ISO 19991:2026

International
Standard
ISO 19991
First edition
Fusion technology — Experimental
2026-08
magnetic confinement fusion
facilities — Supersonic molecular
beam injection fuelling technique
for fusion devices
Technologie pour installations de fusion — Installations
expérimentales de fusion par confinement magnétique
— Technique d’alimentation en combustible par faisceau
moléculaire supersonique pour machines de fusion
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviations . 1
3.1 Terms and definitions .1
3.2 Abbreviations .3
3.3 Symbols .4
4 Method and principle . 4
5 System components . 5
5.1 Components arrangement .5
5.2 Gas source supply .6
5.3 Valve .7
5.4 Injector .8
5.5 Control unit .9
5.6 Other requirements .9
6 Specification for beam properties . 9
6.1 Specification of beam structure .9
6.2 Specification of beam velocity .10
6.3 Specification of injection rate .10
7 Inspection . 10
7.1 General .10
7.2 Inspection of the beam structure .10
7.3 Inspection of the beam velocity .11
7.4 Inspection of the injection rate . 12
Bibliography .13

iii
Foreword
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iv
Introduction
The plasma fuelling system is integral to magnetic confinement fusion devices, including DEMO and other
fusion facilities. Supersonic molecular beam injection (SMBI) is a key candidate for fuel injection, offering
high efficiency with reduced wall retention of gases. SMBI has demonstrated good fuelling efficiency in
many medium and small-sized devices, with higher efficiency compared to conventional methods. Although
its fuelling efficiency may attenuate in large-scale equipment, long-pulse discharges in superconducting
[1]
devices have shown that the good directionality of SMBI can significantly reduce wall retention . This
reduction in wall retention is particularly significant for the utilization of tritium in future fusion facilities.
Unlike conventional gas fuelling systems that utilize gas diffusion and are susceptible to wall retention,
leading to unpredictable plasma density changes and possible facility shutdown, SMBI, with its high beam
velocity, effectively mitigates such risks. Comparing to fuel pellet injection, SMBI has lower fuelling efficiency,
but provides significant system stability. Currently, tritium pellet technology is not yet sufficiently mature.
Therefore, as technology advances, SMBI can be considered as a complementary component of an effective
fuelling system for future fusion facilities, alongside pellet injection.

v
International Standard ISO 19991:2026(en)
Fusion technology — Experimental magnetic confinement
fusion facilities — Supersonic molecular beam injection
fuelling technique for fusion devices
1 Scope
This document specifies the methods and the requirements for the supersonic molecular beam injection
(SMBI) fuelling technique used in experimental magnetic confinement fusion facilities. It outlines the SMBI
system components, specific requirements, and inspection procedures to ensure the effective and controlled
injection of plasma fuel into fusion devices such as international thermonuclear experimental reactor and
demonstration power plant.
This document applies to the formation of supersonic molecular beam (SMB) and usage of the SMBI
technique on the fusion devices, including the specification of gas pressure adjustment, beam characteristics
and injection rate requirements, as well as the procedures for verifying these parameters.
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.
ISO 16646, Fusion installations — Criteria for the design and operation of confinement and ventilation systems
of tritium fusion facilities and fusion fuel handling facilities
3 Terms, definitions, symbols and abbreviations
3.1 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/
3.1.1
supersonic molecular beam injection
SMBI
method of fuelling by generating a supersonic molecular beam through the adiabatic expansion of gas into a
vacuum, maintained by a sufficient pressure differential across a Laval or Laval-like nozzle
3.1.2
beam structure
structure of the beam injected by the SMBI (3.1.1) system, which features a quiet zone as well as a Mach disk
and other characteristics
Note 1 to entry: Figure 1 shows the beam structure.

