Aperture Displacement Research Articles

Simulation of beamlet deflection and its compensation by aperture displacement for a NBI negative ion source prototype

A one-RF-driver prototype of the negative ion source for the NBI of CFETR is being tested and optimized at Southwest Institute of Physics, aiming for H− beam of 200 keV in 200 A/m2. For the NBI negative ion source, it's essential to have a good beam optics with small divergence and aiming trajectory. However, beamlet deflection is unavoidably caused by the magnets inserted in the extraction grid, and it results in the zig-zag beam profile. Measures must be taken to compensate the deflection. In this work, the beamlet deflection and its compensation using the aperture displacement method are studied for the one-RF-driver prototype with numerical simulation. The deflection angle of a single beamlet is calculated and the compensation scheme of displacing the aperture on the steering grid (SG) is designed. A 200 A/m2 uniform beamlet has a deflection angle of 8 mrad at the extraction/acceleration voltage 8 kV/192 kV, EG magnets’ flux peak ±45 mT. The angle can be fully compensated with 0.7 mm displacement of SG aperture. The compensation effect is analyzed for different conditions. The scheme's dependence on the voltage ratio, also the adaptiveness to the magnetic field and the beam property are learnt: For maintaining the compensation effect, a modification within 0.02 mm is needed when the acceleration voltage varies between 152 kV and 232 kV while keeping the extraction voltage unchanged. An increase of 0.1 mm of the displacement is needed when the magnetic field peak is enhanced 5 mT and the necessary increase is 0.15 mm when the non-uniformity is enhanced 10%. The compensation scheme is verified with multi-beamlet simulation in the last, taking the beamlet-beamlet interaction and the long distance transport into account.

Deep/Transfer Learning with Feature Space Ensemble Networks (FeatSpaceEnsNets) and Average Ensemble Networks (AvgEnsNets) for Change Detection Using DInSAR Sentinel-1 and Optical Sentinel-2 Satellite Data Fusion

Differential interferometric synthetic aperture radar (DInSAR), coherence, phase, and displacement are derived from processing SAR images to monitor geological phenomena and urban change. Previously, Sentinel-1 SAR data combined with Sentinel-2 optical imagery has improved classification accuracy in various domains. However, the fusing of Sentinel-1 DInSAR processed imagery with Sentinel-2 optical imagery has not been thoroughly investigated. Thus, we explored this fusion in urban change detection by creating a verified balanced binary classification dataset comprising 1440 blobs. Machine learning models using feature descriptors and non-deep learning classifiers, including a two-layer convolutional neural network (ConvNet2), were used as baselines. Transfer learning by feature extraction (TLFE) using various pre-trained models, deep learning from random initialization, and transfer learning by fine-tuning (TLFT) were all evaluated. We introduce a feature space ensemble family (FeatSpaceEnsNet), an average ensemble family (AvgEnsNet), and a hybrid ensemble family (HybridEnsNet) of TLFE neural networks. The FeatSpaceEnsNets combine TLFE features directly in the feature space using logistic regression. AvgEnsNets combine TLFEs at the decision level by aggregation. HybridEnsNets are a combination of FeatSpaceEnsNets and AvgEnsNets. Several FeatSpaceEnsNets, AvgEnsNets, and HybridEnsNets, comprising a heterogeneous mixture of different depth and architecture models, are defined and evaluated. We show that, in general, TLFE outperforms both TLFT and classic deep learning for the small dataset used and that larger ensembles of TLFE models do not always improve accuracy. The best performing ensemble is an AvgEnsNet (84.862%) comprised of a ResNet50, ResNeXt50, and EfficientNet B4. This was matched by a similarly composed FeatSpaceEnsNet with an F1 score of 0.001 and variance of 0.266 less. The best performing HybridEnsNet had an accuracy of 84.775%. All of the ensembles evaluated outperform the best performing single model, ResNet50 with TLFE (83.751%), except for AvgEnsNet 3, AvgEnsNet 6, and FeatSpaceEnsNet 5. Five of the seven similarly composed FeatSpaceEnsNets outperform the corresponding AvgEnsNet.

