Neutral-beam injection

Last updated

Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral-beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 1990s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative-ion-based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NBI designed for ITER is a substantial challenge [1] (D0, 1 MeV, 40 A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. [2] Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), ion cyclotron resonance heating (ICRH), and lower hybrid resonance heating (LH).

Contents

Mechanism

First, plasma is formed by microwaving gas. Next, the plasma is accelerated across a voltage drop. This heats the ions to fusion conditions. After this the ions are re-neutralizing. Lastly, the neutrals are injected into the machine. Neutral Beam Injection.png
First, plasma is formed by microwaving gas. Next, the plasma is accelerated across a voltage drop. This heats the ions to fusion conditions. After this the ions are re-neutralizing. Lastly, the neutrals are injected into the machine.

This is typically done by:

  1. Making a plasma. This can be done by microwaving a low-pressure gas.
  2. Electrostatic ion acceleration. This is done dropping the positively charged ions towards negative plates. As the ions fall, the electric field does work on them, heating them to fusion temperatures.
  3. Reneutralizing the hot plasma by adding in the opposite charge. This gives the fast-moving beam with no charge.
  4. Injecting the fast-moving hot neutral beam in the machine.

It is critical to inject neutral material into plasma, because if it is charged, it can start harmful plasma instabilities. Most fusion devices inject isotopes of hydrogen, such as pure deuterium or a mix of deuterium and tritium. This material becomes part of the fusion plasma. It also transfers its energy into the existing plasma within the machine. This hot stream of material should raise the overall temperature. Although the beam has no electrostatic charge when it enters, as it passes through the plasma, the atoms are ionized. This happens because the beam bounces off ions already in the plasma [ citation needed ].

Neutral-beam injectors installed in fusion experiments

At present, all main fusion experiments use NBIs. Traditional positive-ion-based injectors (P-NBI) are installed for instance in JET [3] and in ASDEX-U. To allow power deposition in the center of the burning plasma in larger devices, a higher neutral-beam energy is required. High-energy (>100 keV) systems require the use of negative ion technology (N-NBI).

Additional heating power [MW] installed in various fusion power experiments (* design target)
Magnetic confinement deviceP-NBIN-NBI ECRH ICRH LH TypeFirst operation
JET 34107Tokamak1983
JT-60U 403478Tokamak1985
TFTR 4011Tokamak1982
EAST 80.534Tokamak2006
DIII-D 2054Tokamak1986
ASDEX-U 2068Tokamak1991
JT60-SA*24107Tokamak2020
ITER*332020Tokamak2026
LHD [4] 9 (H+)
20 (D+)
15 (H)
6 (D)
 ? ? ?Stellarator1998
Wendelstein 7-X 810 ?Stellarator2015
Legend
  Active
  In development
  Retired
  Active, NBI being updated and revised

Coupling with fusion plasma

Because the magnetic field inside the torus is circular, these fast ions are confined to the background plasma. The confined fast ions mentioned above are slowed down by the background plasma, in a similar way to how air resistance slows down a baseball. The energy transfer from the fast ions to the plasma increases the overall plasma temperature.

It is very important that the fast ions are confined within the plasma long enough for them to deposit their energy. Magnetic fluctuations are a big problem for plasma confinement in this type of device (see plasma stability) by scrambling what were initially well-ordered magnetic fields. If the fast ions are susceptible to this type of behavior, they can escape very quickly. However, some evidence suggests that they are not susceptible.[ citation needed ]

The interaction of fast neutrals with the plasma consist of

Design of neutral beam systems

Beam energy

Maximum neutralisation efficiency of a fast D ion beam in a gas cell, as a function of the ion energy Neutralisation efficiency D ion beam.png
Maximum neutralisation efficiency of a fast D ion beam in a gas cell, as a function of the ion energy

The adsorption length for neutral beam ionization in a plasma is roughly

with in m, particle density n in 1019 m−3, atomic mass M in amu, particle energy E in keV. Depending on the plasma minor diameter and density, a minimum particle energy can be defined for the neutral beam, in order to deposit a sufficient power on the plasma core rather than to the plasma edge. For a fusion-relevant plasma, the required fast neutral energy gets in the range of 1 MeV. With increasing energy, it is increasingly difficult to obtain fast hydrogen atoms starting from precursor beams composed of positive ions. For that reason, recent and future heating neutral beams will be based on negative-ion beams. In the interaction with background gas, it is much easier to detach the extra electron from a negative ion (H has a binding energy of 0.75 eV and a very large cross-section for electron detachment in this energy range) rather than to attach one electron to a positive ion.

