ISO/TS 6737:2023
(Main)Vacuum technology — Vacuum gauges — Characteristics for a stable ionisation vacuum gauge
Vacuum technology — Vacuum gauges — Characteristics for a stable ionisation vacuum gauge
This document describes a special design of an ionisation vacuum gauge which has a well-defined ionising electron path length.[2] Due to the construction design, it leads to good measurement accuracy, long-term stability, as well as gauge independent and reproducible sensitivity for nitrogen and relative sensitivity factors[3][4]. It is designed for the measurement range of 10-6 Pa to 10-2 Pa. This document describes only those dimensions and potentials of the gauge head which are relevant for the electron and ion trajectories. This document does not describe the electrical components necessary to operate the ionisation vacuum gauge in detail. The gauge head can be operated by voltage and power sources and ammeters commercially available, but also by a controller specially built for the purpose of the operation of this gauge head. The ionisation vacuum gauge described in this document can be built by any experienced manufacturer of other ionisation vacuum gauges. It is not subject to intellectual property protection. It is assumed for this document that the applicant is familiar with both the physics and principles of ionisation vacuum gauges as well as high and ultra-high vacuum technology in general.
Technique du vide — Manomètres à vide — Caractéristiques des manomètres à ionisation stable
General Information
Standards Content (Sample)
TECHNICAL ISO/TS
SPECIFICATION 6737
First edition
2023-11
Vacuum technology — Vacuum
gauges — Characteristics for a stable
ionisation vacuum gauge
Technique du vide — Manomètres à vide — Caractéristiques des
manomètres à ionisation stable
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms.2
5 General description of the design . 2
5.1 Components . 2
5.2 Mode of operation . 3
6 Specifications of the ionisation vacuum gauge. 4
6.1 General specifications and requirements for the gauge head . 4
6.2 Electrical equipment . 5
7 Dimensions and potentials .6
7.1 General . 6
7.2 Dimensions and tolerances . . . 7
7.3 Potentials . 9
7.4 Cathode . . 9
7.5 Anode cage . 9
7.6 Gaps between electrodes . 10
7.7 Electrode materials . 10
7.8 Electrical feedthroughs . 10
7.9 Surrounding tube (envelope) and flanges . 10
7.10 Magnetic fields . 10
8 Characteristics of the gauge (informative) .11
8.1 Sensitivity . 11
8.2 Linearity . 11
8.3 Electron transmission efficiency . 11
8.4 Residual current and resolution . 11
8.5 Repeatability . 11
8.6 Reproducibility .12
8.7 Interference .12
8.8 Temperature of electrodes . 12
8.9 Environmental conditions . 12
9 Handling of and measurement with the gauge .12
9.1 Orientation of the gauge head .12
9.2 Environmental conditions . 12
9.3 Conditioning of the gauge .12
9.4 Testing .12
9.5 Warm-up time . 13
9.6 Sensitivity and calibration . 13
9.7 Measurement . 13
9.8 Transport .13
9.9 Cathode exchange . 13
Annex A (Informative) Electron and ion trajectories of recommended gauge dimensions
according to simulation .14
Bibliography .15
iii
Foreword
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iv
Introduction
The ionisation vacuum gauge is the only type of vacuum gauge covering the full range of high and
[1]
ultrahigh vacuum. Important applications need better accuracy, reproducibility and known
sensitivities for many gas species, properties which all current types of ionisation vacuum gauges
lack. This document provides the characteristics for a stable ionisation vacuum gauge so that this
gauge is accurate, robust and long-term stable, with known sensitivity for nitrogen and known relative
sensitivity factors, and can be built by any experienced manufacturer of other ionisation vacuum
gauges.
v
TECHNICAL SPECIFICATION ISO/TS 6737:2023(E)
Vacuum technology — Vacuum gauges — Characteristics
for a stable ionisation vacuum gauge
1 Scope
This document describes a special design of an ionisation vacuum gauge which has a well-defined
[2]
ionising electron path length. Due to the construction design, it leads to good measurement accuracy,
long-term stability, as well as gauge independent and reproducible sensitivity for nitrogen and relative
[3][4] -6 -2
sensitivity factors . It is designed for the measurement range of 10 Pa to 10 Pa.
