Charge Exchange Neutrals Research Articles

Design and initial results of the imaging neutral particle analyzer in large helical device.

A novel Imaging Neutral Particle Analyzer (INPA) was newly installed in early 2024 to enhance the understanding of fast ion confinement on Large Helical Devices (LHDs). This diagnostic system, based on a magnetic spectrometer using a scintillator, provides energy-resolved radial profiles of confined fast ions by measuring charge-exchanged fast neutrals escaping from the plasma. The system utilizes a 100nm thick carbon foil to ionize the fast neutrals, subsequently deflecting the ions toward a scintillator via the existing local magnetic field. The fast ion energy and sightline determine the position of the scintillation, while the light intensity depends on the flux of the fast ions. The INPA features two apertures, facilitating effective measurements in both clockwise and counterclockwise magnetic field directions in the LHD. This INPA was designed as a passive measurement system that measures fast ions charge exchange with background neutrals, focusing on perpendicular beam ions from 5 to 100keV with an energy resolution of about 5.75keV. This paper describes the details of the design, installation, and the initial results of the INPA on the LHD. This work will contribute significantly to a deeper understanding of fast ion transport due to magnetohydrodynamic instabilities.

Commissioning of the imaging neutral particle analyser for the ASDEX Upgrade tokamak

An imaging neutral particle analyser (INPA) diagnostic has been installed and commissioned at the ASDEX Upgrade (AUG) tokamak. The AUG INPA diagnostic provides energy and radially resolved measurements of the fast-ion (FI) distribution, complementing the existing set of diagnostic which measure the confined FI population. To this end, it analyses charge exchange (CX) neutrals produced in reactions between FI and neutrals injected by a neutral beam injector. These CX neutrals are ionised by a 20 nm carbon foil and deflected towards a scintillator by the machine magnetic field. The use of a scintillator as active component allows us to cover the whole plasma radial range with an energy resolution of 9 keV and a spatial of 3 cm for 93 keV deuterons. First measurements demonstrate the high sensitivity of the INPA diagnostic to different AUG fast-ion distribution functions, from NBI and ion-cyclotron resonance heating origin, and show good agreement with the synthetic diagnostic.

Measurements of heat flux components due to charged and non-charged particles in DIII-D divertor near and under detachment

Surfacing Eroding Thermocouples (SETC) have been used to provide high spatial and temporal resolution heat flux measurement in DIII-D. SETCs were first tested in the lower divertor of DIII-D using the Divertor Material Evaluation System (DiMES), then an array of SETCS was permanently installed in the upper SAS-VW divertor for studying the heat flux mitigation in closed divertor geometry. SETCs proved that inducing divertor detachment with various methods is an effective way to reduce the peak heat flux at the divertor targets. In the investigated discharges the heat flux is reduced to less than 30% under detachment compared to attached conditions. A new method of using the combination of recessed SETC and flush SETC was demonstrated to be able to measure the heat flux from volumetric radiation and the charge exchange neutrals. It was observed that the magnitude of the heat flux from radiation and charge exchange neutrals increased during the process of detachment. While 30% of the total incident heat flux is attributable to the volumetric radiation and the charge exchange neutrals in the fully detached divertor condition with B×∇B drift into the divertor, it could reach more than 60% with B×∇B drift away from the divertor.

Modeling of aluminum erosion by neutral particles in dedicated EAST experiments using the 3D-GAPS code

To evaluate the erosion effects of charge exchange (CX) neutrals on first wall materials, aluminum (Al) coated samples installed in a cylindrical sleeve were exposed to EAST plasmas with the Material and Plasma Evaluation System (MAPES). The 3D-GAPS code was firstly used to simulate the dedicated experiments with the neutral energy spectrums measured by the Low Energy Neutral Particle Analyzer located at the MAPES. The modeled Al erosion rates with isotropic angular distributions for incident D neutrals are consistent with the measured erosion thicknesses of Al layer by Rutherford backscattering spectrometry before and after the plasma exposure. The effects of neutral reflection coefficients and the heights of cylindrical sleeve on material erosion are also studied. The measured material erosion rate by CX neutrals in EAST verified by the modeling can help to us understand the effects of neutrals on first wall erosion, which is an important erosion process in future fusion reactors.

