Neutral Beam Ion Research Articles

Deterioration of ion beam neutralization caused by reflection of electrostatic solitary waves within pulsed ion beam

The neutralization of an ion beam pulse directly by electron injection excites the two-stream instability of neutralizing electrons and gives rise to an electrostatic solitary wave (ESW) in the long time limit. The ESW propagates stably and reflects back and forth in the potential well of the ion beam pulse. Through a two-dimensional particle-in-cell code, we numerically simulated the whole neutralization process of the ion beam pulse as well as the excitation and propagation of the ESW. It is found that the reflections of the ESW at the edges of the ion beam pulse cause a lot of neutralizing electrons to be thrown out, forming escaping electrons released in a pulsed manner. Each reflection of the ESW accelerates the reduction of the neutralization degree and brings a small disturbance to the ESW. The accumulative effect of multiple reflections results in rapid collapse of the ESW at the end. The reflections of the ESW at the head and tail of the beam pulse cause the ion beam to lose more neutralizing electrons than the slow attenuation of the ESW inside the ion beam.

Reduction of EUV resist damage by neutral beam etching

Even though EUV lithography has the advantage of implenting a finer pattern compared to ArF immersion lithography due to the use of 13.5 nm instead of 193 nm as the wavelength of the light source, due to the low energy of EUV light source, EUV resist has a thinner thickness than conventional ArF resist. EUV resist having such a thin thickness is more vulnerable to radiation damage received during the etching because of its low etch resistance and also tends to have a problem of low etch selectivity. In this study, the radiation damage to EUV resist during etching of hardmask materials such as Si3N4, SiO2, etc using CF4 gas was compared between neutral beam etching (NBE) and ion beam etching (IBE). When NBE was used, after the etching of 20 nm thick EUV resist, the line edge roughness increase and the critical dimension change of EUV resist were reduced by ∼1/3 and ∼1/2, respectively, compared to those by IBE. Also, at that EUV etch depth, the root mean square surface roughness value of EUV resist etched by NBE was ∼2/3 compared to that by IBE on the average. It was also confirmed that the etching selectivity between SiO2, Si3N4, etc and EUV resist was higher for NBE compared to IBE. The less damage to the EUV resist and the higher etch selectivity of materials such as Si3N4 and SiO2 over EUV resist for NBE compared to IBE are believed to be related to the no potential energy released by the neutralization of the ions during the etching by NBE.

Characterization of a Multicusp Ion Source With Two-Grid Extraction System for Studying Extraction and Transport of Ion Beam

A low-energy multipole line cusp ion source with two-grid extraction system has been developed and characterized for studying the role of the charge exchange processes on ion beam extraction, transport, and neutralization. The ion source is operated for extraction of Ar <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion beam of current 50–240 mA at energies 850–1650 eV. The characterization of the ion beam is carried out at various axial positions along a 200–815-mm beam path with a background pressure of 4– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> Torr. An 11-channel Faraday cup array is used to measure the radial profiles of ion current density and angular beam divergence at these positions. The beam divergences are found to be in the range of 6°–15° for beam perveance of 0.50– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.75\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> AV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3/2</sup> . The attenuation of ion beam current along the beam path is observed due to the charge exchange neutralization of the energetic ions in the presence of high background pressure. Since ion current density along the beam path can be reduced by both the charge exchange processes and beam divergence, the accounting of beam particles, i.e., energetic ions and neutrals, at larger distances is difficult by the measurements of ion current density using the Faraday cup alone. The flux of energetic particles (energetic ions and fast neutrals) is therefore obtained by a thrust measurements on a beam target fixed at a distance of 800 mm from the ion source. The neutralization of the ion beam obtained by the emission of electrons from a hot filament neutralizer is also investigated in the presence of the charge exchange processes. Space-charge neutralization is provided initially by the secondary electrons emitted from the vessel walls or by the charge exchange processes even without the neutralizer operating at the desired temperature. However, the current neutralization is achieved only by the electrons emitted from the hot filament neutralizer operating in a space-charge-limited mode.