Predicting gamma passing rates for portal dosimetry-based IMRT QA using machine learning.

Intensity-modulated radiation therapy (IMRT) quality assurance (QA) measurements are routinely performed prior to treatment delivery to verify dose calculation and delivery accuracy. In this work, we applied a machine learning-based approach to predict portal dosimetry based IMRT QA gamma passing rates. 182 IMRT plans for various treatment sites were planned and delivered with portal dosimetry on two TrueBeam and two Trilogy LINACs. A total of 1497 beams were collected and analyzed using gamma criteria of 2%/2mm with a 5% threshold. The datasets for building the machine learning models consisted of 1269 beams. Ten-fold cross-validation was utilized to tune the model and prevent "overfitting." A separate test set with the remaining 228 beams was used to evaluate model performance. Each beam was characterized by a set of 31 features including both plan complexity metrics and machine characteristics. Three tree-based machine learning algorithms (AdaBoost, Random Forest, and XGBoost) were used to train the models and predict gamma passing rates. Both AdaBoost and Random Forest had 98% of predictions within 3% of the measured 2%/2mm gamma passing rates with a maximum error less than 4% and a mean absolute error<1%. XGBoost showed a slightly worse prediction accuracy with 95% of the predictions within 3% of the measured gamma passing rates and a maximum error of 4.5%. The three models identified the same nine features in the top 10 most important ones that are related to plan complexity and maximum aperture displacement from the central axis or the maximum jaw size in a beam. We have demonstrated that portal dosimetry IMRT QA gamma passing rates can be accurately predicted using tree-based ensemble learning models. The machine learning based approach allows physicists to better identify the failures of IMRT QA measurements and to develop proactive QA approaches.

6 VMAT complexity metrics can reduce patient QA workload

Introduction With the growing use of complex treatments such as VMAT, physics workload has increased in regards to pre-treatment patient specific QA. In this work, we evaluated the ability of 8 RTPLAN-based metrics to predict robust treatment delivery. Methods Ninety-three VMAT plans were delivered on the ArcCHECK system using global 2 %/2 mm (90 % passing points) and 3 %/3 mm (95 %) gamma index criteria. Using patient’s DICOM RTPLAN and Python scripting, we calculated the following aperture-based metrics from Crowe et al. [1] : MFA (Mean Field Area), MAD (mean Aperture Displacement), SAS (Small Aperture Score: threshold 10 mm), CLS (Closed Leaf Score), CAS (Cross Axis Score), and from Mc Niven et al. [2] the MCS (Modulation Complexity Score) deliverability metric. In addition, we developed a novel metric based on TPS second dose calculation using a different leaf offset modelling: LOIC(PTV) (Leaf Offset Impact on Calculation) is defined as the percentage variation of PTV mean dose with respect to the leaf offset parameter in the model (from 0.5 to 0 mm) and aims to quantify the “MU-weighted global narrowness” of MLC aperture. Finally the total MU per Gy delivered was tested. Correlation between gamma passing rates (GPR) and metrics values was assessed using Pearson’s r-coefficient. Receiver-operating characteristic (ROC) analysis was performed to determine appropriate complexity threshold values above which a plan should be considered either for re-optimization (high specificity) or exempt from QA measurements (100 % sensitivity). Results Out of 93 plans, 77 and 41 passed the 3%/3 mm and 2%/2 mm gamma criteria. Table 1 shows absolute Pearson’s r coefficients, associated p-value and ROC Area Under the Curve (AUC) for the 8 metrics and 2 gamma criteria. Download : Download high-res image (374KB) Download : Download full-size image A strong correlation (p 0.001) was observed between GPR and LOIC, CAS, MCS, SAS and MU/Gy. The highest Pearson’s r value was obtained for LOIC (0.66 and 0.69). ROC curves showed the best results for LOIC versus GPR 3%/3 mm, with AUC of 0.92 (Fig. 1.). Download : Download high-res image (149KB) Download : Download full-size image A LOIC threshold of 1.7 % allowed for the identification of robust delivery with a false positive rate of 6.5 % and a true positive rate of 69 %, which makes re-planning a relevant option. On the other hand, a LOIC threshold of 1.25 % provided no false negative (full sensitivity), allowing for a workload reduction of 49 %. Conclusions From the 8 metrics evaluated, LOIC was the most powerful tool in order to identify sparsely/overly modulated plans before time-consuming QA measurements are performed, allowing to halve the patient QA workload and to improve plan accuracy/deliverability.

