A gauge for measuring ultrahigh vacuums with great accuracy is described. It provides a means for ionizing the gas whose pressure is being measured, and consists of a collector electrode, a suppressor, radiation shielding, and a second collector

Torney, F. L., Jr.

English

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https://ntrs.nasa.gov/search.jsp?R=19730023592 2020-03-17T09:32:45+00:00Z
(HASA-Case-LAR-02743) OLTRAHIGH VACUUM N73-32324
GAUGE HAVING TWO COLLECTOR ELECTRODES j
Patent (National Research Corp.) 3 p '
CSCL 14B Onclas
v ' • • - 00-/1U 18072 j
March 21, 1967
 F. L. TORNEY, JR 3,310,699
ULTRAHIGH VACUUM GAUGE HAVING TWO COLLECTOR ELECTRODES
Filed Oct. 16. 1964
ZERO PRI°R
MINUS 300 VOLTS
United States Patent Office 3,310,699
^Patented Mar.-21, 4967
3,310,699
ULTRAHIGH VACUUM GAUGE HAVING TWO
COLLECTOR ELECTRODES
Franklin L. Torney, Jr., South Weymouth, Mass., as- _
signer to National Research Corporation, Cambridge, °
Mass., a corporation of Massachusetts
Filed Oct. 16, 1964, Ser. No. 404,212
1 Claim. (CI. 313—7)
The present invention relates to vacuum gauges and
 1(j
particularly to the measurement of ultra high vacuum.
The invention described herein was made in the perform-
ance of work under a NASA contract and is subject to the
provisions of the National Aeronautics and Space Act of
1958, Public Law 85-568 (72 Stat. 426; 42 U.S.C. 2451) 15
as amended. A license has been granted to the United
States Government for practice of the invention and title
to the invention and this patent have been reversed to the
assignee, subject to voidability by NASA.
It is known that the calibration of vacuum gauges, par- 20
ticularly hot filament ionization gauges without magnets,
departs from linear in the ultra high vacuum range. The
collection current is much higher than the current corre-
sponding to the pressure to be measured.
It is therefore the object of this invention to provide a ,25
technique of pressure measurement which allows the non-
linear gauge readings in the ultra high vacuum range to
be corrected to true pressure with greater accuracy.
It is a further and related object to provide an im-
proved gauge construction which makes possible the prac- 30
tice of this technique.
Other objects of the invention will in part be obvious
and will in part appear hereinafter.
The invention accordingly comprises an improved
gauge and an improved technique of measuring ultra high 35
vacuum pressures.
For a fuller understanding of the nature and objects of
the invention, reference should be had to the following
explanation, taken in connection with the accompanying
drawings wherein: 40
FIG. 1 is a schematic drawing of a prior art suppres-
sor gauge utilized by applicant in making the present in-
vention;
FIG. 2 indicates the calibration curve usually associ-
ated with the gauge of FIG. 1; 45
FIG. 3 indicates an anomalous calibration curve ob-
tained by a variant construction of the gauge of FIG. 1,
which variant is schematically shown in FIG. 4;
FIG. 5 shows an improved gauge according to a pre-
ferred embodiment of the invention. S"
It will be useful in explaining the gauge to explain the
fortuitous manner in which the invention was discovered.
Experiments were being conducted to determine the cali-
bration of a suppressor gauge. Reference is made to the
article by Schuemann at pages 428-430 of the Transac- r>!>
tions of the 1962 Vacuum Symposium (American Vac-
uum Society and McMillan Co., New York, 1962) for
a further discussion of the gauge, per se. A modified ver-
sion of the gauge was made as shown in FIG. 1 wherein a
filament 10 emits electrons which are accelerated towards 60
an anode grid 12. The entire structure is in a glass tube
(not shown) evacuated to the ultra high vacuum range.
The electrons ionize residual gas molecules which are col-
lected at a collector electrode 14. This ion current is a
measure of the residual pressure. A suppressor ring 16 65
prevents secondary emission of electrons from the collec-
tor due to X-rays from'the anode grid. The suppressor
ring is mounted on spaced ceramic insulators 18 in a
shield box 20 which protects the suppressor ring from
X-rays and electrons. An insulator 22 supports an elec- 70
trical lead for biasing the suppressor negative with respect
to the collector.
The experiments were conducted by comparing the
measured collector current (I/n) of the suppressor gauge
with the collector current (Ire/) of a modulated Bayard-
Alpert type gauge after Redhead (Review of Scientific
Instruments, vol. 31: pp. 343-344, 1960). It was ex-
pected that the calibration curve would toe as shown in
FIG. 2. That is, operating without the secondary emis-
sion suppression (suppressor voltage zero; curve A), the
collector current would depart from linear in the range
of 10-10 amperes reference current as X-ray effects be-
came significant compared to ion current. 'Operating with
the suppressor ring at minus 300 volts, the departure from
linear would be attenuated (curve "B).
Instead, the anomalous results of FIG. 3 were encoun-
tered. The curve AA produced without suppression was
similar to the expected curve A of FIG. 2. The curve BB,
produced with suppression, was lower than AA in the
range where X-ray effects are not significant. The differ-
ence in collector currents between curves AA and BB was
essentially constant down to the middle of the 10-10 ref-
erence current range. Then both curves departed from
linear at different rates.
Later the gauge was disassembled and it was observed
that the suppressor ring was tilted away from its intended
position as shown in FIG. 4.
It is concluded that, due to the inadvertent tilting, • and
its negative potential, the suppressor ring 16 intercepted
a portion of the ions destined for the collector 14, in a