Plasma plume expansion with pulsed electron neutralization

Electrons neutralizing the ion beam from a gridded ion source are typically provided by an external cathode. This cathode emits a continuous current that ensures quasi-neutrality of the plume, and current balance of the ion source. A new type of neutralization scheme has recently been identified in the context of radio-frequency (RF) biased ion sources, where instead of a continuous electron current, the plume is neutralized by electron pulses emitted from the same plasma source as the ion beam itself. In contrast to conventional gridded ion sources, experiments have shown that pulsed neutralization produces hot electrons with a strongly anisotropic energy distribution in the plume. By making use of a two-dimensional particle-in-cell (PIC) simulation, we analyze the pulsed neutralization and plasma expansion to understand the fundamental plume physics in these systems, and perform a direct comparison with the expansion observed in typical DC systems. Electron trapping in the near-field plume region is found to be critical for ensuring quasi-neutrality, and the plume potential is observed to be higher than the downstream acceleration grid potential to prevent excessive electron backstreaming into the plasma source. This potential difference results in the formation of high-energy electron beams that generate collective plume oscillations with frequencies above the applied RF frequency. A detailed parametric study is performed to investigate the influence of the pulse frequency, emission current, and capacitance between the source and outer surrounding boundaries. In particular, the pulse frequency and emission current have a significant effect on the resulting plume potential, and the effectiveness of the resulting ion beam neutralization.

Highly Efficient Small Anode Ion Source

We describe some modifications to a Bernas-type ion source that improve the ion beam production efficiency and source operating lifetime. The ionization efficiency of a Bernas type ion source has been improved by using a small anode that is a thin rod, oriented along the magnetic field. The transverse electric field of the small anode causes the plasma to drift in the crossed ExB field to the emission slit. The cathode material recycling was optimized to increase the operating lifetime, and the wall potential optimized to suppress deposition of material and subsequent flake formation. A three-electrode extraction system was optimized for low energy ion beam production and efficient space charge neutralization. An ion beam with emission current density up to 60 mA/cm2 has been extracted from the modified source running on BF3 gas. Space charge neutralization of positive ion beams was improved by injecting electronegative gases.

Thermo-mechanical design of the neutralizer for CRAFT negative ion-based neutral beam injection system

The Comprehensive Research fAcility for Fusion Technology (CRAFT) has been launched in China and a negative ion-based neutral beam injection system (NNBI) is an important component of CRAFT. The primary target of the CRAFT NNBI is to realize a neutral hydrogen beam of >2 MW at the beam energy of 400 keV for the pulse duration of 100 s. In the CRAFT NNBI, the neutralization of the negative ion beam will be accomplished in a gas neutralizer mainly consisting of two vertical channels. Each channel has a rectangular cross-section of 0.145 m × 1.7 m and a length of 3 m, which is formed by several copper panels. A key issue of the neutralizer is to remove the heat load from the incident ion beam. This paper presents the thermo-mechanical design process of the neutralizer of the CRAFT NNBI. Because the heat load densities on the lateral walls of the panels are relatively low, their surface temperature can be well controlled under the safe values of copper by deep drilling of the cooling channels. The largest problem is the leading edge of the panels, where the heat load density is extremely high. Thus, the leading edge element (LEE) is carefully designed to avoid damage to these panels. The LEE is made of CuCrZr alloy and the twisted tape is inserted into the cooling channel to promote the turbulence and consequently the heat transfer capability. Finally, the thermo-mechanical properties of the heat load components of the neutralizer can be controlled under the required values in terms of the surface temperature, water temperature, stress, and deformation.

