Internal transport barriers in tokamak plasmas are explored in order to improve confinement and stability beyond the reference scenario, used for the ITER extrapolation, and to achieve higher bootstrap current fractions as an essential part of non-inductive current drive. Internal transport barriers are produced by modifications of the current profile using external heating and current drive effects, often combined with partial freezing of the initial skin current profile. Thus, formerly inaccessible ion temperatures and QDTeq values have been (transiently) achieved. The present paper reviews the state of the art of these techniques and their effects on plasma transport in view of optimizing the confinement properties. Implications and limits for possible steady state operations and extrapolation to burning plasmas are discussed.

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Internal transport barriers in tokamak plasmas

Negative hydrogen/deuterium ions can be formed by processes occurring in the plasma volume and on surfaces facing the plasma. The principal mechanisms leading to the formation of these negative ions are dissociative electron attachment to ro-vibrationally excited hydrogen/deuterium molecules when the reaction takes place in the plasma volume, and the direct electron transfer from the low work function metal surface to the hydrogen/deuterium atoms when formation occurs on the surface. The existing theoretical models and reported experimental results on these two mechanisms are summarized. Performance of the negative hydrogen/deuterium ion sources that emerged from studies of these mechanisms is reviewed. Contemporary negative ion sources do not have negative ion production electrodes of original surface type sources but are operated with caesium with their structures nearly identical to volume production type sources. Reasons for enhanced negative ion current due to caesium addition to these sources are discussed.

Negative hydrogen ion production mechanisms

The ITER neutral beam injectors are the first injectors to be designed to operate under conditions and constraints similar to those that will be encountered with a fusion reactor. The injectors will use a single large ion source and accelerator that will produce 40A D− 1MeV beams for pulse lengths of up to 3600s. The accelerated ion beams will be neutralized in a gas (D2) neutralizer which is subdivided into four vertical channels to reduce the gas flow into the injectors that is needed to produce optimum target for neutralization. These injectors will have to operate in a hostile radiation environment and they will become highly radioactive due to the neutron flux from ITER. The design has been modified recently to have a rectangular vacuum vessel with a removable lid that allows vertical access to, and maintenance of, the beamline components, the incorporation of an absolute all metal valve at the exit of the injector, the choice of a rf driven ion source as the reference design of ion source, and to have a high voltage deck incorporating the various auxiliary power supplies in air rather that under high pressure SF6. A major development is that it has been agreed that a Neutral Beam Test Facility (NBTF) will be set up at Padua, Italy. The NBTF will consist of two test beds: one of which will be capable of operating a complete injector at full performance. The second will be an ion source test bed, which will be used for the development and testing, to full performance, of the large negative ion source.

Status of the ITER neutral beam injection system (invited)a)

The design of a diode laser frequency stabilization system using the Zeeman effect is described. Various regimes of operation are analyzed using the Jones matrix approach. The system is different from the original Joint Institute for Laboratory Astrophysics design in that the magnetic fields are fully contained and thus it can be used in proximity of magnetically sensitive instruments.

Laser frequency stabilization using linear magneto-optics

Several applications of PIC simulations for understanding basic physics phenomena in low-temperature plasmas are presented: capacitive radiofrequency discharges in Oxygen, dusty plasmas and negative ion sources for heating of fusion plasmas. The analysis of these systems based on their microscopic properties as accessible with PIC gives improved insight into their complex behavior. These studies are results of joint efforts over about one decade of research groups from Greifswald University, Germany; Bari University, Italy; Keio University, Japan and Innsbruck University, Austria. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Particle in Cell Simulation of Low Temperature Laboratory Plasmas

Intense negative ion source producing multimegawatt hydrogen/deuterium negative ion beams has been developed for the neutral beam injector (NBI) in TOKAMAK thermonuclear fusion machines. Negative ions are produced in a cesium seeded multi-cusp plasma generator via volume and surface processes, and accelerated with a multistage electrostatic accelerator. The negative ion source for JT-60U has produced 18.5 A/360 keV (6.7 MW) H− and 14.3 A/380 keV (5.4 MW) D− ion beams at average current densities of 11 mA/cm2 (H−) and 8.5 mA/cm2 (D−). A high energy negative ion source has been developed for the next generation TOKAMAK such as the International Thermonuclear Experimental Reactor (ITER). The source has demonstrated to accelerate negative ions up to 1 MeV, the energy required for ITER. Higher negative ion current density of more than 20 mA/cm2 was obtained in the ITER concept sources. It was confirmed that the consumption rate of cesium is small enough to operate the source for a half year in ITER-NBI without maintenance.

