Cathode ray tube having an electron gun constructed for ready refocusing of the electron beam

Apparatus including a cathode ray tube, comprising:

• (a) a hermetically sealed envelope having a pair of opposite ends and a central axis extending therebetween;
• (b) a target at one end of the envelope;
• (c) a cathode at the other end of the envelope for emitting a beam of electrons;
• (d) a control electrode disposed between the cathode and the target for directing the beam of electrons from the cathode toward the target along the central axis, the control electrode causing the electron beam to focus at a crossover point on the central axis which point is subject to change along the central axis with a variation in a potential on the control electrode;
• (e) an accelerating electrode disposed between the control electrode and the target for accelerating the electron beam;
• (f) deflection means disposed between the accelerating electrode and the target for deflecting the electron beam in two orthogonal directions;
• (g) a series of quadrupolar lenses disposed between the accelerating electrode and the target;
• (h) means for applying potentials to the quadrupolar lenses for focusing the electron beam at the target;
• (i) a refocusing lens disposed between the crossover point of the electron beam and the series of quadrupolar lenses for providing a converging lens action of radial symmetry about the central axis;
• and
• (j) means including a variable potential source for applying a variable potential difference to the refocusing lens for adjustably varying the lens action thereof;
• (k) whereby the electron beam, on being defocused at the target by a change in the potential on the control electrode, can be refocused through adjustment of a single potential on the refocusing lens by the variable potential source rather than through adjustment of the potentials on the quadrupolar lenses.