Charge state of the precursor ion beam

A neutral beam is obtained by neutralisation of a precursor ion beam, commonly accelerated in large electrostatic accelerators. The precursor beam could either be a positive-ion beam or a negative-ion beam: in order to obtain a sufficiently high current, it is produced extracting charges from a plasma discharge. However, few negative hydrogen ions are created in a hydrogen plasma discharge. In order to generate a sufficiently high negative-ion density and obtain a decent negative-ion beam current, caesium vapors are added to the plasma discharge (surface-plasma negative-ion sources). [5] Caesium, deposited at the source walls, is an efficient electron donor; atoms and positive ions scattered at caesiated surface have a relatively high probability of being scattered as negatively charged ions. Operation of caesiated sources is complex and not so reliable. The development of alternative concepts for negative-ion beam sources is mandatory for the use of neutral beam systems in future fusion reactors.

Existing and future negative-ion-based neutral beam systems (N-NBI) are listed in the following table:

N-NBI (* design target)
JT-60U LHD ITER**
Precursor ion beamDH / DH / D
Max acceleration voltage (kV)4001901000
Max power per installed beam (MW)5.86.416.7
Pulse duration (s)30 (2MW, 360kV)128 (at 0.2MW)3600 (at 16.7MW)

Ion beam neutralisation

Neutralisation of the precursor ion beam is commonly performed by passing the beam through a gas cell. [6] For a precursor negative-ion beam at fusion-relevant energies, the key collisional processes are: [7]

D + D2D0 + e + D2(singe-electron detachment, with −10=1.13×10−20 m2 at 1 MeV)
D + D2D+ + e + D2(double-electron detachment, with −11=7.22×10−22 m2 at 1 MeV)
D0 + D2D+ + e + D2(reionization, with 01=3.79×10−21 m2 at 1 MeV)
D+ + D2D0 + D2+(charge exchange, 10 negligible at 1 MeV)

Underline indicates the fast particles, while subscripts i, j of the cross-section ij indicate the charge state of fast particle before and after collision.

Cross-sections at 1 MeV are such that, once created, a fast positive ion cannot be converted into a fast neutral, and this is the cause of the limited achievable efficiency of gas neutralisers.

The fractions of negatively charged, positively charged, and neutral particles exiting the neutraliser gas cells depend on the integrated gas density or target thickness with the gas density along the beam path . In the case of D beams, the maximum neutralisation yield occurs at a target thickness m−2.

Simplified scheme of gas-cell neutraliser for neutral-beam injectors Gas-cell neutraliser for neutral beam injectors.png
Simplified scheme of gas-cell neutraliser for neutral-beam injectors

Typically, the background gas density shall be minimised all along the beam path (i.e. within the accelerating electrodes, along the duct connecting to the fusion plasma) to minimise losses except in the neutraliser cell. Therefore, the required target thickness for neutralisation is obtained by injecting gas in a cell with two open ends. A peaked density profile is realised along the cell, when injection occurs at mid-length. For a given gas throughput [Pa·m3/s], the maximum gas pressure at the centre of the cell depends on the gas conductance [m3/s]:

and in molecular-flow regime can be calculated as

with the geometric parameters , , indicated in figure, gas molecule mass, and gas temperature.

Very high gas throughput is commonly adopted, and neutral-beam systems have custom vacuum pumps among the largest ever built, with pumping speeds in the range of million liters per second. [8] If there are no space constraints, a large gas cell length is adopted, but this solution is unlikely in future devices due to the limited volume inside the bioshield protecting from energetic neutron flux (for instance, in the case of JT-60U the N-NBI neutraliser cell is about 15 m long, while in the ITER HNB its length is limited to 3 m).

See also

Related Research Articles

<span class="mw-page-title-main">Nuclear fusion</span> Process of combining atomic nuclei

Nuclear fusion is a reaction in which two or more atomic nuclei, usually deuterium and tritium, combine to form one or more different atomic nuclei and subatomic particles. The difference in mass between the reactants and products is manifested as either the release or absorption of energy. This difference in mass arises due to the difference in nuclear binding energy between the atomic nuclei before and after the reaction. Nuclear fusion is the process that powers active or main-sequence stars and other high-magnitude stars, where large amounts of energy are released.

<span class="mw-page-title-main">Stellarator</span> Plasma device using external magnets to confine plasma

A stellarator is a device that confines plasma using external magnets. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The name refers to stars as fusion also occurs in stars such as the Sun. It is one of the earliest fusion power devices, along with the z-pinch and magnetic mirror.