Vortex lattices with transverse-mode-locking states switching in a large-aperture off-axis-pumped solid-state laser

We report on optical vortex lattices (OVL) beams with phase-singularity arrays from a large-aperture diode-pumped Yb:CALGO laser. A distinct transverse-mode-locking (TML) effect is found in the OVL formation via controlling pump aperture and off-axis displacement. Various OVL patterns with vortex arrays can be generated in a TML state. While in a non-TML state, the emission is always a high-order Hermite&#x2013;Gaussian or Ince&#x2013;Gaussian mode. Interestingly, a stable TML can be started from a non-TML state by an intentionally induced perturbation. Based on the TML principle, a new closed-form expression is theoretically proposed for interpreting the transverse patterns and the evolution principle of various OVL modes, showing good agreement with the experimental results.

Modification to the accelerator of the NBI-1B ion source for improving the injection efficiency.

Minimizing power loss of a neutral beam imposes modification of the accelerator of the ion source for further improvement of the beam optics. The beam optics can be improved by focusing beamlets. The injection efficiencies by the steering of ion beamlets are investigated numerically to find the optimum modification of the accelerator design of the NBI-1B ion source. The beam power loss was reduced by aperture displacement of three edge beamlets arrays considering power loadings on the beamline components. Successful testing and operation of the ion source at 60 keV/84% of injection efficiency led to the possibility of enhancing the system capability to a 2.4 MW power level at 100 keV/1.9 μP.

A new approach to upscaling fracture network models while preserving geostatistical and geomechanical characteristics

AbstractA new approach to upscaling two‐dimensional fracture network models is proposed for preserving geostatistical and geomechanical characteristics of a smaller‐scale “source” fracture pattern. First, the scaling properties of an outcrop system are examined in terms of spatial organization, lengths, connectivity, and normal/shear displacements using fractal geometry and power law relations. The fracture pattern is observed to be nonfractal with the fractal dimension D ≈ 2, while its length distribution tends to follow a power law with the exponent 2 &lt; a &lt; 3. To introduce a realistic distribution of fracture aperture and shear displacement, a geomechanical model using the combined finite‐discrete element method captures the response of a fractured rock sample with a domain size L = 2 m under in situ stresses. Next, a novel scheme accommodating discrete‐time random walks in recursive self‐referencing lattices is developed to nucleate and propagate fractures together with their stress‐ and scale‐dependent attributes into larger domains of up to 54 m × 54 m. The advantages of this approach include preserving the nonplanarity of natural cracks, capturing the existence of long fractures, retaining the realism of variable apertures, and respecting the stress dependency of displacement‐length correlations. Hydraulic behavior of multiscale growth realizations is modeled by single‐phase flow simulation, where distinct permeability scaling trends are observed for different geomechanical scenarios. A transition zone is identified where flow structure shifts from extremely channeled to distributed as the network scale increases. The results of this paper have implications for upscaling network characteristics for reservoir simulation.

