Neutral-beam injection

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

The adsorption length ${\displaystyle \lambda }$ for neutral beam ionization in a plasma is roughly

${\displaystyle \lambda ={\frac {E}{18\cdot n\cdot M}},}$

with ${\displaystyle \lambda }$ 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.

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:

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⁻ + D₂ → D⁰ + e + D₂ (singe-electron detachment, with ${\displaystyle \sigma }$₋₁₀=1.13×10⁻²⁰ m² at 1 MeV)

D⁻ + D₂ → D⁺ + e + D₂ (double-electron detachment, with ${\displaystyle \sigma }$₋₁₁=7.22×10⁻²² m² at 1 MeV)

D⁰ + D₂ → D⁺ + e + D₂ (reionization, with ${\displaystyle \sigma }$₀₁=3.79×10⁻²¹ m² at 1 MeV)

D⁺ + D₂ → D⁰ + D₂⁺ (charge exchange, ${\displaystyle \sigma }$₁₀ negligible at 1 MeV)

Underline indicates the fast particles, while subscripts i, j of the cross-section ${\displaystyle \sigma }$_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 ${\displaystyle \tau =\int n\,dl,}$ with ${\displaystyle n}$ the gas density along the beam path ${\displaystyle l}$. In the case of D⁻ beams, the maximum neutralisation yield occurs at a target thickness ${\displaystyle \tau _{{\text{D}}^{-},{\text{1 MeV}}}\approx 1.4\cdot 10^{-16}}$ m⁻².

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 ${\displaystyle Q}$ [Pa·m³/s], the maximum gas pressure at the centre of the cell depends on the gas conductance ${\displaystyle C}$ [m³/s]:

${\displaystyle P_{0}=P_{\text{tank}}+{\frac {Q}{2C}},}$

and ${\displaystyle C}$ in molecular-flow regime can be calculated as

${\displaystyle C={\frac {9.7}{L/2}}{\sqrt {\frac {T}{m}}}{\frac {a^{2}\cdot b^{2}}{a+b}},}$

with the geometric parameters ${\displaystyle L}$, ${\displaystyle a}$, ${\displaystyle b}$ indicated in figure, ${\displaystyle m}$ gas molecule mass, and ${\displaystyle T}$ 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 ${\displaystyle L}$ 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).