<span class="mw-page-title-main">Tokamak</span> Magnetic confinement device used to produce thermonuclear fusion power

A tokamak is a device which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially-symmetrical torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. The tokamak concept is currently one of the leading candidates for a practical fusion reactor.

<span class="mw-page-title-main">Fusor</span> An apparatus to create nuclear fusion

A fusor is a device that uses an electric field to heat ions to a temperature in which they undergo nuclear fusion. The machine induces a potential difference between two metal cages, inside a vacuum. Positive ions fall down this voltage drop, building up speed. If they collide in the center, they can fuse. This is one kind of an inertial electrostatic confinement device – a branch of fusion research.

<span class="mw-page-title-main">Fusion power</span> Electricity generation through nuclear fusion

Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2024, no device has reached net power, although net positive reactions have been achieved.

<span class="mw-page-title-main">Inertial electrostatic confinement</span> Fusion power research concept

Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic confinement fusion (MCF) designs. Most IEC devices directly accelerate their fuel to fusion conditions, thereby avoiding energy losses seen during the longer heating stages of MCF devices. In theory, this makes them more suitable for using alternative aneutronic fusion fuels, which offer a number of major practical benefits and makes IEC devices one of the more widely studied approaches to fusion.

Plasma diagnostics are a pool of methods, instruments, and experimental techniques used to measure properties of a plasma, such as plasma components' density, distribution function over energy (temperature), their spatial profiles and dynamics, which enable to derive plasma parameters.

Electron cyclotron resonance (ECR) is a phenomenon observed in plasma physics, condensed matter physics, and accelerator physics. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in magnetic fields. A free electron in a static and uniform magnetic field will move in a circle due to the Lorentz force. The circular motion may be superimposed with a uniform axial motion, resulting in a helix, or with a uniform motion perpendicular to the field resulting in a cycloid. The angular frequency of this cyclotron motion for a given magnetic field strength B is given by

<span class="mw-page-title-main">Lawson criterion</span> Criterion for igniting a nuclear fusion chain reaction

The Lawson criterion is a figure of merit used in nuclear fusion research. It compares the rate of energy being generated by fusion reactions within the fusion fuel to the rate of energy losses to the environment. When the rate of production is higher than the rate of loss, the system will produce net energy. If enough of that energy is captured by the fuel, the system will become self-sustaining and is said to be ignited.

Plasma acceleration is a technique for accelerating charged particles, such as electrons or ions, using the electric field associated with electron plasma wave or other high-gradient plasma structures. These plasma acceleration structures are created using either ultra-short laser pulses or energetic particle beams that are matched to the plasma parameters. The technique offers a way to build affordable and compact particle accelerators.

<span class="mw-page-title-main">Madison Symmetric Torus</span>

The Madison Symmetric Torus (MST) is a reversed field pinch (RFP) physics experiment with applications to both fusion energy research and astrophysical plasmas.

<span class="mw-page-title-main">Tokamak à configuration variable</span> Swiss research fusion reactor at the École Polytechnique Fédérale de Lausanne

The tokamak à configuration variable is an experimental tokamak located at the École Polytechnique Fédérale de Lausanne (EPFL) Swiss Plasma Center (SPC) in Lausanne, Switzerland. As the largest experimental facility of the Swiss Plasma Center, the TCV tokamak explores the physics of magnetic confinement fusion. It distinguishes itself from other tokamaks with its specialized plasma shaping capability, which can produce diverse plasma shapes without requiring hardware modifications.

The polywell is a proposed design for a fusion reactor using an electric and magnetic field to heat ions to fusion conditions.

<span class="mw-page-title-main">Helically Symmetric Experiment</span>

The Helically Symmetric Experiment, is an experimental plasma confinement device at the University of Wisconsin–Madison, with design principles that are intended to be incorporated into a fusion reactor. The HSX is a modular coil stellarator which is a toroid-shaped pressure vessel with external electromagnets which generate a magnetic field for the purpose of containing a plasma. It began operation in 1999.

<span class="mw-page-title-main">Plasma (physics)</span> State of matter

Plasma is one of four fundamental states of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars, but also dominating the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.