New assessment of the fast ion energy in ASDEX upgrade H-mode discharges

Confinement scaling laws such as IPB98(y, 2) are widely used to extrapolate the performance of present tokamaks to next-step devices such as ITER or DEMO. The thermal energy of the plasma (W th), which is used to determine the energy confinement time for most scaling laws, is difficult to measure, due to the sizeable uncertainties in the experimental kinetic profiles. The common approach in the tokamak community is to derive W th as the difference between the measured magnetohydrodynamic (MHD) energy and some simulation-based estimate of the fast ion energy W fi. In H-mode plasmas W fi can be as high as W th, in presence of neutral beam injection (NBI) or ion cyclotron radio frequency heating (ICRF), therefore an accurate assessment of W fi is crucial to have a somewhat reliable H-factor, regardless of the power-scaling of a given scaling law. In this paper we aim at evaluating the current approach to estimate W fi, by comparing its predictions with a wide database of calculations using validated NBI codes. Systematic deviations and trends, as well as statistical scatter are discussed. We use a comprehensive database of AUG H-mode deuterium plasmas, with significant variations of plasma current, NBI power and plasma density. We neglect thereby the fast-ion losses caused by MHD modes and the synergy effect between NBI and ICRF. A new approach is proposed based on the newly developed fast NBI code RABBIT.

Excitation of the beta-induced Alfvén-acoustic eigenmode during sawtooth-like oscillation in EAST

The characteristics and excitation conditions of the beta-induced Alfvén-acoustic eigenmode (BAAE) have been investigated during sawtooth-like oscillation in the Experimental Advanced Superconducting Tokamak (EAST). The sawtooth-like events are triggered periodically by the double-tearing reconnection crash (DTRC), and the oscillations with period τo≥ 100 ms, where m and n are the poloidal and toroidal mode numbers respectively. The BAAE coexists with beta-induced Alfvén eigenmodes (BAEs) and reversed-shear Alfvén eigenmodes (RSAEs), and the BAAE has similar characteristics to the pairing of BAEs–RSAEs. The radial structures of the BAAE and BAEs are deformed into analogous triangle shapes along the poloidal direction, and the BAAE is located more outward with broadened radial coverage. The frequencies of the BAAE and RSAEs sweep upward synchronously with decreasing ( < 1), and the BAAE can also provide valuable constraint conditions for the q-profile measurements. Furthermore, the mode structure of the BAAE is achieved experimentally with m = 4/n ≈ 4.The period τo of DTRC is an important factor, which can be adopted for evaluating the excitation conditions of BAAE; namely, the BAAE can be excited when τo achieves the threshold condition. τo increases when the total injected power of neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) increases, where the NBI beam direction is perpendicular to the plasma current. The gradient of electron temperature at the vicinity of increases with τo, and the portion of energetic ions in the central region increases as well. Therefore, the BAAE in EAST should be excited by the special distribution of trapped energetic ions through toroidal precessional resonances, and is also an important constraint condition for the excitation.

Identification of an Optimized Heating and Fast Ion Generation Scheme for the Wendelstein 7-X Stellarator.

A Doppler shifted resonance minority species ion cyclotron range of frequency (ICRF) scheme for heating neutral beam ions has been identified and optimized for the Wendelstein 7-X stellarator. Compared with more conventional methods, the synergetic scheme increases the normalized core collisional power transfer to the background plasma, and induces larger concentrations of energetic ions. Simulations in the intricate 3D magnetic stellarator geometry reveal an energetic distribution function that is only weakly anisotropic, and is thus relevant to fast ion and alpha particle driven Alfvén eigenmode experimental preparation. Quasilinear theory and simulations of the Joint European Torus indicate that the excellent confinement properties are due to increased velocity diffusion from ICRF interaction along the magnetic field lines. Agreement is found between SCENIC simulations and Joint European Torus experimental measurements for the total neutron rate and the energy distribution of the fast ions.