Imaging Neutral Particle Analyzer Engineering Design and Installation for the ASDEX Upgrade Tokamak

An imaging neutral particle analyzer (INPA) has been installed in the ASDEX Upgrade (AUG) tokamak to provide simultaneous measurements of the radial profile and the energy of the confined fast-ion (FI) population. The INPA diagnostic leverages the advantages of neutral particle analyzers (NPAs) and FI loss detectors (FILDs) by measuring charge exchange (CX) neutrals ionized by an ultrathin carbon foil is deflected into a scintillator by the local magnetic field. The design of this diagnostic has been developed under a series of constraints, such as the lack of space and hazardous environment (high level of neutron and gamma radiation), the need of high precision in the alignment of the nonsequential optical system (with six optical axes), and the optimization of the scintillator plate (signal emitter). To this end, a modular and adjustable design has been pursued with several degrees of freedom to ensure precise positioning during the installation and optical calibration. The induced electromagnetic and thermal stresses have been assessed and compared against the material limits of the detector, showing that no significant stress is expected for the system. The thermal analysis shows that the diagnostic scintillator can work efficiently during normal operation conditions. The CAD design, a comparison between the synthetic and real optical signals and the mechanical assessment [based on finite element analysis (FEA)], is presented in this contribution.

Estimation of the electrodes sputtering of the miniature linear accelerator

In this paper, electrode sputtering of the miniature linear accelerator (MLA)is investigated. The main part of this study is numerical simulations of the deposited metal layers formation on the insulator. It consist of three parts: ion beam trajectory analysis in MLA using COMSOL Multiphysics, simulations of surface sputtering by BCA codes (SRIM, SDTrimSP, and SCATTER) and sputtered particles trajectory analysis in MLA using COMSOL Multiphysics. The main sources of deposited particles are identified using numerical simulations of the surface sputtering by the ions extracted from the Penning ion source (PIS) and charge-exchange neutrals originated inside the ion-optical system (IOS) and subsequent analysis of the sputtered particle trajectories. Experimental data were obtained by digitizing MLA photos during operation by ImageJ software. Simulations of the electrodes sputtering of MLA were performed. The simulations allowed to determine the erosion zones responsible for the deposited metal layer formation on the insulator surface. The experimental data are in good qualitative agreement with the simulated ones.

Fully kinetic model of plasma expansion in a magnetic nozzle

Abstract A self-consistent model is presented for performing steady-state fully kinetic particle-in-cell simulations of magnetised plasma plumes. An energy-based electron reflection prevents the numerical pump instability associated with a typical open-outflow boundary, and is shown to be sufficiently general that both the plume kinetics and plasma potential demonstrate domain independence (within 6%). This is upheld by non-stationary Robin-type boundary conditions on the Poisson’s equation, coupled to a capacitive circuit that allows physical evolution of the downstream potential drop in the transient. The method has been validated against experiments, providing results that fall within the uncertainty of measurements. Simulations are then carried out to study collisional xenon discharges into axisymmetric diverging magnetic nozzles (MNs). Particular discussion is given to the identification of a collision-enhanced potential well arising from charge separation at the plume periphery, the role of ion–neutral charge exchange, and a three-region piecewise polytropic cooling regime for electrons. The polytropic index is shown to depend on the degree of magnetisation. Specifically, in the region near the thruster outlet, the plume is weakly-magnetised due to the cross-field diffusion of electron-heavy particle collisions. Downstream, a strongly-magnetised region of near-isothermal expansion occurs. Finally, in the detached region, the polytropic index tends to that of a more adiabatic unmagnetised case. With an increasing MN field strength, an inferior limit is found to the average polytropic index of γ ¯ e ∼ 1.16 .

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.

Exposures of bulk W and nanostructured W coatings to medium flux D plasmas

• Exposures of nanostructured W coatings and bulk W was performed to high energy D plasmas in GyM. • Formation of nanostructures was observed after exposures. • Nanostructures strongly depend on the structure of W. • Development of blisters was observed on compact coatings. In this work, we discuss the exposures of polycrystalline tungsten (W) and nanostructured W coatings to high energy ( ~ 30 ÷ 300 eV ) medium-flux D ( ~ 10 20 m - 2 s - 1 ) deuterium plasmas of the linear machine GyM at an overall fluence of 10 24 m - 2 . The energy and particle flux are tailored to that of charge-exchange neutrals at the vessel main chamber of present-day tokamaks and expected in ITER. Scanning Electron Microscopy measurements show the formation of surface nanostructures that are found to strongly depend on the W morphology and crystallinity, as well as on the energy of the ion species. The obtained nanostructures are few nanometres deep and similar to the one obtained after exposure of bulk W to divertor-like conditions. Dedicated annealing experiments are performed on selected samples, showing that the nanostructures developed on W coatings are thermally stable up to their recrystallisation temperature. The development of micrometric-sized blisters is also observed and found to strongly depend on the features of the W coatings, the substrate and the applied bias voltage.

