Automatic light-controlling system for dc arc emission spectrographs controls the vaporization rate of the sample and stabilizes the dc arc. The output energy is regulated such that advantage can be taken of the highly sensitive dc arc source without sacrificing the desired precision

Gordon, W. A.

English

NASA Technical Reports Server

Stem 
Refractory Metal Bead
raphite Rod 
xial Recess 
pered End 
November 1967
	 Brief 67-10404 
NASA TECH BRIEF 
I
-	 .. 
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. 
Control Apparatus for Spectral Energy Source 
DC Arc	 Direct-Reading 
Source
Photomultiplier 
Sensitive Supply	 Rectifier	
5jg Cat SPectromet 
Error 
AC Power	 Phase  
Graphi te Ho ld er 
Differential 
LAmplifier	 Command Signal 
J::^FGenerator 
CONTROL SYSTEM 
CATHODE ELECTRODE 
The problem: 
To devise a method for controlling the energy 
source of an emission spectrograph or spectrometer, 
particularly the dc arc source which excites the sample 
material. A dc arc source is extremely sensitive, but no 
satisfactory method has been found to precisely con-
trol its output energy. Uncontrolled arcs are subject 
to large relative errors which severely restrict the 
precision with which trace analyses can be made. Ana-
lytical spectroscopists use other sources to achieve 
high precision and thereby sacrifice sensitivity. 
The solution: 
An automatic, light-controlling system for dc arc 
emission spectrographs, which Controls the vaporiza-
tion rate of the sample, and also stabilizes the dc arc. 
The output energy is regulated such that advantage 
can be taken of the highly sensitive dc arc source, 
without sacrificing the desired precision.
(continued overleaf) 
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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.
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The vaporization rate of the sample, and therefore, 
the output energy of the dc arc source, are controlled 
in accordance with a programmed signal—either a 
constant or a time-dependent command signal. The 
output energy of the dc arc source is compared with a 
programmed command signal and the difference 
between them produces an error signal. This signal 
regulates the dc current supply to the arc source, such 
that the output energy is in accordance with the pro-
grammed command signal. 
The system also stabilizes the arc of the dc arc 
source by means of a cathode structure having a 
refractory metal tip. This produces a steady, rather 
than flickering, source of light. 
How it's done: 
The control system consists of a dc arc source as-
sociated with a direct reading spectrometer. The arc 
source includes a cathode and an anode having a 
graphite holder which contains the sample material. 
To produce an arc between the anode and cathode, 
these electrodes are connected to a phase sensitive 
rectifier, energized from an ac power supply. 
A photomultiplier tube senses the light output of a 
selected spectral line of the arc source and supplies an 
electrical signal through a line to a differential ampli-
fier. The signal is proportional to the light energy of 
the arc. The photomultiplier tube signal is compared 
by the differential amplifier with the output from a 
command signal generator. The difference produces 
an error signal which is fed to the phase sensitive recti-
fier to control the rectifier's output magnitude. The 
error signal increases or decreases the rectifier output
to control the vaporization rate of the sample 
material. 
The cathode electrode is a. graphite rod having a 
tapered end. An axial recess, drilled into the tapered 
end of the rod, receives the stem of a refractory metal 
bead. This bead enables a stable arc discharge to be 
maintained. 
Notes: 
1. Although this apparatus was designed for analysis 
of trace constituents in a sample by emission 
spectroscopy, it also can be used for determining 
major constituents of a sample. 
2. This apparatus can be used in mass spectrometry 
and atomic absorption spectroscopy and in any 
application in which a constant or time-dependent 
energy output is required. 
3. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B67-10404 
Patent status: 
This invention is owned by NASA and a patent
application has been filed. Royalty-free, nonexclusive 
licenses for its commercial use will be granted by 
NASA. Inquiries concerning license rights should be 
made to. NASA, Code GP, Washington, D.C. 20546. 
Source: William A. Gordon
(Lewis-391) 
Brief 67-10404	 Category 01


Control apparatus for spectral energy source

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