Examination of the properties of IMRT and VMAT beams and evaluation against pre-treatment quality assurance results

This study aimed to provide a detailed evaluation and comparison of a range of modulated beam evaluation metrics, in terms of their correlation with QA testing results and their variation between treatment sites, for a large number of treatments. Ten metrics including the modulation index (MI), fluence map complexity, modulation complexity score (MCS), mean aperture displacement (MAD) and small aperture score (SAS) were evaluated for 546 beams from 122 intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans targeting the anus, rectum, endometrium, brain, head and neck and prostate. The calculated sets of metrics were evaluated in terms of their relationships to each other and their correlation with the results of electronic portal imaging based quality assurance (QA) evaluations of the treatment beams. Evaluation of the MI, MAD and SAS suggested that beams used in treatments of the anus, rectum, head and neck were more complex than the prostate and brain treatment beams. Seven of the ten beam complexity metrics were found to be strongly correlated with the results from QA testing of the IMRT beams (p < 0.00008). For example, values of SAS (with multileaf collimator apertures narrower than 10 mm defined as ‘small’) less than 0.2 also identified QA passing IMRT beams with 100% specificity. However, few of the metrics are correlated with the results from QA testing of the VMAT beams, whether they were evaluated as whole 360° arcs or as 60° sub-arcs. Select evaluation of beam complexity metrics (at least MI, MCS and SAS) is therefore recommended, as an intermediate step in the IMRT QA chain. Such evaluation may also be useful as a means of periodically reviewing VMAT planning or optimiser performance.

Long-pulse beam acceleration of MeV-class H− ion beams for ITER NB accelerator

In order to realize neutral beam systems in International Thermonuclear Experimental Reactor whose target is to produce a 1 MeV, 200 A/m(2) during 3600 s D(-) ion beam, the electrostatic five-stages negative ion accelerator so-called "MeV accelerator" has been developed at Japan Atomic Energy Agency. To extend pulse length, heat load of the acceleration grids was reduced by controlling the ion beam trajectory. Namely, the beam deflection due to the residual magnetic field of filter magnet was suppressed with the newly developed extractor with a 0.5 mm off-set aperture displacement. The new extractor improved the deflection angle from 6 mrad to 1 mrad, resulting in the reduction of direct interception of negative ions from 23% to 15% of the total acceleration power, respectively. As a result, the pulse length of 130 A/m(2), 881 keV H(-) ion beam has been successfully extended from a previous value of 0.4 s to 8.7 s. This is the first long pulse negative ion beam acceleration over 100 MW/m(2).

Vacuum Insulation and Achievement of 980 keV, 185 A/m2 H− Ion Beam Acceleration at JAEA for the ITER Neutral Beam Injector

Vacuum insulation of −1 MV is a common issue for the HV bushing and the accelerator for the ITER neutral beam injector (NBI). The HV bushing as an insulating feedthrough has a five-stage structure and each stage consists of double-layered insulators. To sustain −1 MV in vacuum, reduction of electric field at several triple points existing around the double-layered insulators is a critical issue. To reduce electric field simultaneously at these points, three types of stress ring have been developed. In a voltage holding test of a full-scale mockup equipped with these stress rings, 120% of rated voltage was sustained and the voltage holding capability required in ITER was verified. In the MeV accelerator, whose target is the acceleration of a H− ion beam of 1 MeV, 200 A/m2, the gap between the grid support was extended to suppress breakdowns triggered by electric field concentration at the edge and corner of the grid support. This modification improved the voltage holding capability in vacuum, and the MeV accelerator succeeded in sustaining −1 MV stably. Furthermore, it appeared that the H− ions beam was deflected and a part of the beam was intercepted at the acceleration grid. This causes high heat load on the grids and breakdowns during beam acceleration. To suppress the direct interception, a new grid was designed with proper aperture displacement based on a three dimensional beam trajectory analysis. As a result, 980 keV, 185 A/m2 H− ion beam acceleration has been demonstrated, which is close to the ITER requirement.