Key
1 injector
2 jet boundary
3 barrel shock
4 Mach disk shock
α divergence angle
X effective distance
M
a
Flow direction.
Figure 1 — Schematic diagram of the beam structure
3.1.3
divergence angle
α
angle at the injector outlet between the two tangents to the boundary of the beam's central cross-section
3.1.4
effective distance
X
M
length of quiet zone, expressed as the distance from the injector outlet to the Mach disk along the central
axis of the beam
3.1.5
working distance
L
w
distance measured along the central axis of the beam, extending from the injector outlet to the boundary of
the working chamber's inner wall
3.1.6
time-of-flight
TOF
time interval for beam particles to travel from the injector outlet to the measurement location
3.1.7
beam velocity
maximum velocity of the beam measured by the time-of-flight (3.1.6) method

3.1.8
ideal beam velocity
v
m
theoretical maximum velocity that the beam can reach when, under ideal conditions, the gas’s entire internal
energy is completely converted into directed kinetic energy, as described by Formula (1):
2RT
v  (1)
m
 1 M

3.1.9
standard sound speed
sound speed of a specific gas (H , D , T , N , Ne, Ar, etc.) at 288,15 K and 1,013 × 10 Pa
2 2 2 2
3.1.10
mean free path
λ
average distance a gas particle travels between successive collisions with other particles
3.1.11
gas source pressure
p
pressure of the gas supply at the injector inlet
3.1.12
background pressure
p
b
pressure of the environment surrounding the beam downstream of the injector outlet
3.1.13
maximum permissible working pressure
p
max
lowest value among the maximum allowable pressures of individual components in the gas circuit including
valve, gas pipe and pressure vessel
3.1.14
safety gas pressure ratio
r
ratio of the maximum permissible working pressure (3.1.13) to the normal operating gas source pressure
(3.1.11)
3.1.15
injection rate
I
inj
number of gas particles flowing through the injector outlet per second
3.2 Abbreviations
SMBI supersonic molecular beam injection
TOF time-of-flight
3.3 Symbols
α divergence angle
X effective distance
M
v standard sound speed
c
v ideal beam velocity
m
d diameter of the injector throat
L working distance
w
p gas source pressure at the inlet of the valve
p background pressure at the injector outlet
b
p maximum component operating pressure
max
λ mean free path
N Avogadro's constant
A
N number of injected particles
∆p
pressure change
V volume of the inspection container
ideal gas constant (8,314 J/(mol⋅ K))
R
T absolute temperature (K) of gas source
t SMB pulse duration
I injection rate
inj
max
maximum required injection rate
I
inj
γ
adiabatic index/specific heat ratio of applied gas source species
M molar mass of gas source (kg/mol)
r safety gas pressure ratio
4 Method and principle
The supersonic molecular beam is generated through an adiabatic expansion in a Laval or Laval-like nozzle/
injector, which ensures a sufficient pressure differential across its ends. The expansion process converts
a portion of the gas source's internal energy into translational energy, resulting in a directed motion at
velocities exceeding the local speed of sound.
For the system to operate properly, specific provisions shall be established for its key components, structure,
and operating conditions, along with beam characteristics, specifically the operating flow rate range, beam
velocity, and beam structure. Furthermore, a defined inspection method shall be established, particularly
for verifying the supersonic molecular beam (SMB) velocity at the working distance.

5 System components
5.1 Components arrangement
The SMBI system is designed with flexibility, incorporating either separate or integrated functional
components that are crucial for its operation. These components include the gas source supply, valve, injector,
control unit and other necessary components as illustrated in Figure 2. In experimental fusion devices, the
valve and injector are typically integrated, with the valve outlet being directly and rigidly connected to the
injector inlet. For large-scale, reactor-level fusion devices, the valve and the injector are usually separated.
In such configurations, the valve is located outside the vacuum vessel to facilitate maintenance, while the
fixed, robust injector is placed inside the vacuum vessel with background pressure at the injector outlet, p ,
b
such as within an inspection chamber or a working chamber.
The workflow of the SMBI system is arranged as follows: the pressure reducing valve located at the rear end
of the gas cylinder
...