This document describes only those dimensions and potentials of the gauge head which are relevant for
the electron and ion trajectories. This document does not describe the electrical components necessary
to operate the ionisation vacuum gauge in detail. The gauge head can be operated by voltage and power
sources and ammeters commercially available, but also by a controller specially built for the purpose of
the operation of this gauge head.
The ionisation vacuum gauge described in this document can be built by any experienced manufacturer
of other ionisation vacuum gauges. It is not subject to intellectual property protection.
It is assumed for this document that the applicant is familiar with both the physics and principles of
ionisation vacuum gauges as well as high and ultra-high vacuum technology in general.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN ISO 13920, Welding - General tolerances for welded constructions - Dimensions for lengths and angles -
Shape and position (ISO 13920:1996)
ISO 2768-1, General tolerances — Part 1: Tolerances for linear and angular dimensions without individual
tolerance indications
ISO 3669, Vacuum technology — Dimensions of knife-edge flanges
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
Wehnelt electrode
Wehnelt
an electrode with cylindrical symmetry around the electron emitting cathode, mainly used for focusing
of the electron beam
3.2
ionisation space
the space in which ions generated by collision of gas molecules with high energy electrons reach the ion
collector by means of a suitable electrostatic field
3.3
Faraday cup
metal cup or other piece of metal designed to catch charged particles in vacuum
Note 1 to entry: In the ionisation vacuum gauge described in this document, the Faraday cup is designed to
capture the electrons emitted from the cathode.
3.4
envelope
the metallic wall at zero (earth) potential surrounding the gauge head at least in its full length
3.5
electron transmission
the ratio of electron current measured at the Faraday cup divided by the electron current emitted from
the cathode
4 Symbols and abbreviated terms
I ion current at pressure p [A]
I ion current at residual pressure p [A]
0 0
I electron emission current [A]
e
p pressure [Pa]
p residual pressure [Pa]
r relative sensitivity factor as defined in ISO 27894
x
S sensitivity (coefficient) [1/Pa]
S sensitivity for nitrogen [1/Pa]
N2
5 General description of the design
5.1 Components
The ionisation vacuum gauge consists of the following functional parts:
a) electron emitting cathode,
b) Wehnelt electrode,
c) anode cage in two parts,
d) ion collector,
e) electron deflector,
f) Faraday cup,
g) envelope.
The functional components a) to f) need to be exactly dimensioned.
In addition, the gauge needs electrical feedthroughs, wires, mounting parts and insulators. The gauge
shall be mounted on a DN40CF or on a DN63CF flange according to ISO 3669 with corresponding tube
sizes DN40 or DN63 as envelope (see 7.9).
5.2 Mode of operation
A schematic for the illustration of the operation of the gauge is shown in Figure 1, for a detailed drawing
see Figure 2 in 7.2. For simplification, in Figure 1 the anode cage (3) has not been divided in two parts
as in Figure 2 with (3a) and (3b) and the collector ring (6) in between (see also Figure 3) .
Key
1 cathode with emitter disk (red)
2 Wehnelt cylinder
3 anode cage
4 electron deflector
5 Faraday cup
6 ion collector
+
I ion
-
e electron
Figure 1 — Simplified illustration of mode of operation (informative)
Electrons are emitted from the hot thermionic cathode (1 in Figure 1 and Figure 2), which is preferably
an indirectly heated disk emitter on a potential of 50 V. The Wehnelt electrode (2 in Figure 1)
surrounding the cathode has a lower potential and controls and focuses the electron beam into the
opening of the anode cage at 250 V.
Note that in the future, it can be possible that the thermionic cathode can be replaced by a so-called
cold field emission cathode. This cathode shall be long-term stable and provide an electron current of
about 100 µA. In addition, it shall be ensured that the energy of the electrons along their path in the
ionization space is not changed compared to the design with thermionic cathode.