Voltage holding study of 1 MeV accelerator for ITER neutral beam injector

Voltage holding test on MeV accelerator indicated that sustainable voltage was a half of that of ideal quasi-Rogowski electrode. It was suggested that the emission of the clumps is enhanced by a local electric field concentration, which leads to discharge initiation at lower voltage. To reduce the electric field concentration in the MeV accelerator, gaps between the grid supports were expanded and curvature radii at the support corners were increased. After the modifications, the accelerator succeeded in sustaining -1 MV in vacuum without beam acceleration. However, the beam energy was still limited at a level of 900 keV with a beam current density of 150 A∕m(2) (346 mA) where the 3 × 5 apertures were used. Measurement of the beam profile revealed that deflection of the H(-) ions was large and a part of the H(-) ions was intercepted at the acceleration grid. This causes high heat load on the grids and the breakdowns during beam acceleration. To suppress the direct interception, new grid system was designed with proper aperture displacement based on a 3D beam trajectory analysis. As the result, the beam deflection was compensated and the voltage holding during the beam acceleration was improved. Beam parameter of the MeV accelerator was increased to 980 keV, 185 A∕m(2) (427 mA), which is close to the requirement of ITER accelerator (1 MeV, 200 A∕m(2)).

Numerical Simulation of Two-Dimensional Acoustic Liners with High-Speed Grazing Flow

Resonant acoustic liners are used to dissipate acoustic energy in engine ducts in the presence of grazing flow. Future applications have been proposed that will expose liners to much higher grazing flow velocities than are typically encountered. To study their characteristics at these conditions, a fully predictive liner eduction technique is developed by solving the compressible Navier-Stokes equations with accurate boundary conditions. Validation of the numerical approach is demonstrated by comparing the predicted absorption characteristics of a 3 kHz resonant liner geometry without any background flow against existing computational and experimental data. Using the validated approach, simulations at more realistic liner operating conditions are performed. Incident grazing acoustic waves of different intensity and frequency are performed at grazing flow Mach numbers up to 0.85. The liner impedance is evaluated and compared with existing semiempirical models; reasonable agreement is found in most cases. The simulation databases are then used to examine candidate quantities for recently suggested time-domain liner models. It is found that, at high grazing Mach numbers, the vortex shedding characteristics continue to be a strong function of frequency, with incident waves of lower frequency leading to stronger vortex shedding. It is also found that the aperture wall shear stress and displacement thickness are strongly coupled to the in-aperture flow details, while the mass flux through the aperture is more closely linked to the incident sound frequency.

Long pulse H− ion beam acceleration in MeV accelerator

A multiaperture multigrid accelerator called "MeV accelerator" has been developed for neutral beam injection system of international thermonuclear experimental reactor. In the present work, long pulse H(-) ion beam acceleration was performed by the MeV accelerator equipped with new water-cooled grids. At present, the pulse length was extended to 5 s for the beams of 750 keV, 221 mA, and 10 s for the beams of 600 keV, 158 mA. Energy density, defined as products of beam energy (keV), current (mA), and pulse (s) divided by aperture area (m(2)), increased more than one order of magnitude higher compared with original MeV accelerator without water cooling in its grids. At higher energy and current, the grid was melted by beam deflection. Due to this grid melting, breakdowns occurred between the grids, and hence, the pulse length was limited. Beam deflection will be compensated by aperture displacement in next experiment.