Negative hydrogen ion source for TOKAMAK neutral beam injector (invited)

The production mechanism of negative ions has been investigated experimentally in a cesium seeded volume type negative ion source. The cesium light emission intensity, Cs ion volume density, plasma parameters, H- current, and work function of the plasma grid were measured. The change in plasma parameters was small even in a high Cs ion density region(up to 1012 cm-3) which was also confirmed by zero-dimensional simulation. Upon heating the plasma grid, the work function decreased and the negative ion current increased to become three times larger than that of the low plasma grid temperature condition. This increase was in good agreement with the decrease of the work function of the plasma grid surface. Therefore, it is considered that the Cs contribution for the negative ion production is the surface effect in the negative ion source.

https://iopscience.iop.org/article/10.1143/JJAP.40.4709/pdf

Mechanism of negative ion production in a cesium seeded ion source

Two Monte Carlo simulation codes: (a) neutral transport code and (b) negative ion (H−) transport code, have been developed to understand transport phenomena in negative ion sources. In the neutral transport code, Boltzmann equations for hydrogen molecules (H2) and atoms (H) are solved. Three dimensional (3D) spatial distributions of H2, H, and H− production are obtained for a tandem negative ion source. The volume production of H− is limited to the area around the gas inlet in the first chamber and near the plasma grid in the second chamber. On the other hand, distribution of H− surface production is shown to be almost uniform over all the plasma grid surface. In the negative ion code, H− trajectories are calculated by numerically solving the 3D equation of motion for H− ions. The effects of the magnetic filter on the extraction probability of surface produced H− ions are mainly studied. The dependence of the extraction probability on the field strength is small.

Monte Carlo simulation of negative ion production in the negative hydrogen ion source

Development of the negative-ion sources has been conducted to realize a high power neutral beam injector for International Thermonuclear Experimental Reactor (ITER). A high negative-ion current density of 31 mA/cm2 (H−) at a very low pressure of 0.1 Pa has been produced in a cesium seeded multicusp plasma generator which has the same concept of the ITER source. For a vacuum insulated accelerator, a voltage holding experiment of long distance vacuum gaps up to ∼1.8 m has been performed. It was clarified that the transition region of product pressure distance (pd) from the vacuum breakdown to the gas discharge is about 0.2 Pa m which is high enough from the operating region of the ITER source. A prototype vacuum insulated accelerator was fabricated based on the experiment and tested. A high-energy H− beam acceleration up to 970 keV, 37 mA, and 1 s has been successfully demonstrated.

Development of a large volume negative-ion source for ITER neutral beam injector

Development of an electrostatic accelerator for 1 MeV, 1 ampere class H− ion beams has been carried out as a Proof-of-Principle (PoP) of high current accelerators required in neutral beam system of fusion reactors such as ITER A unique feature of such fusion oriented accelerators is to utilize vacuum insulation not only inside but also outside of the accelerator for high voltage insulation, since conventional gas insulation is not applicable due to excess radiation induced conductivity in the gas The PoP accelerator is surrounded by an insulator stack as a vacuum boundary with a vacuum gap of 50 mm all around the accelerator One of the key technologies to achieve insulation of MV voltage was reduction of electric field strength at triple junction of the insulator (made of FRP, fiber reinforced plastic) stack By lowering the stress to 17 kV/mm, the accelerator sustained 1 MV stably for more than 2 hours By tuning the ion source with/without cesium under stable accelerator operation, the PoP accelerat

T. Morishita

Papers

Negative hydrogen ion source for TOKAMAK neutral beam injector (invited)

Mechanism of negative ion production in a cesium seeded ion source

Monte Carlo simulation of negative ion production in the negative hydrogen ion source

Development of a large volume negative-ion source for ITER neutral beam injector

Acceleration of 1 MeV, 100 mA Class H− ion beams in a proof-of-principle accelerator for ITER