Neutral beam injection systems are conceptually quite straightforward. Following the schematic given in Figure 1.4, ions (either positive or negative) are created within a plasma source before being electrostatically extracted into a beam line. The ions are then electrostatically accelerated between high voltage acceleration grids. Once the ions attain the specified energy, they are passed through a neutral gas chamber where charge exchange reactions with the background gas neutralise the ion beam, creating a high energy neutral beam. This neutral beam is then injected directly into the plasma volume of fusion devices where direct collisional heating occurs. extraction grid plasma source acceleration grids neutraliser to reactor
Figure 1.4: Schematic of a neutral beam injection system.
The neutralisation process is crucial due to the fact that a charged beam would both interact with existing magnetic confinement fields and generate its own magnetic fields in addition to disrupting quasineutrality. The neutralisation efficiency of ions is a non-linear function of particle energy, where the neutrali- sation of positive ions becomes prohibitively inefficient for energies greater than
∼ 100 keV[4]. As fusion reactors are expected to operate with neutral beam energies on the order of 1 MeV (with ITER’s neutral beam system targeted for 1 MeV in deuterium and 0.87 MeV in hydrogen[3]), it is therefore necessary to operate with negative ions rather than positive ions. This presents a significant challenge, owing to the fact that negative hydrogen ions are much more difficult to produce than positive hydrogen ions[6]. Furthermore, the extraction of nega-
tively charged particles from the ion source introduces the problem of coextracted electrons which can both interfere with the negative ion beam and damage beam line components[36]. Additionally, negative ions are only weakly bound systems where electrons can easily be stripped through collisions with background neutrals within the beam line, limiting the operational neutral pressure of ion sources[36]. The development of high throughput negative ion sources capable of supplying the exceptionally high extracted negative ion currents while meeting other rigor- ous operational requirements of magnetic confinement fusion reactors remains an open problem and stands as a high priority for the magnetic confinement fusion research effort[7].
1.2.1
Negative Ion Sources
As mentioned above, the production of negative hydrogen ions is distinctly non- trivial. Hydrogen is only weakly electronegative, with an electron affinity of just 0.754 eV. The negative ion is therefore highly susceptible to collisional detachment due to electrons and even in collisions with neutral atomic or molecular species[37]. While it is relatively straightforward to achieve plasma densities of 1017 −1019 m−3 in low temperature laboratory plasma devices, the negative charge carriers
are typically strongly dominated by electrons, with negative ions often making up only a small fraction of the total plasma density[9]. It is therefore necessary to take special measures to engineer favourable conditions for negative ion production in order to achieve the high densities of negative ions required to supply fusion relevant neutral beam systems.
There are two main processes which are generally accepted as being the princi- pal mechanisms for the generation of negative ions in hydrogen plasmas[6]: a vol- ume production mechanism, in which electrons interact with vibrationally excited molecular hydrogen, causing dissociative attachment resulting in the production
of atomic hydrogen and negative hydrogen ions; and a surface production mech- anism, in which hydrogen atoms adsorbed onto a surface acquire an additional electron from the surface material before being released back into the plasma volume.
In typical plasma devices, the two mechanisms are of roughly equal importance, however each mechanism involves its own distinct limitations which prevent the efficient production of negative ions at high densities[6]. The volume mechanism relies on high densities of both electrons and vibrationally excited molecular hy- drogen to occur. This in turn requires a high frequency of ionisation and excitation events which is most efficient at relatively high electron temperatures. High neg- ative ion densities, however, can only be sustained at low electron temperatures, where electron collisions can be limited. This inherent contradiction severely lim- its the efficiency of the volume production mechanism. On the other hand, the surface mechanism is limited by the propensity of the surface material to give up electrons to the weakly electronegative hydrogen atoms. The work function of common plasma facing materials such as stainless steel or tungsten is typically in excess of 4 eV which strongly impedes the electron transfer process required for negative ion formation.
While the limitations of volume production are inherently difficult to address, surface production can be readily improved with the use of low work function materials on plasma facing surfaces[38]. Materials with low work functions do not typically have the structural, thermal, and chemical properties required for the bulk construction of internal device structures and surfaces. It is therefore common to evaporate a thin layer of low work function material onto the plasma facing surfaces in order to achieve improved performance of the surface produc- tion mechanism. With caesium having the lowest work function of any elemental material, it is commonly used for the conditioning of surfaces in conventional neg-
ative ion sources, typically achieving a 2-8 times improvement in performance[6]. Due to plasma material interactions, the thin layer of caesium is easily damaged, necessitating a continuous evaporation of caesium into the plasma chamber to replace it. In addition to being able to readily lower the work function of plasma facing materials, the surface production mechanism also offer the advantage of be- ing easily separated from the high electron temperature region where the plasma is generated. This allows plasma to be cooled by a magnetic filter, thus reducing collisional losses without strongly affecting negative ion generation rates.
As described in Figure 1.5 a typical conventional negative ion source operating on the principle of surface production will consist of the following[7]: an induc- tively coupled plasma source to generate the plasma; a caesium dispenser oven, from which caesium is continuously evaporated into the plasma chamber; a trans- verse magnetic filter which both cools and redirects electrons (limiting coextracted electron flux); a positively biased plasma grid which both provides a surface for negative ion production and separates out negative species; and an extraction grid which accelerates the negative species into the beam line.
ICP source Cs oven transverse magnetic filter
plasma grid extraction grid
to beam line
Figure 1.5: Schematic of a conventional negative ion source for NBI.
While the use of caesium conditioning has allowed conventional negative ion sources to meet the stringent throughput requirements of fusion relevant neutral
This is largely attributable to the fact that caesium must be continuously evapo- rated into the device during operation. In order for the uniformity of the negative ion beam to be maintained, caesium must be uniformly distributed across the plasma grid surface and must remain so throughout operation (with ITER requir- ing operational durations of at least 1 h)[3]. Due to interactions with the plasma and the magnetic filter field, it has proven difficult to control caesium dynamics and ensure uniformity during extended operation[39–41]. Even if sufficient unifor- mity can be maintained within an extended pulse, even slight asymmetries would accumulate between pulses, necessitating regular reconditioning. Additionally, the caesium reservoir must be regularly replenished. This is distinctly unfavourable within operational fusion reactors where the strong neutron flux (and subsequent activation of device materials) requires that all maintenance be performed re- motely.
With caesium dynamics limiting the long term performance of surface based negative ion sources, there has been a renewed interest in the development of vol- ume based negative ion sources which could be operated without caesiation and run continuously without active maintenance requirements. This would require a significant improvement in the performance of the volume production mechanism. A recent proposal for achieving this is the use of helicon plasma drivers for nega- tive ion sources[8]. As helicon devices can achieve an order of magnitude higher plasma densities than ICP devices and at lower electron temperatures, it is ex- pected that they might offer higher volume production rates and lower collisional losses than conventional negative ion sources with ICP drivers. This approach doesn’t really address the intrinsic inefficiencies of the volume production method, but rather acts as a ‘brute force’ approach to increase negative ion production. This proposal has spawned a significant research interest, with numerous groups currently developing exploratory helicon based negative ion sources to investigate their viability[11–14].