\section{Electron Optics}
-The electron gun used for this study contains a total of ten electrodes, with six independently adjustable groups. Figure \ref{egun_simulation1.pdf} illustrates a cross section of the gun, using colour coding to indicate groups of electrodes which are kept at the same potential.
-
-
The important electrode groups are, in order from left to right:
\begin{enumerate}
- \item {\bf Wenhalt Cylindar}
+ \item {\bf Wenhalt Cylindar} $Vw$
- The first electrode, which houses the cathode, providing a narrow apparture for electrons to exit. A positive potential (of the order of $10V$ applied to the Wenhalt causes electrons leaving the cathode to be accelerated into a narrow beam.
- It is difficult to control the focusing properties of the gun using the Wenhalt alone; the main purpose of the Wenhalt is to create a high current, narrow beam of electrons, which can be focused by the other electrodes in the gun. If the potential applied to the Wenhalt is too high, electrons will be drawn into its surface. If the Wenhalt potential is too low, then fewer electrons are able to leave the aparture.
+ The first electrode, which houses the cathode, providing a narrow apparture for electrons to exit. A positive potential (of the order of $10V$) applied to the Wenhalt causes electrons leaving the cathode to be accelerated into a narrow beam.
- \item {\bf Einzel Lens }
+ The main purpose of the Wenhalt is to create a high current, narrow beam of electrons. If the potential applied to the Wenhalt is too high, electrons will be drawn into its surface. If the Wenhalt potential is too low, then fewer electrons are able to leave the aparture.
+
+ \item {\bf Accelerating Electrodes } $V_a$
+
The six central electrodes are an example of an Einzel lens, used for acceleration and focusing of the electron beam.
The first and last pair of electrodes are held at a large positive potential, causing electrons to accelerate. A smaller potential (often negative, but not necessarily) applied to the central pair of electrodes has the effect of altering the angular dispersion of the beam.
In preparation for Total Current Spectroscopy experiments, the effect of each of the controllable potentials was investigated by focusing the electron gun on its original flurescent screen. However, when repurposed for total current spectroscopy, the gun needed to be refocused several times (with changing sample holder design).
-The gun was focused using an iterative process, by which each potential was altered in turn to maximise the current.
-
\pagebreak
\subsection{A two dimensional electron gun simulation}
-The below figures \ref{egun_simulation1.pdf} and \ref{egun_simulation2.pdf} are the results of a simplistic electron gun simulation. The results of this simulation were not used to focus the actual electron gun; the images shown here are purely presented as a visual aid.
+The below figures \ref{egun_simulation1.pdf} and \ref{egun_simulation2.pdf} are the results of a simplistic electron gun simulation. The results of this simulation were not used to focus the actual electron gun; however Figure \ref{egun_simulation1.pdf} was extremely useful, as it shows the possibility for electrons to strike the insulating posts holding the gun together. When these posts were covered with Ta strips connected to the final electrode, the stability of the electron gun was improved.
-\begin{center}
+\begin{figure}[H]
+ \centering
\includegraphics[scale=0.45, angle=270]{figures/egun/egun_simulation1.pdf}
-
- \captionof{figure}{{\bf 2D Simulation of trajectories of electrons accelerated through an electron gun}}
+ \caption{Simulated electron trajectories}
\label{egun_simulation1.pdf}
+\end{figure}
+
+\begin{figure}[H]
+ \centering
\includegraphics[scale=0.45, angle=270]{figures/egun/egun_simulation2.pdf}
\captionof{figure}{{\bf 2D Simulation of the electrostatic potential produced by the electron gun}}\label{egun_simulation2.pdf}
-
-\end{center}
+\end{figure}
+
\pagebreak
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