Neutralization of ion beam by electron injection: Accumulation of cold electrons

Ion beam charge neutralization by electron injection is a complex kinetic process. Recent experiments show that the resulting self-potential of the ion beam after neutralization by plasma is much lower than the temperature of plasma electrons [Stepanov et al., Phys. Plasmas 23, 043113 (2016)], indicating that kinetic effects are important and may affect the neutralization of the ion beam. We performed a numerical study of the charge neutralization process of an ion beam making use of a two-dimensional electrostatic particle-in-cell code. The results show that the process of charge neutralization by electron injection is composed of two stages. During the first stage, the self-potential of the beam is higher than the temperature of injected electrons (Te/e) and all injected electrons are captured by the ion beam. During the second stage, hot electrons escape from the ion beam and the beam self-potential (φ) decreases because cold electrons slowly accumulate resulting in the beam self-potential φ to become much lower than Te/e in agreement with previous experimental observations at Princeton Advanced Teststand. We also determined that the resulting φ scales as φ∼Te, in agreement with previous experimental observations from Gabovich's group. In addition, the results show that the transverse position of the electron source has a great impact on ion beam neutralization. A slight shift of the electron source as relevant to the ion thrusters leads to a large increase in the beam self-potential because of an increase in potential energy of injected electrons.

Neutralization of ion beam by electron injection: Excitation and propagation of electrostatic solitary waves

The charge neutralization of an ion beam by electron injection is investigated using a two-dimensional electrostatic particle-in-cell code. The simulation results show that electrostatic solitary waves (ESWs) can be robustly generated in the neutralization process and last for a long time (for more than 30 μs) and therefore, ESWs can strongly affect the neutralization process. The ESWs propagate along the axis of the ion beam and reflect from the beam boundaries. The simulations clearly show that two ESWs can pass through each other with only small changes in amplitude. Partial exchange of trapped electrons in collisions of two ESWs is observed in the simulations and can explain interaction during collisions of two ESWs. Coalescence of two ESWs is also observed.

On the limitations of hybrid particle-in-cell for ion thruster plume simulations

A commonly used approach to simulate ion thruster plumes is a hybrid particle-in-cell (PIC) with the Boltzmann relation to model electrons. The limitations of such an approach are investigated by benchmarking hybrid PIC against fully kinetic PIC simulations using the real Xe+ to electron mass ratio (mi/me = 241073). The results show that both full PIC and hybrid PIC lead to similar plume density structures but very different plume potentials. If one could ignore the ion beam neutralization region near the thruster exit, the main physics missing from a hybrid PIC plume model is that from the anisotropic electron characteristics. Including the beam neutralization region leads to further changes in electron temperature and plume potential. Compared to full PIC simulations, the hybrid PIC model results in an overestimation of the plume potential by about 30%–40% in the transverse direction for the simulations considered. Since the electrons in the plume are nonequilibrium and anisotropic, simply replacing the Boltzmann relation with the more general polytropic thermodynamic relation for electrons in a hybrid PIC plume model will not lead to a significant improvement in accuracy.

Physical design of the neutralizer for CFETR negative ion based neutral beam injection prototype

A research project of the CFETR Negative ion based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI protype are to produce a negative hydrogen ion beam of >20 A to 200–400 kV for 3600 s and to attain a neutralization efficiency of >50%. Thus, a neutral beam power of >2 MW is foreseen onto the calorimeter (i.e., beam dump). A gas neutralizer is applied inside the beamline of CFETR NNBI prototype, to achieve the neutralization of negative ion beam through the charge-exchange collisions with the gas target. The most critical issues for the gas neutralizer are to balance the contradictive requirements of high neutralization efficiency, low gas inflow, and low heat load. During the physical design of the neutralizer for CFETR NNBI prototype, two proposals have been considered and studied in detail. One has single beam channel and other one has two narrow channels separated by a panel. The narrow channel design is benefic to decrease gas conductance, and thus reduce the required gas inflow to attain the maximal neutralization efficiency. However, the inserted panel will restrict the extraction area of the beam source, where the aperture columns corresponding to the panel should be masked to avoid the direct beam interception. But the leading edge of the inserted panel will still suffer a high thermal load due to the stray particles from the beam source. A 3D model of the whole beamline of CFETR NNBI prototype has been developed to study the gas flow and the beam transport in the beamline. Based on this model, these two neutralizer designs have been evaluated in detail, in terms of the relation between neutralization efficiency and gas inflow, and the power load on the neutralizer components.