Due to the penetration of the anode potential into that of the Wehnelt electrode the electrons can be
extracted and accelerated into the inner part of the cylindrical anode cage. The first part of the anode
cage (3a at 250 V, see Figure 2), the ion collector ring (0 V, 6 in Figure 1 and Figure 2) and the second
remaining part of the anode cage (250 V, 3b in Figure 2) form an electrostatic lens which focuses the
electron beam into the circular exit of the anode cage. Behind this exit the electron beam is deflected
by the electron deflector electrode (45 V, 4 in Figure 1 and Figure 2) in a U-turn onto the capturing part
of the Faraday cup (280 V, 5 in Figure 1 and Figure 2). When the electrons hit the Faraday cup, they will
generate X-rays. By the U-turn, it is ensured that these X-rays have a very low probability to reach the
ion collector or the anode where they would generate secondary electrons.
The ions generated by the electron beam inside the anode cage are accelerated towards the ion collector
which consists of the mentioned ring and a rod reaching into the larger space of the anode cage. Ions
generated behind the exit of the cage are accelerated towards the electron deflector electrode. The
ionisation space is well defined in this design.
The measured ion current will be proportional to the gas density in contact with the electron beam,
the ionisation probability of the gas molecules by the electron impacts along their path, the mean path
length of the electrons inside the ionisation space and the electron current.
Due to the focused electron beam inside the anode cage, the electron current should not exceed 200 µA.
Higher currents can cause non-linearities.
The mean electron path length is defined by the length of the anode cage and the potential inside it. Any
changes of the emission points of the electrons on the cathode will not significantly change the path
length. A replacement of the same cathode type will have an insignificant influence on the path length.
Space charge effects of ions will also have an insignificant influence on the path length within the
specified measurement range up to 0,01 Pa. Due to the well-defined electron path length, the nitrogen
sensitivity and the relative sensitivity factors will not significantly vary from gauge to gauge except
within the uncertainty due to variation of secondary electrons produced by the ion impingement on the
[3]
collector.
As for an ionisation vacuum gauge of the extractor type, in this gauge, X-rays have a low probability
of reaching the ion collector. This ensures that the secondary electron current on the ion collector
produced by X-rays is rather small. Such a current would be indistinguishable from the measured ion
current.
Ions desorbed by electron impact in the Faraday cup will be attracted to the electron deflector and will
not reach the ion collector. Electron stimulated desorption of neutrals, however, will contribute to the
gas density in the gauge and therefore to the ion current.
6 Specifications of the ionisation vacuum gauge
6.1 General specifications and requirements for the gauge head
a) The electrons shall have a direct and well-defined path from their source, the cathode, through
the ionisation space to the target, the Faraday cup. The path length shall not be increased by
oscillations through the ionisation space, e.g. by a magnetic field.
b) The two parts of the anode cage, the ion collector ring and the Wehnelt electrode shall have a
common cylindrical axis according to best possible practice. The center of the cathode shall be
aligned to this axis.
c) The electron emitting cathode shall be surrounded by a Wehnelt electrode.
d) It is recommended that the cathode is an indirectly heated disk emitter to ensure well defined and
stable starting points on equal potential of the electron trajectories over the emission area.
e) The shape, dimensions, position and potentials of the anode cage, Wehnelt electrode and electron
emitting part of the cathode shall be such that the emitted electrons are accelerated parallel to or
with a small maximum angle (typically < 5°) to the cylindrical axis described in b).
f) The two parts of the anode cage and the ion collector ring shall form an electrostatic lens for the
electrons to focus them to the circular exit of the anode cage. The electron beam shall be focused
in such a way that less than 5 % of the total electron current hits the anode cage. The collector
electrode consists of a ring with a long rod reaching into the anode cage in the direction of the
Faraday cup to efficiently collect ions from inside of the electron beam.
g) It is necessary that openings in the cylindrical anode cage allow a free exchange of molecules inside
and outside of the cage so that there is no significant difference in gas density. This can be achieved
by slotted holes or similar. It is, however, required that there is no significant potential penetration
from the envelope into the anode cage.
h) The electr
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