24.81 \\ \hline OF all seven foil widths & \(2.2198\times 10^{-2}\) & \(1.0145\times 10^{-2}\) & \(1.5177\times 10^{-2}\) & \(\sum\)4.7520 \(\times\) 10\({}^{-2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 18: Calculated parameters and OF for three foil widths and for all foil widths based on three foil widths for F1 using TP2 data. Figure 21: \(U_{\text{Smax}}\) as a function of _fv_ for TP2. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Function F1**} & \multicolumn{3}{c}{**Used Foil Width**} \\ \cline{2-4}  & **2 m, 4 m, 8 m** & **2 m, 5 m, 8 m** & **2 m, 6 m, 8 m** \\ \hline OF three foil widths & 0 & 0 & 0 \\ \(a\) & 2546.86 & 2462.06 & 2374.51 \\ \(b\) & 1.4973 & 1.4606 & 1.4185 \\ \(c\) & 601.69 & 583.65 & 557.67 \\ \hline OF all seven foil widths & \(1.4916\times 10^{-3}\) & \(1.5242\times 10^{-3}\) & \(3.3845\times 10^{-3}\) & \(\sum\)6.4003 \(\times\) 10\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 19: Calculated parameters and OF for three foil widths and for all foil widths based on three foil widths for F2 using TP2 data.