System locates the focal point of an extremely high intensity electron beam. The electron beam is swept and scanned cyclically with deflection coils under a focusing lens, causing the beam focal point to move so the locus of its positions is a spherical surface symmetrical to the beam axis

Downing, R. D.

English

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December 1967	 Brief 67-10649 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Electron Beam Deflected to Determine Focal Point Location 
Electron I 
Bearn
Sweep 
Coil 
/ I, 
Swee 
S	 EuIsPacedDete> AngIe 
Electrical	 Insulation	 I Connector
— 
Cooling Water	 Beam Focal Point I 
Workpiece	 —
Radius of Sphere 
The problem:	 The solution: 
The electron beam is swept and scanned cyclically 
To design a system that locates the focal point of an 	 with deflection coils located beneath a focusing lens. 
extremely high intensity electron beam. This focal	 This causes the beam focal point to move in such a 
point must be positioned precisely with respect to a 	 way that the locus of its positions will be a spherical 
workpiece, such as for welding, which is to be sub- 	 surface symmetrical with respect to the beam axis. 
jected to the beam power. It is necessary that the 	 Faraday cages or other suitable detectors are sym-
relative position be determined prior to bringing the 	 metrically spaced on a plane normal to the at-rest axis 
full intensity of the beam to bear so as to properly 	 of the electron beam. If the beam raster is symmetrical 
weld and prevent damage to the workpiece. During	 with respect to the detectors, each detector will collect 
these determinations, power input to the workpiece	 charge at an equal rate (equal current) when the axis 
must be reduced to a safe level. 	 of symmetry of the detectors coincides with the at-rest 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19670000648 2020-03-11T21:32:44+00:00Z
axis of the beam. Suitable circuitry is employed to 
time average the charge collected by each detector. 
Adjustment of the beam position is made (by steering 
yokes or other means) until these currents are equal. 
The unswept focal point will then lie along the axis 
of symmetry of the detectors. The position of the focal 
point along this line is set by adjusting the focal point 
(using the lens or mechanical position) so that the in-
tegrated charge reading to the detectors is a maximum. 
How it's done: 
This locator system causes the focused beam to 
sweep cyclically in such a fashion as to periodically 
enter two or more small Faraday cages whose location 
with respect to. a desired point in space is known pre-
cisely. The pulsed charges received by the Faraday 
cages are then integrated, by a suitable circuit, and the 
beam location is deduced from the relative signals in-
dicated by such detectors. Suitable adjustment is made 
to equalize these signals by steering the beam with 
electrostatic, electromagnetic, or mechanical means. 
When the deflection circuits are then interrupted, the 
beam axis will be located midway between the de-
tectors. 
In a like manner, one can with another sweep direc-
tion deduce the unswept beam axis location with re-
spect to another axis in the plane of the detectors. 
The intersection of these two sweep-direction axes is 
a point through which the unswept beam axis will fall. 
The X—Y position of the beam can thereby be deter-
mined if these detectors are located precisely with 
respect to each other on-the corners of a square and
the swept beam is steered or directed so that the 
charges collected by the detectors are equal. The un-
swept beam axis will then pass perpendicularly 
through the center of the square. This system uses de-
tectors located at the same spherical surface that the 
focused beam generates to deduce the location of the 
focal point along the center axis. The maximum 
charge flow Occurs when the beam is focused on de-
tectors which have small acceptance areas. 
Notes: 
I. A refinement of this system is to utilize the detec-
tors in an automatic positioning system to control - 
the location of a beam to a specific place. Unbal-
ance signals derived from.X, Y, and Z sensors may 
be utilized to automatically steer the beam focal 
point to a predetermined position. 
2. A related system is described in NASA Tech Brief 
67-10650. Inquiries may also be directed-to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10649 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: R. D. Downing
of General Electric Company

under contract to

Marshall Space Flight Center 
(M FS- l4l07) 
Brief 67-10649	 -	 Category 01


Electron beam deflected to determine focal point location

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