Performance test results of ion beam transport for SST-1 neutral beam injector

A neutral beam injector is built at IPR to heat the plasma of SST-1 and its upgrade. It delivers a maximum beam power of 1.7 MW for 55 kV Hydrogen beam or 80 kV Deuterium beam. At lower beam voltage, the delivered power falls to 500 kW at 30 kV Hydrogen beam which is adequate to heat SST-1 plasma ions to ~ 1 keV. Process of acceleration of ions to the required beam voltage, conversion of ions to neutrals and removal of un-neutralized ions and the beam diagnostic systems occupy a large space. The consequence is that linear extent of the neutral beam injector is at least a few meters. Also, port access provides a very narrow duct. Even a very good injector design and fabrication practices keep beam divergence at a very low but finite value. The result is beam transport becomes an important issue. Since a wide area beam is constructed by hundreds of beam lets, it becomes essential they be focused in such a way that beam transport loss is minimized. Horizontal and vertical focal lengths are two parameters, in addition to beam divergence, which give a description of the beam transport. We have obtained these two parameters for our injector by using beam transport code; making several hundred simulation runs by varying optical parameters of the beam. The selected parameters set has been translated into the engineering features of the extractor grid set of the ion source. Aperture displacement technique is used to secure the horizontal beam focusing at 5.4 m. Combination of both aperture displacement and inclining of two grid halves to ~ 17 mrad are secured for vertical beam focusing at 7 m from earth grid of the ion source. The gaps between the design, engineered and performance tested values usually arise due to lack of exercising control over fabrication processes or due to inaccuracies in the assumption made in the model calculations of beam optics and beam transport. This has been the case with several injectors, notably with JET injector. To overcome this problem, we have validated and scaled our design calculations with performance parameters of the Neutral Beam Injector at IPP, Julich, Germany. The performance test of the SST-1 PINI ion source was done at MARION Test Stand at IPP, Julich. Analyses of these results indicate that the measured power profile and the optical parameters of the beam are in good agreement with the simulation results. These parameters are stable over the beam pulse of 14s with extracted beam energy of 31 MJ at 41 kV. This paper presents these results and details out future work need to be done in order to assess the steady state stability of the beam parameters.

Optimization of negative ion extractor in a JAERI 400 keV H− ion source

The influence of a dipole magnetic field in the extractor on the beamlet deflection has been investigated using a JAERI 400 keV H− ion source. The beam deflection angle decreased from 10.2 to 7.0 mrad when the integrated value of the magnetic field was changed from 2530 to 910 G cm. The measured deflection angle was larger than the estimated value from the ion trajectory considering the magnetic field. To understand the dominant factor that enhances the beam deflection angle, three-dimensional trajectory simulation was performed. It was confirmed that the axis of the beamlet deflected by the magnetic field in the extractor is displaced from the center of the aperture at the grounded grid (GRG). This displacement enhances the beam deflection angle due to the effect of the electrostatic lens at the GRG. This phenomenon is similar to the beam deflection by aperture displacement of the GRG.

Effects of aperture displacement in broad-beam ion extraction systems

Low-energy broad-beam ion sources cover a wide range of applications. Geometrical perturbations of the axial-symmetric extraction channels by off-axis shifts of the accelerator grid holes give rise to an ion beamlet tilt which under certain conditions is dependent on the beamlet current. In this study, the characteristics of the beamlet deflection are presented which were obtained by a three-dimensional trajectory simulation.

Improvement of a large negative ion source for the Large Helical Device neutral beam injector

For compensation of the magnetically deflected beamlets, a new type of grid system has been applied to a large negative ion source of a negative-ion-based neutral beam injector in the Large Helical Device (LHD) fusion experimental machine. Corresponding to deviation of the negative ion beamlet trajectory due to the electron suppression magnetic field at the extraction grid, the exit aperture of the extraction grid is displaced to cancel the beamlet deflection by electrostatic lens effect. As a result, the beamlet deflection was suppressed less than 2 mrad in a wide range of energy of 110–180 keV, and the port-through efficiency of the injected beam was improved to 90%–95% from 70%–75% in the case of the previous compensation by the grounded grid aperture displacement. The injected beam power was increased to 3.5 MW with an energy of 165 keV using one injector. To improve beam uniformity of the large area beam, individual arc currents through filaments are controlled for uniform plasma production, with independent 12 arc power supplies connected to 12 filament power supplies. The initial results show good controllability of the individual arc current distribution at a high arc power. The improvement of the ion source performance would lead to an increase in the injection power in the following LHD experimental campaign.