<span class="mw-page-title-main">Hybrid Illinois Device for Research and Applications</span> Toroidal magnetic fusion device

The Hybrid Illinois Device for Research and Applications (HIDRA) is a medium-sized toroidal magnetic fusion device housed in the Nuclear Radiation Laboratory and operated by the Center for Plasma-Material Interactions (CPMI) within the Department of Nuclear, Plasma and Radiological Engineering at the University of Illinois at Urbana–Champaign, United States. HIDRA had its first plasma at the end of April 2016 and started experimental campaigns by December of that year. HIDRA is the former WEGA classical stellarator that was operated at the Max Planck Institute for Plasma Physics in Greifswald Germany from 2001 to 2013.

The Gas Dynamic Trap is a magnetic mirror machine being operated at the Budker Institute of Nuclear Physics in Akademgorodok, Russia.

<span class="mw-page-title-main">ITER Neutral Beam Test Facility</span>

The ITER Neutral Beam Test Facility is a part of the International Thermonuclear Experimental Reactor (ITER) in Padova, Veneto, Italy. The facility will host the full-scale prototype of the reactor's neutral beam injector, MITICA, and a smaller prototype of its ion source, SPIDER. SPIDER started its operation in June 2018. SPIDER will be used to optimize the ion beam source, to optimize the use of caesium vapor, and to verify the uniformity of the extracted ion beam also during long pulses.

Colliding beam fusion (CBF), or colliding beam fusion reactor (CBFR), is a class of fusion power concepts that are based on two or more intersecting beams of fusion fuel ions that are independently accelerated to fusion energies using a variety of particle accelerator designs or other means. One of the beams may be replaced by a static target, in which case the approach is termed accelerator based fusion or beam-target fusion, but the physics is the same as colliding beams.

Low to High Confinement Mode Transition, more commonly referred to as L-H transition, is a phenomenon in the fields of plasma physics and magnetic confinement fusion, signifying the transition from less efficient plasma confinement to highly efficient modes. The L-H transition, a milestone in the development of nuclear fusion, enables the confinement of high-temperature plasmas. The transition is dependent on many factors such as density, magnetic field strength, heating method, plasma fueling, and edge plasma control, and is made possible through mechanisms such as edge turbulence, E×B shear, edge electric field, and edge current and plasma flow. Researchers studying this field use tools such as Electron Cyclotron Emission, Thomson Scattering, magnetic diagnostics, and Langmuir probes to gauge the PLH and seek to lower this value. This confinement is a necessary condition for sustaining the fusion reactions, which involve the combination of atomic nuclei, leading to the release of vast amounts of energy.

References

  1. L. R. Grisham, P. Agostinetti, G. Barrera, P. Blatchford, D. Boilson, J. Chareyre, et al., Recent improvements to the ITER neutral beam system design, Fusion Engineering and Design 87 (11), 1805–1815.
  2. V. Toigo; D. Boilson; T. Bonicelli; R. Piovan; M. Hanada; et al. (2015). "Progress in the realization of the PRIMA neutral beam test facility". Nucl. Fusion. 55 (8): 083025. Bibcode:2015NucFu..55h3025T. doi:10.1088/0029-5515/55/8/083025. hdl: 10281/96413 . S2CID   124477971.
  3. "Neutral beam powers into the record books, 09/07/2012". Archived from the original on 2017-03-24.
  4. Ikeda, K.; Tsumori, K.; Kisaki, M.; Nakano, H.; Nagaoka, K.; Osakabe, M.; Kamio, S.; Fujiwara, Y.; Haba, Y.; Takeiri, Y. (2018). "First results of deuterium beam operation on neutral beam injectors in the large helical device". Proceedings of the 17th International Conference on Ion Sources. AIP Conference Proceedings. 2011 (1): 060002. Bibcode:2018AIPC.2011f0002I. doi: 10.1063/1.5053331 .
  5. Schiesko, L; McNeely, P; Fantz, U; Franzen, P (2011-07-07). "Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source". Plasma Physics and Controlled Fusion. 53 (8): 085029. Bibcode:2011PPCF...53h5029S. doi:10.1088/0741-3335/53/8/085029. ISSN   0741-3335. S2CID   33934446.
  6. G. Serianni; et al. (April 2017). "Neutralisation and transport of negative ion beams: physics and diagnostics". New Journal of Physics. 19 (4): 045003. Bibcode:2017NJPh...19d5003S. doi: 10.1088/1367-2630/aa64bd . hdl: 11577/3227451 .
  7. IAEA Aladdin database.
  8. G. Duesing (1987). "The vacuum systems of the nuclear fusion facility JET". Vacuum. 37 (3–4): 309–315. Bibcode:1987Vacuu..37..309D. doi:10.1016/0042-207X(87)90015-7.