Imaging Neutral Particle Analyzer (INPA) measurements of confined fast ions in DIII-D

A novel imaging neutral particle analyzer (INPA) which provides energy-resolved radial profiles of confined fast ions on the DIII-D tokamak is discussed. The INPA measures charge-exchanged energetic neutrals by viewing an “active” neutral beam through a 1D pinhole camera with a rear collimating slit that defines the neutral particle collection sightlines. The incident neutrals are ionized by 10 nm thick carbon stripping foils and the local tokamak magnetic field acts as a magnetic spectrometer to disperse ions onto a phosphor scintillator. A fast (160 Hz) CCD camera provides 2D images of the escaping neutrals mapped to energy and radial position in the plasma. The INPA typically probes passing orbits with an energy resolution of ≈7.5 keV (E= 20−80 keV) and a spatial resolution that ranges from 4 cm half width at half max (HWHM) in the core to 3 cm at the plasma edge. The INPA clearly resolves fast ion transport in localized regions of phase space due to individual sawteeth and a replenishing before each event. Extension to proton and triton DD fusion product measurements in high-performance DIII-D plasmas is analyzed and simulations show peak signals which are 10−6 lower than that from neutral beam ions for the same configuration. Possible modifications to increase fusion product signals are discussed along with upgrades to improve the overall diagnostic performance.

Electrostatic solitary waves in ion beam neutralization

The excitation and propagation of electrostatic solitary waves (ESWs) are observed in two-dimensional particle-in-cell simulations of ion beam neutralization by electron injection by a filament. Electrons from the filament are attracted by positive ions and bounce inside the ion beam pulse. Bouncing back and forth electron streams start to mix, creating two-stream instability. The instability saturates with the formation of ESWs. These ESWs reach several centimeters in longitudinal size and are stable for a long time (≫τb, the duration of the ion beam pulse). The excitation of large-amplitude ESWs reduces the degree of neutralization of the ion beam pulse. In addition, the dissipation of ESWs causes heating of neutralizing electrons and their escape from the ion beam, leading to a further reduction of neutralization degree. The appearance of these waves can explain the results of previous experimental studies, which showed poor ion beam neutralization by electro-emitting filaments.

Measurement of edge plasma parameters at W7-X using alkali beam emission spectroscopy

A 60 keV neutral Alkali beam system was designed, built and installed for beam emission spectroscopy measurement of edge plasma on W7-X.As the decay length of the first excited state of sodium is much shorter than lithium it was selected as beam species. This way the steep density gradient at the bean shaped cross section of the Wendelstein 7-X plasma can be resolved.Unexpectedly a thermal sodium beam was also observed in the far scrape off layer of W7-X plasma during the first measurements in 2017. It was found to originate from the recirculating neutralizer which uses sodium vapour. To resolve this issue the neutralizer material was changed to Potassium.The observation system consists of two parts: a 40 channel avalanche photo diode (APD) camera unit and a CMOS camera which run in parallel: 95% of the collected light goes to the APD unit which is digitized with 2 MHz sampling rate while the CCD camera is operated in the 100 Hz range. Chopping the beam up to 250 kHz is also possible for precise background measurement on the time scale of the turbulence.In this paper the main improvements of the sodium beam system using potassium for ion beam neutralization and the first measurement results are described.

Development of Idea of Plasma-Optical Mass Separation

The paper presents the history of the origination and development of A.I. Morozov’s idea of plasma-optical mass separation (POMS-E). The problems that arose in the initial stage of its practical implementation are noted. Changes in Morozov’s POMS-E scheme of the collectors of separated ions and the system for the formation of neutralized ion beams are described. Accompanying plasma effects leading to the generation of ions with energies exceeding the energies equivalent to the discharge voltage are reported. The results of the main stage of separation of a three-component ion mixture in the POMS-E magnetic field are presented. A method to increase the number of ions passing through the POMS-E magnetic barrier is proposed. A conclusion is made on good promises of the idea of a plasma-optical mass separator.