Measurement of grid heat loading in a negative ion source for producing intense H− ion beams by aperture displacement focusing technique

Heat loading of acceleration grids has been investigated using a JAERI 400 keV negative hydrogen ion source, which has a three-stage multiaperture accelerator. To investigate the effect of beamlet steering on the heat loading, the apertures in the grounded grid were displaced up to 5 mm. The steering angle increased linearly with the displacement distance of the grid. The obtained steering angle was 7.5 mrad at the displacement distance of 5 mm. It was confirmed that the steering angle agrees well with the thin lens theory. Degradation of beam optics due to the steering was not observed. The heat load of the grounded grid was scarcely changed up to the displacement distance of 2 mm and then increased from 2.5% to 8% of the total input power when the grid was displaced from 2.5 to 5 mm. Heat loads of the other unsteered grids were not changed.

Compensation of beam deflection due to the magnetic field using beam steering by aperture displacement technique in the multibeamlet negative ion source

A beam steering technique using aperture displacement was examined to correct the negative ion beam deflection due to the magnetic field for electron suppression in a large-area multibeamlet H− source. The total deflection angle was estimated, including the effect of the deflection by the electron suppression magnetic field and the beam steering by the aperture displacement, both by linear optics theory and by three-dimensional beam trajectory simulation. Two methods were compared; one used the displacement both of the grounded grid (GG) apertures and of the exit part of the extraction grid (EG exit) apertures, and the other used the displacement only of the EG exit. The beam steering experiments were performed using a large-area multibeamlet H− source with both displacement methods, and the results were compared with the theoretical estimations. As a result, both methods were effective to correct the beam deflection. In particular, the displacement of only the EG exit with a simplified displacement structure achieved a large steering angle by a small displacement. The steering angle in the experiment was a few mrad smaller than the estimations. Based on these results, the aperture displacement of the EG exit was applied to the 1/5 segment of a H− source in the Large-Helical-Device neutral beam injector, where the GG apertures are displaced only focusing of the large-area multibeamlet. In this case, 1.0 mm of the displacement is concluded to be proper to sufficiently compensate the beam deflection at 180 keV, from extrapolation of the beam energy characteristics at 100–140 kev.

Investigation of beam deflection reduction and multi-beamlet focus at a large-area negative ion source for a neutral beam injector with 3-D beam trajectory simulation

We investigated the reduction of ion beam deflection caused by electron deflection magnets, and focus of multi-beamlets at a large-area negative ion source of a neutral beam injector (NBI) in order to reduce beam loss during long-distance beam transport (>10 m) and beam injection into a nuclear fusion device. The electrostatic lens effect by displacement of the beam extraction aperture of a grounded grid (GG) was utilized for the beam deflection reduction and the multi-beamlet focus. We proposed an analysis process to adjust the aperture displacement which avoids beam collision with the GG by too much displacement. The analysis process includes a 3-D beam trajectory simulation used for analyzing the beam deflection angle and beam radius as well as theoretical calculations, which are used to calculate the aperture displacement based on the 3-D simulation results. Applicability of the analysis process was examined for a large-area high-current H − ion source of an NBI (0.25 m×1.25 m, 40 A, 180 keV). The analysis process was proved to be useful for adjusting the GG aperture displacement and ion source parameters (acceleration gap length g acc, extraction voltage V ext, etc.) so as to reduce the beam deflection and to focus the multi-beamlets avoiding beam collision with the GG. 3-D simulation results were compared with the experimental results of the H − ion source to confirm validity of the analysis process. The beam deflection angle was reduced to almost zero in the simulation with a displacement equal to the experiment. This simulation result was consistent with the experimental results. The analysis process will allow design work of future negative ion sources to be speeded up, with a minimum of experimental work.