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.

On the use of rhodium mirrors for optical diagnostics in ITER

The first mirrors of optical diagnostics in ITER are exposed to high radiation and fluxes of particles which escape the plasma, in the order of 1020 m−2s−1. At the position of the mirror, the flux may still reach about 1018 m−2s−1. First mirrors are thus the most vulnerable in-vessel optical components, being subject to erosion, esp. by fast charge-exchange neutrals, or to deposition of impurities at flux rates which can reach 0.05 nm/s. The material selected for the reflecting surface must combine a high optical reflectivity in a wide spectral range and a sufficient resistance to physical sputtering during normal operation and during mirror cleaning discharges, if any is installed. Rhodium (103Rh) was identified early as a possible or even promising candidate. It combines several attractive properties, for instance a mass which leads in most cases to low sputtering yields together with an optical reflectance (RRh≈75%) which is much higher than of some other options. RRh is insensitive to large temperature changes. Rhodium is fairly inert and its low oxidation is an appreciable advantage in case of steam ingress events.The core-plasma CXRS diagnostic in ITER (UPP 3) have now turned to Rh as a baseline. The aim is to procure monocrystalline rhodium (SC-Rh) to mitigate the increase of the diffuse reflection with the damage due to physical sputtering.

Development and verification of a novel scintillator-based, imaging neutral particle analyzer in DIII-D tokamak

A compact scintillator-based imaging neutral particle analyzer (INPA) that provides energy and radially resolved measurements of confined fast ions has been designed and built in the DIII-D tokamak. The system measures charge-exchanged energetic neutrals by viewing an active neutral beam source through a 1D pinhole camera with a rear collimating slit that defines the neutral particle collection sightline and radial positions probed in the plasma. The incident neutrals are ionized by ultra-thin carbon stripping foils of 10 nm thickness with the local tokamak magnetic field acting as a magnetic spectrometer to disperse the ions onto a scintillator. The strike position on the phosphor is determined by the fast ion energy and sightline while the intensity of emitted light from the phosphor is proportional to the ion flux. Fast camera measurements of the scintillator provide 2D images of the escaping neutrals mapped to energy and radial position in the plasma. The INPA system images a broad radial range from the plasma core to edge and deuterium energies up to 80 keV, with energy resolution of ∼7.5 keV and pitch angle resolution of ∼5°. The first data obtained from the INPA demonstrates that the system has excellent signal to noise ratio and provides unprecedented details of phase space dynamics.

Characterization of xenon ion and neutral interactions in a well-characterized experiment

Interactions between fast ions and slow neutral atoms are commonly dominated by charge-exchange and momentum-exchange collisions, which are important to understanding and simulating the performance and behavior of many plasma devices. To investigate these interactions, this work developed a simple, well-characterized experiment that accurately measures the behavior of high energy xenon ions incident on a background of xenon neutral atoms. By using well-defined operating conditions and a simple geometry, these results serve as canonical data for the development and validation of plasma models and models of neutral beam sources that need to ensure accurate treatment of angular scattering distributions of charge-exchange and momentum-exchange ions and neutrals. The energies used in this study are relevant for electric propulsion devices ∼1.5 keV and can be used to improve models of ion-neutral interactions in the plume. By comparing these results to both analytical and computational models of ion-neutral interactions, we discovered the importance of (1) accurately treating the differential cross-sections for momentum-exchange and charge-exchange collisions over a large range of neutral background pressures and (2) properly considering commonly overlooked interactions, such as ion-induced electron emission from nearby surfaces and neutral-neutral ionization collisions.

