Electron beam control assembly for a scanning electron beam computed tomography scanner

An electrode for removing ions from an electron beam having a beam envelope and propagating along an axis, the electrode being of substantially cylindrical configuration and extending substantially co-axially with the electron beam and comprising:

• a pair of end rings and four lengthwise-extending cylinder sections each spanning predetermined arcs of the cylinder periphery;
• oppositely positioned first and second such cylinder sections being electrically isolated from one another and from the end rings and the third and fourth such sections;
• the end rings being electrically connected in common with the third and fourth cylinder sections;
• and the resulting electrically separate entities being connected to voltages of selected values to thereby provide highly effective removal of positive ions from the electron beam and at the same time provide a uniform electric field across the cross-section of the beam:
• envelope, the vertical electric field on the y-axis in the electrode being ##EQU7## wherein the y-axis is vertical, perpendicular to the axis of the electrode and passes through the centers of the first and second cylinder sections, y is the distance along the y-axis, V.sub.o is an ion-extraction voltage applied to the first or second cylinder section and R is the radius of the electrode.

An improved ion clearing electrode for extracting ions from an electron beam propagated within a vacuum chamber and for providing a uniform electric field across the cross-section of the beam, comprising:

• at least a first cylinder assembly extending substantially co-axially with the electron beam comprising four sections extending substantially the length thereof;
• a first pair of first and second of such sections defining substantially equal arcs of the cylinder cross-section on opposite sides thereof;
• a second pair of third and fourth of such sections lying on opposite sides of the cylinder axis and each spanning substantially equal arcs between the first pair of sections;
• and a pair of circular rings defining opposite ends of the cylinder assembly;
• the end rings forming an electrically common arrangement with the third and fourth cylinder sections, and said first and second sections being electrically isolated from one another and from said electrically common arrangement;
• the first and second sections and said electrically common arrangement being adapted to receive respective voltages to provide a substantially uniform electric field within said first cylinder assembly across the cross-section of the electron beam.

An improved ion clearing electrode for extracting ions from an electron beam propagated within a vacuum chamber and maintaining electric field uniformity across the beam, comprising:

• at least a first generally cylindrical ion clearing electrode structure substantially co-axial with the envelope of the electron beam, said first cylindrical structure comprising a pair of substantially circular end rings defining the opposite ends thereof;
• a pair of first and second cylinder sections substantially spanning the distance between the end rings and being substantially concentric with the associated end rings and defining substantially equal arcs;
• the end rings being electrically connected to a pair of cylinder center sections spanning the distance between the end rings on opposite sides of the beam axis and each spanning 60.degree. of the arc of the associated end rings and being substantially concentric with the associated end rings;
• and wherein the first section, the second section, and the electrically connected center sections and end rings are adapted for receiving respective predetermined voltages such that the vertical electric field E.sub.y on the y-axis in the ion clearing electrode is given by:
• ##EQU8## where the y-axis is vertical, perpendicular to the axis of the ion clearing electrode and passes through the centers of the first and second cylinder sections, y is the distance along the y-axis, R is the radius of the ion clearing electrode, and the magnitude of V.sub.o is selected for ion extraction, and wherein the voltages on (1) the first section, (2) the second section and (3) the center sections and end rings are respectively (1) system ground, (2) -V.sub.o and (3) -V.sub.o /2.

An improved ion clearing electrode assembly for extracting ions from an electron beam propagated within a vacuum chamber and for maintaining electric field uniformity across the beam, the electrode assembly comprising first and second cylinder assemblies substantially co-axial with the electron beam, each said cylinder assembly comprising at least four corresponding sections extending substantially the length of each said assembly;

• the first and second sections thereof lying on opposite sides of the cylinder and each spanning substantially the associated cylinder arc between third and fourth sections;
• the third and fourth such sections each defining approximately 60.degree. arcs of the cylinder on opposite sides thereof;
• and at least a pair of circular end rings defining opposite ends of the cylinder assembly;
• the end rings forming an electrically common arrangement with the third and fourth cylinder sections, and the first and second cylinder sections being electrically isolated from one another and from said electrically common arrangement;
• and wherein the first and second electrode sections and the electrically common arrangement are adapted for receiving respective predetermined voltages such that the vertical electric field E.sub.y on the y-axis (vertical axis) in the first cylinder assembly is given by:
• ##EQU9## wherein y is the distance along the y-axis;
• R is the radius of the ion clearing electrode;
• the magnitude of V.sub.o is selected for ion extraction;
• the vertical electrical field E.sub.y on the y-axis of the second cylinder assembly is given by the above expression, with the sign thereof being reversed;
• and wherein the predetermined voltages applied to said first cylinder assembly are
• ______________________________________ first section:
• system ground second section:
• -V.sub.o common assembly:
• -V.sub.o /2, ______________________________________
• and those applied to said second cylinder assembly are
• ______________________________________ first section:
• -V.sub.o second section:
• system ground common assembly:
• -V.sub.o /2. ______________________________________