Many different gas discharges and plasmas exhibit bistable states under a given set of conditions, and the history-dependent hysteresis that is manifested by intensive quantities of the system upon variation of an external parameter has been observed in inductively coupled plasmas (ICPs). When the external parameters (such as discharge powers) increase, the plasma density increases suddenly from a low- to high-density mode, whereas decreasing the power maintains the plasma in a relatively high-density mode, resulting in significant hysteresis. To date, a comprehensive description of plasma hysteresis and a physical understanding of the main mechanism underlying their bistability remain elusive, despite many experimental observations of plasma bistability conducted under radio-frequency ICP excitation. This fundamental understanding of mode transitions and hysteresis is essential and highly important in various applied fields owing to the widespread use of ICPs, such as semiconductor/display/solar-cell processing (etching, deposition, and ashing), wireless light lamp, nanostructure fabrication, nuclear-fusion operation, spacecraft propulsion, gas reformation, and the removal of hazardous gases and materials. If, in such applications, plasma undergoes a mode transition and hysteresis occurs in response to external perturbations, the process result will be strongly affected. Due to these reasons, this paper comprehensively reviews both the current knowledge in the context of the various applied fields and the global understanding of the bistability and hysteresis physics in the ICPs. At first, the basic understanding of the ICP is given. After that, applications of ICPs to various applied fields of nano/environmental/energy-science are introduced. Finally, the mode transition and hysteresis in ICPs are studied in detail. This study will show the fundamental understanding of hysteresis physics in plasmas and give open possibilities for applications to various applied fields to find novel control knob and optimizing processing conditions.

Review of inductively coupled plasmas: Nano-applications and bistable hysteresis physics

Negative ion sources for fusion are high densities plasma sources in large discharge volumes. There are many challenges in the modeling of these sources, due to numerical constraints associated with the high plasma density, to the coupling between plasma and neutral transport and chemistry, the presence of a magnetic filter, and the extraction of negative ions. In this paper we present recent results concerning these different aspects. Emphasis is put on the modeling approach and on the methods and approximations. The models are not fully predictive and not complete as would be engineering codes but they are used to identify the basic principles and to better understand the physics of the negative ion sources.

https://iopscience.iop.org/article/10.1088/1367-2630/19/1/015002/pdf

Modeling of plasma transport and negative ion extraction in a magnetized radio-frequency plasma source

Neutral beam injection is one of themost importantmethods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions aswell as driving the plasma current. Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into themagnetised plasma. At the particle energy required for themost advanced thermonuclear devices and particularly for ITER, neutralisation of positive ions is very inefficient so that negative ions are used. The present paper is devoted to the description of the phenomena occurringwhen a high-powermulti-ampere negative ion beam travels from the beam source towards the plasma. Simulation of the trajectory of the beam and of its features requires various numerical codes, whichmust take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beamwith the background gas. Themain outcome is the partial neutralisation of the beamparticles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnosticmethods capable of investigating the beamproperties and of assessing the relevance of the various phenomenawill be discussed. Examples will be given regarding themeasurements collected in the smallflexibleNIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection betweenmeasurements and simulations in view of the operation of the beam is highlighted.

/pdf/neutralisation-and-transport-of-negative-ion-beams-physics-88z184k0f9.pdf

Neutralisation and transport of negative ion beams: physics and diagnostics

In the next generation of fusion reactors, such as DEMO, neutral beam injectors (NBIs) of high energy (0.8-1 MeV) deuterium atoms with high wall-plug efficiency (>50%) will be required to reach burning plasma conditions and to provide a significant amount of current drive. The present NBI system for DEMO assumes that 50 MW is delivered to the plasma by 3 NBIs. In the Siphore NBI concept, negative deuterium ions are extracted from a long, thin ion source 3 m high and 15 cm wide, accelerated and subsequently photo-neutralized. This requires the development of a new generation of negative ion sources. At the Swiss Plasma Center, a novel radio frequency helicon plasma source, based on a resonant network antenna source delivering up to 10 kW at 13.56 MHz, has been developed and is presently under study on the Resonant Antenna Ion Device (RAID). RAID is a linear device (1.9 m total length, 0.4 m diameter) and is equipped with an extensive set of diagnostics for full plasma characterization. In this work, the principles of operation of resonant antennas as helicon sources are introduced. We present absolute spectroscopy, Langmuir probe, and interferometry measurements on helicon plasmas. We characterize the performance of the source in terms of hydrogen/deuterium dissociation and negative ion production as a function of the input power. Furthermore, first results with the helicon birdcage antenna installed on the Cybele negative ion source at CEA-IRFM are presented, as a first step towards the validation of the Siphore concept.

/pdf/helicon-wave-generated-plasmas-for-negative-ion-beams-for-2hu58a0wwn.pdf

Helicon wave-generated plasmas for negative ion beams for fusion

Cavity Ring Down Spectroscopy (CRDS) is used to measure the D− absolute density produced in the helicon plasma reactor RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center. The birdcage geometry of the helicon antenna produces a homogeneous, high-density plasma column (ne ≅ 1.5 × 1018 m−3 in H2 and D2 at 0.3 Pa and 3 kW of input power) 1.4 m long. We present the CRDS experimental setup, its positioning on the RAID reactor, and how the mechanical and thermal effects of the plasma affect the measurement. First results in deuterium plasma confirm the production of negative ions (D−) with a significant density: an average value of 3.0 × 1016 m−3 of D− is obtained at 0.3 Pa and 5 kW of power input in Cs-free plasma. This result is in good agreement with calculations performed with the collisional radiative code YACORA.

Cavity ring-down spectroscopy to measure negative ion density in a helicon plasma source for fusion neutral beams

In parallel to the developments dedicated to the ITER neutral beam (NB) system, CEA-IRFM with
laboratories in France and Switzerland are studying the feasibility of a new generation ofNBsystem
able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steadystate
scenario, theNBsystem will have to provide a highNBpower level with a high wall-plug
efficiency (η∼60%). Neutralization of the energetic negative ions by photodetachment (so called
photoneutralization), if feasible, appears to be the ideal solution to meet these performances, in the
sense that it could offer a high beam neutralization rate (>80%) and a wall-plug efficiency higher than
60%. The main challenge of this new injector concept is the achievement of a very high power photon
flux which could be provided by 3MWFabry–Perot optical cavities implanted along the 1 MeVD−
beam in the neutralizer stage. The beamline topology is tall and narrow to provide laminar ion beam
sheets, which will be entirely illuminated by the intra-cavity photon beams propagating along the
vertical axis. The paper describes the presentR&D(experiments and modelling) addressing the
development of a new ion source concept (Cybele source) which is based on a magnetized plasma
column. Parametric studies of the source are performed using Langmuir probes in order to
characterize and compare the plasma parameters in the source column with different plasma
generators, such as filamented cathodes, radio-frequency driver and a helicon antenna specifically
developed at SPC-EPFL satisfying the requirements for the Cybele (axial magnetic field of 10 mT,
source operating pressure: 0.3 Pa in hydrogen or deuterium). The paper compares the performances
of the three plasma generators. It is shown that the helicon plasma generator is a very promising
candidate to provide an intense and uniform negative ion beam sheet.

https://iopscience.iop.org/article/10.1088/1367-2630/18/12/125005/pdf

J M Bernard

Papers

Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors