Design of improved compact decoupler based on adjustable capacitor for EAST-ICRF antenna

The Ion Cyclotron Radio Frequency (ICRF) heating antenna on EAST adopts a decoupling device to constrain power coupling among the radiation straps, which was discovered shortcomings such as long size, poor contact, and etc. In order to improve these weak points, a new type decoupler with terminal-loaded tunable capacitor is designed to replace the previous design. Besides the capability of the tunable admittance parameters of decoupler, the withstand voltage of the capacitor is the most significant consideration for working under high power. Therefore, the theoretical analysis carefully elaborates the capacitor withstand voltage, and the detailed analytical equations and criteria for design are given. After the comparative analysis of theoretical calculation and 3D simulation results, the decoupler design scheme is finalized. The capacitor-loaded decoupler has been successfully adopted for ICRF antenna at port N on EAST, and achieved the optimization of adjacent port isolation from −22 to −58 dB at 37 MHz without plasma to restrict mutual coupling. The new design of the decoupler has greatly improved its compactness and automatic adjustment performance, and could be good solution for the decoupling network of ICRF antennas.     

Mitigation of radio frequency sheaths through magnetic field-aligned ICRF antenna design

One of the primary challenges of auxiliary heating of tokamaks in the ion cyclotron range of frequencies (ICRF) is the reduction of impurities associated with ICRF operation. On Alcator C-Mod, a new magnetic field-aligned antenna was optimized for magnetic flux coupling, power handling, and minimized integrated parallel electric field (E_∥). Initial simulations performed using both slab and cylindrical geometry suggested nearly complete cancellation of E_∥ in front of the antenna structure for certain toroidal phasings. Using toroidal models, the cancellation of E_∥ is more modest, suggesting 3-D geometrical effects are important. Using finite element method simulations with a 3-D toroidal cold plasma model, multiple antenna phases were analyzed: [0, π, 0, π], [0, 0, π, π], [0, π, π, 0], [0, 0, 0, 0], [0, π/6, 0, π/6], and [0, π/2, π, 3π/2]. In each case, the field-aligned antenna had reduced integrated E_∥ relative to the existing non-aligned antenna geometry, with the greatest reduction for monopole [0, 0, 0, 0] phasing.     

Plasma and antenna coupling characterization in ICRF-wall conditioning experiments

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the divertor tokamaks ASDEX Upgrade (AUG) and JET to simulate the scenario of ITER wall conditioning at half-field (AUG) and full-field (JET). ICWC-plasma and antenna coupling characterization results obtained during the Ion Cyclotron Resonance Frequency (ICRF)-Wall Conditioning experiments performed in helium-hydrogen mixture in AUG and helium-deuterium mixtures in JET are presented here. Safe operational regimes for optimum ICWC in ITER could be explored for different magnetic fields. Satisfactory antenna coupling in the Mode Conversion scenario along with reproducible generation of ICRF plasmas and reliable wall conditioning were achieved by coupling RF power from one or two ICRF antennas at two (AUG, JET) different resonant frequencies. These results are in qualitative agreement with the predictions of 1-D TOMCAT code. Present study of ICWC indicates towards the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B_V ? B_T) magnetic field on antenna coupling and plasma parameters. The results obtained from JET and AUG tokamaks, presented in this paper, emphasizes the proposed phenomenological schemes for further development of ICWC in superconducting tokamaks.     

Resonant loop system for the KSTAR ICRF current drive

The phased current distribution at current straps for the KSTAR ICRF antenna causes a power imbalance at each strap owing to the mutual couplings between current straps. In order to mitigate the effect of coupling, a decoupler connecting two phased feeding lines are designed based on both a lumped element antenna model and a distributed transmission line model. Though the decoupler parameter is dependent on the loading resistance, which depends on plasma condition, an analysis shows that the decoupling is effective in the wide range of loading resistance assuming the low variation of mutual inductance between straps. A circuit analysis also shows that the RF characteristics of a complex RF transmission system are matched well for the asymmetric antenna current spectrum aiming for a non-inductive current drive of KSTAR. The calibration result of decoupler after installation is also discussed.     

Power Compensation for ICRF Heating in EAST

The source system covering a working frequency range of 24 MHz to 70 MHz with a total maximum output power of 12 MW has already been fabricated for Ion Cyclotron Range of Frequency(ICRF) heating in EAST from 2012. There are two continuous wave(CW) antennas consisting of four launching elements each fed by a separate 1.5 MW transmitter. Due to the strong mutual coupling among the launching elements, the injection power for launching elements should be imbalance to keep the k||(parallel wave number) spectrum of the launcher symmetric for ICRF heating. Cross power induced by the mutual coupling will also induce many significant issues,such as an uncontrollable phase of currents in launching elements, high voltage standing wave ratio(VSWR), and impedance mismatching. It is necessary to develop a power compensation system for antennas to keep the power balance between the feed points. The power balance system consists of two significant parts: a decoupler and phase control. The decoupler helps to achieve ports isolation to make the differential phase controllable and compensate partly cross power. After that, the differential phase of 0 or π will keep the power balance of two feed points completely. The first power compensation system consisting of four decouplers was assembled and tested for the port B antenna at the working frequency of 35 MHz. With the application of the power compensation system, the power balance, phase feedback control, and voltage standing wave ratio(VSWR) had obviously been improved in the 2015 EAST campaign.     

Effects of External Resonant Fields on the Tokamak Edge Plasma Fluctuations

In this contribution, the effects of external resonant electric and magnetic fields on the tokamak edge plasma fluctuations have been investigated. For this purpose, the radial and poloidal electric fields and ion saturation current have been measured by two arrays of the Langmuir probes. An external resonant electric field was applied with the limiter biasing system. The biased electric voltage has been restricted to 0 < V _bias  < +320 and it has been applied with the limiter that is fixed in the r/a = 0.9. Moreover, the power spectra of particle flux Γ _r , Γ _p , Reynolds stress, coherency between E _p and I _s have been calculated. Fourier-based techniques have been employed to analyze the frequency of the Reynolds stress and particle flux. The results show that after positive biasing application (V _bias  = +200v), the Reynolds stress increases about 50 % while it doesn’t change remarkability after positive biasing with V _bias  = +320v. The Reynolds stress power spectrum confirms these results. The effect of positive biasing on Γ _r has been displayed a decrease about 60, 20 % by V _bias  = +200v, V _bias  = +320v respectively. Γ _p and I _s increase about 30 and 4 % in the present of biased (V _bias  = +200v) while they don’t change remarkability in another voltage. The power spectrum of Γ _r decreases about 15 and 10 % while the power spectrum of Γ _p increases about 80 and 20 % after positive biasing (V _bias  = +200v, V _bias  = +320v). Consequently, a better confinement obtained for biased with V _bias  = +200v. It means that, the magnitude of biased is important factor in modifying and control plasma turbulence. Also, results were compared with the effects of external resonant magnetic field.     

Plasma Internal Inductance in the Presence of External Resonant Fields in IR-T1 Tokamak

In this paper we presented experimental investigation of effects of local limiter biasing (V_biasing = +200 v, V_biasing = +320 v) on the plasma parameters as plasma current, loop voltage, poloidal beta, plasma pressure, plasma energy, plasma resistance, plasma temperature, plasma displacement, Shafranov parameter and plasma internal inductance in IR-T1 tokamak. For these purposes, array of magnetic probes and also a diamagnetic loop have been used. The results show that applied biased voltage V_biasing = +200 v causes to decrease of about 40 % in plasma internal inductance. The plasma resistance and the plasma displacement have been decreased by V_biasing = +200 v. The main result of the application of V_biasing = +200 v is flatting the plasma parameters profiles. In other words, the addition of biasing voltage V_biasing = +200 v to plasma could be effective for improving the quality of tokamak plasma discharge by creating the steady state plasma. The plasma current, plasma pressure, plasma energy, plasma temperature and shift parameter have increased after the application of limiter biasing with V_biasing = +320 v but they decrease rapidly.     

Analysis of double stub tuner control stability in a phased array antenna with strong cross-coupling

Active stub tuning with a fast ferrite tuner (FFT) has greatly increased the effectiveness of fusion ion cyclotron range of frequency (ICRF) systems (50–100 MHz) by allowing for the antenna system to respond dynamically to changes in the plasma load impedance such as during the L–H transition or edge localized modes (ELMs). A high power waveguide double-stub tuner is under development for use with the Alcator C-Mod lower hybrid current drive (LHCD) system at 4.6 GHz. The amplitude and relative phase shift between adjacent columns of an LHCD antenna are critical for control of the launched n_|| spectrum. Adding a double-stub tuning network will perturb the phase and amplitude of the forward wave particularly if the unmatched reflection coefficient is high. This effect can be compensated by adjusting the phase of the low power microwave drive for each klystron amplifier. Cross-coupling of the reflected power between columns of the launcher must also be considered. The problem is simulated by cascading a scattering matrix for the plasma provided by a linear coupling model with the measured launcher scattering matrix and that of the FFTs. The solution is advanced in an iterative manner similar to the time-dependent behavior of the real system. System performance is presented under a range of edge density conditions from under-dense to over-dense and a range of launched n_||. Simulations predict power reflection coefficients (Γ²) of less than 1% with no contamination of the n_|| spectrum. Instability of the FFT tuning network can be problematic for certain plasma conditions and relative phasings, but reducing the control gain of the FFT network stabilizes the system.     

Vacancy-type defects near Al surface studied by slow positron annihilation spectroscopy before and after He implantation

The vacancy-type defects He_nV_m near Al surface before and after He⁺ implantation and their evolutions with annealing temperatures and aging time have been investigated by mono-energy slow positron annihilation spectroscopy (SPAS) with S parameters. The results show that many vacancies are produced during the sample preparation process, which can be re-occupied by Al atoms during annealing, Al⁺ and MeV He⁺ implantation. S parameters denote the concentration and size of He_nV_m clusters induced by He⁺ implantation in Al. The higher fluence of He implanted, the larger S parameters will be, indicating more He_nV_m clusters produced. S parameters decrease with the increase of annealing temperatures until the fastest change temperature, and then an opposite or minor change occurs depending on the fluence of He implanted in Al, showing that the concentration and size of He_nV_m clusters will vary with the annealing temperatures. Aged at RT for some time, the concentration and mean size of He_nV_m clusters in Al will get smaller and larger, respectively, resulting in the decrease of S parameters with the aging time. In conclusions, the evolution of vacancy-type defects He_nV_m near Al surface after He⁺ implantation depends on the annealing temperatures, He concentration and aging time.     

ICRF physics aspects of wall conditioning plasma characterization in TEXTOR

This paper focuses on encouraging results obtained on the characterization of RF produced plasmas during pulsed-mode wall conditioning discharges in ion cyclotron resonance frequency (ICRF) regime in the limiter tokamak TEXTOR. Recent Ion Cyclotron Wall Conditioning (ICWC) experiment carried out in TEXTOR tokamak, lead to the identification of various dependences of the antenna-plasma coupling efficiency on the plasma parameters for possible ICWC-discharge cleaning in ITER at half field. Our ICWC experiments emphasize on (i) study of antenna coupling during the mode conversion scenario, (ii) reproducible generation of ICRF plasmas for wall conditioning, by coupling RF power from one or two ICRF antennas and (iii) effect of application of an additional (along with toroidal magnetic field) stationary vertical (B_V ? B_T) or oscillating poloidal magnetic field (B_p ? B_T) on antenna coupling and relevant plasma parameters.     

Real-time measurement and feedback control of ion temperature profile and toroidal rotation using fast CXRS system in JT-60U

A fast charge exchange recombination spectroscopy (CXRS) system has been developed for the real-time measurement and feedback control of ion temperature (T_i) profile and toroidal rotation velocity (V_t) in JT-60U. In order to control T_i and V_t in real-time, the charge exchange recombination spectroscopy with high time resolution, the real-time processor system, and the real-time control system have been developed. Utilizing this system, real-time control of the T_i gradient between r/a ∼ 0.25-0.5 has been demonstrated with neutral beams at high beta plasmas (normalized beta β_N ∼ 1.6-2.8). The strength of the internal transport barrier is controlled. Moreover, the real-time control of V_t has been demonstrated from counter (anti-parallel to the plasma current, I_p) to co (parallel to the I_p) direction. Then the behavior of edge localized mode (ELM) is changed by controlling the V_t.     

Comparison of measured and calculated reaction rate distributions in an scwr-like test lattice

High resolution gamma-ray spectroscopy measurements were performed on 61 rods of an SCWR-like fuel lattice, after irradiation in the central test zone of the PROTEUS zero-power research reactor at the Paul Scherrer Institute in Switzerland. The derived reaction rates are the capture rate in ²³⁸U (C₈) and the total fission rate (F_tot), and also the reaction rate ratio C₈/F_tot. Each of these has been mapped rod-wise on the lattice and compared to calculated results from whole-reactor Monte Carlo simulations with MCNPX. Ratios of calculated to experimental values (C/E’s) have been assessed for the C₈, F_tot and C₈/F_tot distributions across the lattice. These C/E’s show excellent agreement between the calculations and the measurements. For the ²³⁸U capture rate distribution, the 1σ level in the comparisons corresponds to an uncertainty of ±0.8%, while for the total fission rate the corresponding value is ±0.4%. The uncertainty for C₈/F_tot, assessed as a reaction rate ratio characterizing each individual rod position in the test lattice, is significantly higher at ±2.2%. To determine the reproducibility of these results, the measurements were performed twice, once in 2006 and again in 2009. The agreement between these two measurement sets is within the respective statistical uncertainties.     

The ternary system: silicon-uranium-vanadium

Phase equilibria in the system Si-U-V were established at 1100 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed, U₂V₃Si₄ and (U_1−xV_x)₅Si₃, for which the crystal structures were elucidated by X-ray powder data refinement and found to be isotypic with the monoclinic U₂Mo₃Si₄-type (space group P2₁/c; a = 0.6821(3), b = 0.6820(4), c = 0.6735(3) nm, β = 109.77(1)°) and the tetragonal W₅Si₃-type (space group I4/mcm, a = 1.06825(2), c = 0.52764(2) nm), respectively. (U_1−xV_x)₅Si₃ appears at 1100 °C without any significant homogeneity region at x ∼ 0.2 resulting in a formula U₄VSi₃ which corresponds to a fully ordered atom arrangement. DTA experiments clearly show decomposition of this phase above 1206 °C revealing a two-phase region U₃Si₂ + V₃Si. At 1100 °C U₄VSi₃ is in equilibrium with V₃Si, V₅Si₃, U₃Si₂ and U(V). At 800 °C U₄VSi₃ forms one vertex of the tie-triangle to U₃Si and V₃Si. Due to the rather high thermodynamic stability of V₃Si and the corresponding tie-lines V₃Si + liquid at 1100 °C and V₃Si + U(V) below 925 °C, no compatibility exists between U₃Si or U₃Si₂ and vanadium metal.     

Commissioning of 3.7 GHz LHCD system on ADITYA tokamak and some initial results

To drive plasma current non-inductively, a lower hybrid current drive (LHCD) system has been designed, fabricated and successfully installed on ADITYA tokamak. The system is designed to launch 120 kW of RF power, at a frequency of 3.7 GHz. The system mainly consists of a high power CW klystron source, a long waveguide transmission line of about 100 m length, a UHV compatible modular waveguide line of about 2.65 m, and a conventional grill type antenna. Independent phase shifters, one each in the eight lines, are used to adjust the antenna phasing and also provides the flexibility to launch a composite spectrum. The antenna is designed to launch lower hybrid waves (LHW) with parallel refractive index (N_||), in the range, 1 < N_|| < 4.5, by appropriately phasing the antenna. Antenna is positioned in the shadow of the poloidal limiter and is provided with 100 mm radial movement to achieve optimum coupling conditions.The complete system development includes design, fabrication and testing of number of waveguide components, modular waveguide lines and their integration. Different cost effective fabrication techniques are adopted to achieve good RF performance. Special attention is paid on the flanged joint seals in the long transmission line to minimize the RF losses. The entire LHCD system is calibrated, especially, in terms of phase, insertion loss and return loss measurements.After the successful integration of the system on ADITYA tokamak, some initial experiments have been carried out to assess the system commissioning and its performance. The experiments were done with a plasma (hydrogen) density of 2-5 × 10¹² cm⁻³ at a toroidal magnetic field of 0.8 T with 10-25 kA of plasma current. Initial results indicate that, good coupling is achieved in the presence of proper edge density. Measurements obtained from second harmonic electron cyclotron emission (ECE) and hard X-ray diagnostics suggest generation of suprathermal electrons in the presence of LH pulse. Plasma current pulse elongation with LH power is observed but needs further investigation to derive conclusions.This paper presents the design, fabrication, testing and integration of the waveguide lines, waveguide components and UHV compatible modular transmission lines of the LHCD system on ADITYA tokamak and discusses some of the initial results.     

Au L-shell ionization by Si and S ions: Integral and differential cross sections and alignment

We present the following experimental results on Au L-shell ionization: (1) in the bombarding energy range 0.25—2.5 $\text{MeV}\text{u}$, absolute X-ray yields and the L₃-vacancy integral alignment for Si, and cross section ratios $\text{σ2}\text{σ3}$ for Si and S as projectiles; (2) MeV sulphur energy, subshell ionization probability ratios and the components A₂₀(b) and A₂₂(b) of the statistical tensor descr the L₃-vacancy for impact parameters b = 20–450 fm. A comparison of the data to SCA calculations reveals, except perhaps for the differential alignment, important discrepancies relative to the theoretical predictions. These discrepancies, increasing at lower projectile velocities, are most probable due to the mutual L-subshell coupling, as well as to the rotational coupling between the L₃ magnetic substates during the collision.     

Effects of γ-ray irradiation on the C-V and G/ω-V characteristics of Al/SiO2/p-Si (MIS) structures

The effect of the ⁶⁰C_o (γ-ray) exposure on the electrical characteristics of Al/SiO₂/p-Si (MIS) structures has been investigated using capacitance-voltage (C-V) and conductance-voltage (G/ω-V) measurements. The MIS structures were stressed with a bias of 0 V during ⁶⁰C_oγ-sources irradiation with the total dose range from 0 to 25 kGy. The C-V and G/ω-V characteristics were measured at 500 kHz and room temperature before and after ⁶⁰C_oγ-ray irradiation. The results indicated that γ-irradiation caused an increase in the barrier height Φ_B, interface states N_ss and depletion layer width W_D obtained from reverse bias C-V measurements. The series resistance R_s profile for various radiation doses was obtained from forward and reverse bias C-V and G/ω-V measurements. Both C-V and G/ω-V characteristics indicate that the total dose radiation hardness of MIS structures may be limited by the decisive properties of the SiO₂/Si interface to radiation-induced damage. After γ-irradiation, the decrease in capacitance of MIS structure results in the increase in the semiconductor depletion width.     

Phase relations for reactions of hydrogen with sodium oxide between 500 and 900°C

The equilibrium pressure technique, commonly used for investigating the phase relations in binary gas-condensed phase equilibria, was applied to the Na-Na₂O-NaOH-NaH corner of the ternary Na-O-H system. Measured amounts of hydrogen were reacted with Na₂O (sealed in vacuo in a thin-walled nickel crucible) and the equilibrium hydrogen pressure (0.03 < P < 100 kPa) was determined as a function of temperature(510 < T < 879°C) and hydrogen atom fraction(0 < X_H < 0.275). Discontinuities encountered in the isothermal(√P vs X_H) or fixed-hydrogen-concentration (InP vs 1/T) plots were used to delineate the condensed-phase boundaries that occur within theP-T-X_H ranges studied. Data were fitted to analytical equations which permitted construction of a portion of the phase diagram and yielded pertinent thermodynamic information. Limitations of the technique in applications to other similar systems are discussed.     

Experimental validation of control rod related perturbations of moderator regions in an SCWR-like fuel lattice

Two different types of perturbations of an SCWR-like fuel lattice have been investigated experimentally in the central test zone of the PROTEUS zero-power research reactor at the Paul Scherrer Institute in Switzerland. In each case, a campaign of high-resolution gamma-ray spectroscopy measurements was carried out on 34 fuel pins of the test lattice. In the first case, the test lattice was perturbed by inserting aluminum rods into the four central moderator regions, while in the second case, the perturbation was affected using steel absorber rods (instead of aluminum). The derived reaction rates are the capture rate in ²³⁸U (C₈) and the total fission rate (F_tot), as also the reaction rate ratio C₈/F_tot. Each of these has been mapped on the lattice and compared to calculated results from whole-reactor Monte Carlo simulations with MCNPX. Excellent agreement has been obtained, for both perturbed lattices, between the calculated and experimental distributions of C₈, F_tot and C₈/F_tot. Considering that control rods in an SCWR assembly are foreseen to be inserted into the central moderator regions, these results may be considered as generic validation of Monte Carlo simulations for the two different types of lattice perturbations which inserted control rods imply, viz. moderator displacement and strong neutron absorption.In a second step, calculated C₈, F_tot and C₈/F_tot distributions for the two perturbed lattices (as well as for the unperturbed lattice) have been compared, at assembly level, between MCNPX and the deterministic LWR lattice code CASMO-4E. In the case of the unperturbed lattice, as well as for the lattice with steel rods, the agreement between the codes is found to be within ～1% for all pins and each reaction rate. However, for the lattice with aluminum rods, i.e. the case with mainly just moderator displacement involved, CASMO overestimates the reaction rates in the vicinity of the perturbations by up to 2–3%, when employing the standard input options. The reason for this discrepancy has been found to be the leakage treatment, which uses the fundamental-mode buckling applied in a homogenized sense across the lattice. In this way, global leakage gradients get averaged out over the entire assembly. The optional input card BZ2 for CASMO resolves this problem, and the codes then agree within 1% even for the aluminum case.     

Triple-probe Diagnostic Measurements in Plasma of GLAST Spherical Tokamak

Triple-probe has been developed and operated successfully to characterize ECRH-assisted argon as well as hydrogen microwave plasmas in GLAST Spherical Tokamak. This technique enables to determine transient plasma parameters such as floating potential, electron temperature and electron number density in rapidly time-varying plasmas. An effective electron heating mechanism is applied to produce microwave plasma by injecting radiofrequency (RF) radiation at a frequency of 2.45 GHz in the presence of resonant toroidal magnetic field. Plasma parameters and corresponding fluctuations are measured as a function of time in different gas fill pressures for various applied magnetic fields. The results demonstrate the dependence of plasma parameters such as V _f , T _e , n _e and their fluctuations on gas fill pressure during the pre-ionization phase of the GLAST operation. Plasma behavior is observed to be closely depending on the coupling of RF power during microwave discharge. Additionally, the hydrogen plasma shows pronounced fluctuations in comparison with argon plasma with some decrease in electron temperature and densities.     

Pure ionization cross section of helium and ionization mechanism

Pure target ionization was investigated for 0.4-6.4 MeV C^q+(q = 1-4) + He and O^q+(q = 1-4) + He collisions. The double-to-single target ionization ratios R₂₁ were measured using coincidence techniques. We compare our results with existing experimental results and find they are in good agreement. The ratio R₂₁ is nearly independent of projectile charge state. The relation of R₂₁ ∼ V is analyzed using the over barrier model (OBM) and ionization probability, which is described in our extended over barrier model. Our calculation agrees well with the experimental results.