manner analogous to the operation of a modulated Bay-
ard-Alpert gauge, thus producing the lower calibration
curve BB. It must be that a uniform percentage of ions
are collected since the difference (about 10%) between
curves AA and BB is constant in the high vacuum range.
A modulator can be provided in a suppressor gauge,
either as part of the suppressor as in the fortuitous con-
figuration above, or as a separate element. • The discov-
ery that the modulation fraction is constant allows one to
(use suppression and modulation functions together to pro-
vide more accurate readings of ultra high vacuum pres-
sures than would be obtainable from the use of a sup-
pressor gauge, a modulation gauge, or both operated
simultaneously.
A preferred embodiment of the invention is shown in
FIG. 5. The gauge elements are essentially the same as -
in FIG. 1 (including filament 110, anode grid 112, col-
lector 114, suppressor 116 mounted via ceramic spaces
118 to a shield box 120 corresponding to the elements 10,
12, 14, 16, 18, 20 of FIG. 1 respectively), except that a
modulator ring 124 is added, as shown in FIG. 5. The
modulator ring 124 may be maintained at a potential of
about —100 to —300 volts in typical configurations when
in use for intercepting a portion of the ions, and at ground
potential when not in use. The suppressor ring 116 can
operate at about —300 volts and ground. The modulator
.ring 124 is located in a zone within the shield box 120
well shielded from radiation emanating from the anode
grid 112.
Before describing the best mode of using the invention,
the nature of the interelectrode currents in an ionization
gauge is restated in simple fashion. The total current
measured at the collector (Im) is due to positive ions
and secondary emission of electrons IN. The positive ion
current is made up of positive ions formed from mole-
cules of the gas to be studied (Is), highly energetic ions
of gas (oxygen and other contaminant gases) sorbed in
the anode grid and other gauge elements and desorbed by
operation of the gauge (IH) and low energy ions (Ic)
formed by the ionization of neutral molecules of con-
taminant gas. The contaminant gas is any gas in the
tube whose partial pressure has no relation to the partial
pressures of gas under study.
3,310,699
The total collector current in a gauge (no modula-
tion, no suppression), may then be described as follows:
(1)
If the gauge is modulated, without suppression, Equation
1 must be revised as follows:
7ifi,=a(fo+/c) + (/w+/H) (2)
where a is the constant of modulation which can be de-
termined. The collector current of a suppressor gauge
(no modulation) is:
/W3=/J+/C+/H, 7N=0 (3)
The operation of the suppressor grid reduces secondary
emission to zero.
Subtracting Equation 3 from Equation 1 :
1m— /m3=/N .(4)
That is, the difference between the gauge readings with
and without suppression gives the true secondary emis-
sion current of the gauge.
Further information can be developed from the read-
ings I/Ti, Im2, Im3, and follows:
Equation 1 is multiplied by a :
Subtracting Equation 2 from Equation 1'
10
15
20
25
*
H
 ^1 'N (5)
Since IN and a can be determined for a gauge, one can
solve for IH.
It has been proposed by Redhead (see e.g., Vacuum
Magazine, vol. 13, p. 253, 1963) that the ratio 7c:7H is
about 100. Experimental data indicates that this figure
is correct. Therefore, once the reading, Im, Im2 and Im3
have been made and a is determined for a gauge, one can
compute Ic, IN, IH as noted above. Then, returning to
Equation 1 above, Is, which is the measure of gas density,
can be computed.
The above'finding that the deviation in collector cur-
rent due to inadvertent modulation is constant leads to
the conclusion that the factors IH and Ic are independent
of pressure. This allows the present invention to be used
30
4
in the following manner. Operating at known pressures
in the range of 10~9 torr based on a reference current of
a gauge having a sensitivity of 0.1 amp./torr, the gauge
of FIG. 5 is operated with modulation and without modu-
lation. The uniform difference between curves
(Im—Im3)
gives the current IN. Then, at lower pressures, the gauge
is operated with suppression to eliminate IN. The meas-
ured collector current at lower pressures (Im2) will be
utilized in Equations 2 and 5 with a known value of IN,
to find IH. Ic is then determined as 100IH- Equation 1
can then be utilized to find the ionization current (Is)
related to the gas under study. Thus, a pressure gauge
with suppressor and modulator, constructed as described
above, affords advantages beyond those attainable by
measuring gas density with a suppressor gauge and a
modulator gauge together.
Many variations can be made from the preferred em-
bodiment of the above description. It is therefore in-
tended that the above description shall be regarded as il-
lustrative and not in a limiting sense.
What is claimed is:
An improved ultra high vacuum gauge comprising in
combination means for ionizing molecules of the gas
whose pressure is to be measured, a first collector elec-
trode for collecting ions, means for suppressing secondary
emission from the first collector electrode, means for
shielding the suppressing means from radiation from the
ionizing means, and a second collector means interposed
between said first collector and said suppressor and biassed
negatively with respect to said first collector, said second
collector arranged to selectively collect ions and reduce
ion flow to said first collector.
35
40
References Cited by the Examiner
UNITED STATES PATENTS
2,639,397 5/1953 Clark 313—7
3,001,128 9/1961 Nottingham , 315—111 X
JAMES W. LAWRENCE, Primary Examiner.
GEORGE N. WESTBY, S. SCHLOSSER,
Assistant Examiners.


Ultrahigh vacuum gauge having two collector electrodes

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