+ \item Wenhault
+
+ The Wenhault is a small cylindrical electrode which surrounds and houses the filament.
+
+ \item Einzel Lens
+
+ \item Deflection Plates
+
+ \item Final Electrode
+\end{enumerate}
+
+As discussed in Section \ref{}, the resolution of total current spectroscopy (and energy resolved secondary electron spectroscopy) is intrinsically linked to the distribution in energies of electrons arriving at the sample. Although the dist
+
+A two dimensional electron gun simulator has also been written in order to help produce figures for qualitative purposes; results of some simulations are shown in Figure \ref{}.
+
+
+
+
+\subsection*{Primary Energy Control and Current Measurement}
+
+In order to collect data on the large number of planned samples for the study, some form of automation was required. The automated system needed to be able to set the primary energy by adjusting the potential of the cathode relative to the sample, and simultaneously record the total current through the sample.
+
+The available power supplies at CAMSP featured analogue inputs for external control. This meant that a Digital to Analogue Convertor (DAC) card was needed to interface between the control computer and the power supply. In addition, the available instruments for current measurement produced analogue outputs. As a result, Analogue to Digital Convertors (ADCs) were required to automate the recording of total current.
+
+Although an external DAC/ADC box was already available for these purposes, initial tests showed that the ADCs on the box did not function. The decision was made to design and construct a custom DAC/ADC box, rather than wait up to two months for a commercial box to arrive. The design of the custom DAC/ADC box is discussed in detail in Appendix B, and the software written for the on-board microprocessor and the controlling computer are included in Appendix D.
+
+Figure \ref{block_diagram.pdf} shows a block diagram including all aspects of the Total Current Spectroscopy experiments. The emission current measurement point was included to allow for monitoring the behaviour of the filament, and confirm the assumptions of constant emission current. After the malfunctioning of one of the two available ammeters, it was only possible to measure sample current. However, earlier tests suggested that for short time periods (several minutes at most) the emission current's dependence upon time was negligable.
+
+\begin{center}
+ \includegraphics[scale=0.80]{figures/block_diagram}
+ \captionof{figure}{Block Diagram for TCS Experiments}
+ \label{block_diagram.pdf}
+\end{center}
+
+
+\begin{landscape}
+
+
+ \includegraphics[scale=0.85]{figures/electron_gun.pdf}
+ \captionof{figure}{Electron Gun and Control Circuit}
+ \label{electron_gun.pdf}
+
+
+\end{landscape}