Preliminary study on heat load using calorimetric measurement during long-pulse high-performance discharges on EAST

Experimental Advanced Superconducting Tokamak (EAST) aims to demonstrate steady-state advanced high-performance H-mode plasmas with an ITER-like configuration, plasma control and heating schemes. The plasma-facing components in EAST are actively cooled, providing good conditions for researching long-pulse and high-energy discharges. A long-pulse high-performance plasma discharge (#59892 discharge) of up to 103 s with a core electron temperature of up to 4.5 keV was sustained with an injected energy exceeding 0.22 GJ in the 2015–2016 experimental campaign. A calorimetric measurement utilizing the temperature increment of cooling water is carried out to calculate the heat load on the strike point region of the lower divertor during long-pulse discharges in EAST. For the long-pulse and high-energy discharges, the comparison of the measurement results for the heat load measured by divertor Langmuir probes and the calorimetry diagnostic indicates that most of the heat load is delivered to the divertor panels as plasma, not radiation, and charge exchange neutrals. The ratio of the heat load on the strike point region of the lower divertor to the total injected energy is on average 42.5% per discharge with the lower single null divertor configuration. If the radiated energy loss measured by the fast bolometer diagnostic is taken into consideration, the ratio is found to be 61.6%. The experimental results and the analysis of the physics involved in these discharges are reported and discussed.

Beryllium migration in JET ITER-like wall plasmas

JET is used as a test bed for ITER, to investigate beryllium migration which connects the lifetime of first-wall components under erosion with tokamak safety, in relation to long-term fuel retention. The (i) limiter and the (ii) divertor configurations have been studied in JET-ILW (JET with a Be first wall and W divertor), and compared with those for the former JET-C (JET with carbon-based plasma-facing components (PFCs)). (i) For the limiter configuration, the Be gross erosion at the contact point was determined in situ by spectroscopy as between 4% (Ein = 35 eV) and more than 100%, caused by Be self-sputtering (Ein = 200 eV). Chemically assisted physical sputtering via BeD release has been identified to contribute to the effective Be sputtering yield, i.e. at Ein = 75 eV, erosion was enhanced by about 1/3 with respect to the bare physical sputtering case. An effective gross yield of 10% is on average representative for limiter plasma conditions, whereas a factor of 2 difference between the gross erosion and net erosion, determined by post-mortem analysis, was found. The primary impurity source in the limiter configuration in JET-ILW is only 25% higher (in weight) than that for the JET-C case. The main fraction of eroded Be stays within the main chamber. (ii) For the divertor configuration, neutral Be and BeD from physically and chemically assisted physical sputtering by charge exchange neutrals and residual ion flux at the recessed wall enter the plasma, ionize and are transported by scrape-off layer flows towards the inner divertor where significant net deposition takes place. The amount of Be eroded at the first wall (21 g) and the Be amount deposited in the inner divertor (28 g) are in fair agreement, though the balancing is as yet incomplete due to the limited analysis of PFCs. The primary impurity source in the JET-ILW is a factor of 5.3 less in comparison with that for JET-C, resulting in lower divertor material deposition, by more than one order of magnitude. Within the divertor, Be performs far fewer re-erosion and transport steps than C due to an energetic threshold for Be sputtering, and inhibits as a result of this the transport to the divertor floor and the pump duct entrance. The target plates in the JET-ILW inner divertor represent at the strike line a permanent net erosion zone, in contrast to the net deposition zone in JET-C with thick carbon deposits on the CFC (carbon-fibre composite) plates. The Be migration identified is consistent with the observed low long-term fuel retention and dust production with the JET-ILW.

Experimental evidence of energetic neutrals production in an ion diode

The paper presents several experimental proofs of the formation of energetic charge-exchange neutrals in a self-magnetically insulated ion diode with a graphite cathode. The energetic neutrals are thought to be produced as a result of charge exchange process between accelerated ions and stationary neutral molecules. The experiments have been carried out using both a diode with externally applied magnetic insulation (single-pulse mode: 100ns, 250–300kV) and a diode with self-magnetic insulation (double-pulse mode: 300–500ns, 100–150kV (negative pulse); 120ns, 250–300kV (positive pulse)). The motivation for looking at the neutral component of the ion beam came when we compared two independent methods to measure the energy density of the beam. A quantitative comparison of infrared measurements with signals from Faraday cups and diode voltage was made to assess the presence of neutral atoms in the ion beam. As another proof of charge-exchange effects in ion diode we present the results of statistical analysis of diode performance. It was found that the shot-to shot variation of the energy density in a set of 50–100 shots does not exceed 11%, whilst the same variation for ion current density was 20–30%; suggesting the presence of neutrals in the beam. Moreover, the pressure in the zone of ion beam energy dissipation exceeds the results stated in cited references. The difference between our experimental data and results stated by other authors we attribute to the presence of a low-energy charge-exchange neutral component in the ion beam.

A reconsideration of the noise equivalent power and the data analysis procedure for the infrared imaging video bolometers.