Beam extraction characteristics of a low-energy, high-current pulsed ion source of a versatile experiment spherical torus neutral beam injector

A neutral beam (NB) ion source aimed to inject a 0.5-MW NB has been developed for Versatile Experiment Spherical Torus (VEST) plasma heating. Beam energy is generally limited to less than ∼20 keV to reduce particle orbit loss and beam shine-through. To meet this power demand, high-current ion beam extraction is important to achieve the required performance of the ion source. In this study, a 0.7-MW ion source was designed, fabricated, assembled, and optimized at the NB test stand in Korea Atomic Energy Research Institute (KAERI) considering the beam power losses in the beamline components. The ion source comprises a multicusped-bucket-type plasma generator and a multislot-triode-type ion accelerator with an extraction area of 12 cm × 46 cm. The characteristics of arc discharge and beam extraction were investigated using hydrogen gas to obtain the optimum operating parameters of the arc power, filament voltage, gas pressure, extraction voltage, and suppression voltage. The ion source was then conditioned and performance tests were primarily completed. Results of the experiments show that the ion source can extract sufficient ion beam power that is than 0.75 MW (50 A/15 keV) with 100-kW arc power. The arc efficiency and beam perveance were estimated to be 0.6 A/kW and 20–25 μP, respectively. Moreover, the design goal of delivering a 0.5-MW NB into the VEST plasma will be achievable based on the optimum operating conditions. This ion source has been installed in the VEST NB injection system at Seoul National University and its commissioning is ongoing.

A rotary and reciprocating scintillator based fast-ion loss detector for the MAST-U tokamak.

The design and unique feature of the first fast-ion loss detector (FILD) for the Mega Amp Spherical Tokamak - Upgrade (MAST-U) is presented here. The MAST-U FILD head is mounted on an axially and angularly actuated mechanism that makes it possible to independently adapt the orientation [0°, 90°] and radial position [1.40 m, 1.60 m] of the FILD head, i.e., its collimator, thus maximizing the detector velocity-space coverage in a broad range of plasma scenarios with different q95. The 3D geometry of the detector has been optimized to detect fast-ion losses from the neutral beam injectors. Orbit simulations are used to calculate the strike map and predict the expected signals. The results show a velocity-space range of [4 cm, 13 cm] in gyroradius and [30°, 85°] in pitch angle, covering the entire neutral beam ion energy range. The optical system will provide direct sight of the scintillator and simultaneous detection with two cameras, giving high spatial and temporal resolution. The MAST-U FILD will shed light on the dominant fast-ion transport mechanisms in one of the world's two largest spherical tokamaks through absolute measurements of fast-ion losses.

Potentialities of fast ion neutralization at grazing incidence angles from crystal surfaces for development of new generation of uv laser sources

Abstract. The paper considers processes of electron capture by fast ions scattered from metal surfaces at grazing incidence angles. It discusses the possibilities of excitation of high atomic levels in neutralization process and laser generation. The Doppler compression effect for ion beams is shown to be rather essential for laser generation in the ion energy range of several tens keV. It is shown that the four-level laser generation scheme is the preferable one. The values of critical currents for laser generation are evaluated. The experimental setup for ion beam neutralization is observed. The experimental results on nitrogen ion beam neutralization on silicon (001) wafers at grazing incident angles are presented. The dependence of the neutralization coefficient on the grazing incidence angle is measured, a good coincidence between peak maximum and Lindhard angles is demonstrated. It is shown that in case of neutralization without special vacuum surface cleaning the neutralization coefficient makes approximately 30% for ion energy of 40 keV. The one-electron resonant neutralization to nitrogen 2P1/2, 2D3/2, 4S3/2 atomic terms is discussed. The mechanism of resonant coherent excitation of fast nitrogen atoms in front of a Si (001) surface is observed, the resonant energy being estimated about 70 keV.