Electronic Optic Accelerating Focusing System

Charged particles (CP) are being recently applied not only in scientific research but in technology as well (TV sets and computer displays). The majority of devices and appliances based on CP include the following functional systems: the charged particles transport system (CPTS), the receiver of charged particles (CPR), the power supply and the control system.

Any CPTS consists of two separate parts: a part that accelerates CP to the pre-defined energy level and a part that forms a beam of CP with pre-defined parameters. The main function of any CPTS is the creation of a beam of CP with necessary parameters and with minimum losses of the transported CP in a vicinity of the receiver.

In many devices and appliances the length of the last drift interval (a distance from the outlet of the CPTS to the receiver of particles) varies depending on the point of the receiver (e.g. the screen) the beam of the charged particles (CPB) interacts with. To provide the approximately equal cross-section of the CPB in any point of the receiver one should either vary the position of the cross-over of the CPB synchronously with its movement across the plane of the receiver or use a paraxial CPB (in which CP trajectories are parallel to the beam axis). The provision of such conditions of the CP transportation leads to the increase of the linear dimensions of the CPTS which is undesirable in many cases. That is why the collimating of the CPB is conducted by means of diaphragms in which case the losses of CP are large.

Original electrostatic systems with the spiral symmetry [1] enable decreasing losses of CP. In these systems, a quadrupole electric field twists, e.g. screws along the CPB trajectory. Such systems effect the dimensions and the form of the CPB cross-section and the distribution of the CP density over the cross-section much stronger. But inspite of the high efficiency and large advantages of the systems with the spiral structure of the field, they also cannot provide obtaining small-diameter paraxial CPB without essential losses of CP within the beam.

This results from the main principle of the CPB transportation, namely from the fact that the emmittance of the CPB does not vary unless the CP pulse varies within the beam [2].

To get over this disadvantage we developed an Accelerating Focusing System (AFS) with a spiral structure of the electrostatic field [3], Fig.1.



The both functions of the CPTS (accelerating the CP to the necessary energy level and shaping CP into a beam with the necessary parameters) are combined in the AFS.

As it is seen in Fig.1, the AFS contains electrodes positioned on a 2nd order surface (the cone or the cylinder). The AFS can be obtained from a screw (spiral) lens if it is dissected by some planes perpendicular to the axis of the lens into an odd number of alternating parts of the lengths Lm, Lm' (Lm>Lm'), the parts Lm' being then deleted.

Potential differences are created between all neighboring electrodes of the AFS. The potential differences applied to the electrodes positioned within the same "ring" make up the radial gradients of the potential, while the potential differences applied between the "rings" make up the axial gradient of the potential. The superposition of these fields provides the value and the direction of the vector of the AFS electrostatic field intensity. Variations of the potential differences between the electrodes lead to the variations of the CPB parameters including the emmittance area because the pulse of the CP varies within the AFS.

Model testing of the AFS showed that a proton beam with a rectangular cross-section of 3.5 * 0.5 cm at the inlet of the AFS is transformed into a paraxial, round in the cross-section beam of <0.5 cm in diameter (the Faraday cylinder diameter), practically without any loss of protons.

The paraxial feature of the proton beam was tested in the drift interval of 35 cm in length, with same Faraday cylinder.

Note that the AFS can work in the mode of conventional electrostatic lenses and accelerating systems.

References

1. Strashkewitz A.M., Electrostatic systems with the spiral symmetry, Radiotechnika I electronica, 1985, 10, No 2, p 341. USSR.
2. Benford A., Transportation beams of the charged particles, M., Atomizdat, 1969. USSR.
3. Tatus et al., Electrostatic electron lens, USSR author's certificate, 1612968A1, 1990.