
\chapter*{Electron Gun Control Circuit}

The control circuit diagram for the electron gun is shown in Figure \ref{electron_gun.pdf}. The wiring of the circuit, including resistors and potentiometers, was incoroprated into a single box, with external connections available for the power supplies, ammeters, electron gun, and sample holder. Both the components and operation of this circuit are straightforward; we will give a brief overview here for completeness.

\begin{itemize}
	\item {\bf Filament Heating} 
	A constant current power supply is used to heat the filament. The inability to directly attach a wire to the filament leads to the requirement for biasing resistors in parallel with the filament. If the resistors are equal valued, assuming that each half of the filament has equal resistance, it is trivial to show that the potential of the emitting tip of the cathode is equal to that at the midpoint of the resistors.

	\emph{NOTE: I suspect the periodically changing emission current may be due to temperature dependence of the resistance of the {\bf resistors}, since the current through the parallel circuit is constant, but not necessarily the filament if $R$ is not constant. However, I have not had time to test this.}

	\item {\bf Applied Potential}
	As discussed in Section \ref{tcs_theory1}, the energy of electrons arriving at the sample is proportional to $U$ (plus a constant).
	For this experiment, the power supply for setting $U$ has been chosen to allow for serial control using a Digital to Analogue Convertor (DAC). Refer to Appendix \ref{} for more information.

	\item {\bf Electrode Potentials}
	Seperate power supplies have been used for each independent electrode potential. The power supplies are biased to the cathode, rather than the sample; this ensures that changes in $U$ do not effect the optics of the gun.
	


	\begin{itemize}
		\item {\bf Deflection Plates}
	The deflection plates are referenced to the accelerating electrodes (connections not shown). By using a dual gang potentiometer, with one electrode wired in the opposite direction to the other, the deflection plates will always be at $\frac{V_d}{2} \pm V_a$.
	\end{itemize}

	\item {\bf Current Measurement}
	Three current measurement points are available:

	\begin{itemize}
	
		\item  {\bf Sample Current}
		This is the current measured during Total Current Spectroscopy experiments (see \ref{tcs}). The ammeter used for this measurement provides an analogue output signal; Appendix \ref{} discusses the use of Analogue to Digital Conversion for automating the Total Current Spectroscopy experiments.

		\item {\bf Primary Electron Current}
		By applying Kirchoff's law, it can be seen that the sum of currents passing through a gun electrode or the sample is equal to the current flowing through this measurement point. This measurement point was used to verify that the primary current was constant, as assumed by Total Current Spectroscopy theory. Although some variation in primary electron current was observed, this occured over an extremely long timescale compared to the timescales involved with sample current measurement.

		\item {\bf Leak Current}
		The third measurement point includes electrons which travel through the accelerating electrodes or deflection plates. Knowledge of the current lost through electrodes before the surface was useful for optimising the current incident on the surface.
	\end{itemize}
\end{itemize}
\begin{landscape}
\begin{center}
	\includegraphics[scale=0.80]{/home/sam/Documents/University/honours/thesis/figures/electron_gun.pdf}
	\captionof{figure}{Circuit Diagram for Electron Gun Control}
	\label{electron_gun.pdf}
\end{center}
\end{landscape}