The infrared imaging video bolometer (IRVB) used for measurement of the two-dimensional (2D) radiation profiles from the Large Helical Device has been significantly upgraded recently to improve its signal to noise ratio, sensitivity, and calibration, which ultimately provides quantitative measurements of the radiation from the plasma. The reliability of the quantified data needs to be established by various checks. The noise estimates also need to be revised and more realistic values need to be established. It is shown that the 2D heat diffusion equation can be used for estimating the power falling on the IRVB foil, even with a significant amount of spatial variation in the thermal diffusivity across the area of the platinum foil found experimentally during foil calibration. The equation for the noise equivalent power density (NEPD) is re-derived to include the errors in the measurement of the thermophysical and the optical properties of the IRVB foil. The theoretical value estimated using this newly derived equation matches closely, within 5.5%, with the mean experimental value. The change in the contribution of each error term of the NEPD equation with rising foil temperature is also studied and the blackbody term is found to dominate the other terms at elevated operating temperatures. The IRVB foil is also sensitive to the charge exchange (CX) neutrals escaping from the plasma. The CX neutral contribution is estimated to be marginally higher than the noise equivalent power (NEP) of the IRVB. It is also established that the radiation measured by the IRVB originates from the impurity line radiation from the plasma and not from the heated divertor tiles. The change in the power density due to noise reduction measures such as data smoothing and averaging is found to be comparable to the IRVB NEPD. The precautions that need to be considered during background subtraction are also discussed with experimental illustrations. Finally, the analysis algorithm with all the improvements is validated and found to reproduce the input power well within 10% accuracy. This article answers many fundamental questions relevant to the IRVB and illustrates the care to be exercised while processing the IRVB data.

Modelling of the material transport and layer formation in the divertor of JET: Comparison of ITER-like wall with full carbon wall conditions

Impurity transport within the inner JET divertor has been modelled with ERO to estimate the transport to and the resulting deposition at remote areas. Various parametric studies involving divertor plasma conditions and strike point position have been performed. In JET-ILW (beryllium main chamber and tungsten divertor) beryllium, flowing from the main chamber into the divertor and then effectively reflected at the tungsten divertor tiles, is transported to remote areas. The tungsten flux to remote areas in L-Mode is in comparison to the beryllium flux negligible due to small sputtering. However, tungsten is sputtered during ELMs in H-Mode conditions. Nevertheless, depending on the plasma conditions, strike point position and the location of the remote area, the maximum resulting tungsten flux to remote areas is at least ∼3 times lower than the corresponding beryllium flux. Modelled beryllium and tungsten deposition on a rotating collector probe located below tile 5 is in good agreement with measurements if the beryllium influx into the inner divertor is assumed to be in the range of 0.1% relative to the deuterium ion flux and erosion due to fast charge exchange neutrals is considered. Comparison between JET-ILW and JET-C is presented.

Low energy ion distribution measurements in Madison Symmetric Torus plasmas

Charge-exchange neutrals contain information about the contents of a plasma and can be detected as they escape confinement. The Florida A&amp;M University compact neutral particle analyzer (CNPA), used to measure the contents of neutral particle flux, has been reconfigured, calibrated, and installed on the Madison Symmetric Torus (MST) for high temperature deuterium plasmas. The energy range of the CNPA has been extended to cover 0.34–5.2 keV through an upgrade of the 25 detection channels. The CNPA has been used on all types of MST plasmas at a rate of 20 kHz throughout the entire discharge (∼70 ms). Plasma parameter scans show that the ion distribution is most dependent on the plasma current. Magnetic reconnection events throughout these scans produce stronger poloidal electric fields, stronger global magnetic modes, and larger changes in magnetic energy all of which heavily influence the non-Maxwellian part of the ion distribution (the fast ion tail).

Preliminary test results on tungsten tile with castellation structures in KSTAR

A bulk tungsten tile with conventional and shaped castellation structures was exposed to various plasmas in KSTAR during 2012 campaign, in order to verify the functions of the shaped castellation designed for ITER divertor. The thermal response of the tile during the campaign was measured by thermocouples. The tungsten tile was collected after the campaign and the castellation structures were examined. The deposition inside the gap was studied to identify the contributions of ions and charge exchange neutrals. The shaped castellation shows a penetration depth of ions smaller than that of conventional one by a factor of ∼2, thus less deposition inside the gap which is consistent with the prediction made by SPICE2 code. It seems that blister formation and cracking have occurred during plasma shots.