Passive Active Multijunction Research Articles

LH launchers for tokamaks at IPR

Earlier, a conventional grill launcher-based LHCD system has been developed to drive plasma current non-inductively in tokamaks at IPR (ADITYA and SST1) at a frequency (fo = ωo/2π) of 3.7 GHz, which has the ability to launch pure spectrum with high directivity. Grill having large number of sub-waveguides launches a sharp spectrum, which drive LH current more efficiently and therefore was preferred for carrying out plasma physics studies in a controlled manner. An over-dense plasma near the mouth of the launcher provides good coupling of LHWs with plasma and therefore the grill launcher needs to be placed close to the last close flux surface (LCFS). The grill launcher at IPR allows to launch LHWs having a parallel refractive index (N||) from 1.5 to 4.0 by varying the phase difference between adjacent sub-waveguides from 60° to 160°. For ADITYA tokamak, owing to space constraints, the grill launcher with eight sub-waveguides (arranged in two rows, each having four sub-waveguides) could be developed to launch of power up to 120 kW, and therefore, the topology of the ADITYA LHCD was not very complex. However, for the 1 MW LHCD system for SST1 tokamak, the grill launcher with sixty-four sub-waveguides (arranged in two rows, each having thirty-two sub-waveguides) was developed and the SST1 LHCD system, thus conceived, was huge and very complex offering tremendous engineering design challenges. For reactor-grade plasmas, where rf power of a few tens of MW would be launched in to the plasmas, a more compact and reactor-relevant LH launcher, often known as passive–active multijunction (PAM) is foreseen. The PAM launcher offers very good coupling with low reflection co-efficient (∼2–5%) when the plasma density near the mouth of the launcher is near cutoff densities for a given frequency (ωo = ωpe, the electron plasma frequency), and thus allowing it to be placed far away from the LCFS of the plasma, thereby minimizing deleterious effects on launcher due to hostile edge plasma conditions and makes this launcher relevant for reactor-grade plasmas. Further, the space behind the passive waveguide can be utilized for neutron shielding and efficient thermal management. On the contrary, it offers reduced directivity and the spectrum can be varied over a narrow range. Further, the network of transmission lines feeding this launcher is relatively simpler and offers less technological challenges. Thus, 250-kW PAM launcher (comprising of two modules)-based LHCD system for ADITYA-U tokamak has been designed and developed. The launched N|| may be varied over a narrow range (2.25 ± 0.375) by varying the phase difference between adjacent modules over 180° ± 90°. The physics requirements, design philosophy, engineering challenges/solutions, the key outcome and achievements as we embark on to PAM launcher are presented and discussed. Based on our experience of PAM launcher, for ADITYA-U tokamak, conceptual design of the PAM launcher for SST1 is also presented.

Design and analysis of a PAM launcher at 4.6 GHz for a new LHCD system on EAST

To improve the Current Drive (CD) capability in long-pulse (up to ∼1000 s) H-mode operation, it has been decided to develop a new Lower Hybrid Current Drive system at 4.6 GHz with an active cooling Passive Active Multijunction (PAM) launcher on EAST. In this paper, both the radio frequency (RF) and the physical properties of this PAM are studied numerically. The same nominal parallel refractive index (N || = k ||c/ω, where k || is the parallel wavenumber, c the velocity of light, and ω the wave angular frequency) of 2.04 as the existing 4.6 GHz Full Active Multijunction (FAM) is chosen. Ray-tracing calculations indicate that good accessibility could be achieved when the LH waves radiate with this nominal N || in typical long-pulse H-mode plasmas. The coupling performance in terms of power reflection coefficient (R C), power spectrum, maximum electric field, power directivity (D P) and global CD capability is evaluated with the ALOHA code based on the linear coupling theory. Good coupling performance with averaged R C ⩽ 1% and D P ∼ 70% could be expected with the density (n e) in front of the PAM close to the cut-off value (n e_co). The simulated R C remains below 6.5% over a wide density range 0.5 ⩽ n e/n e_co ⩽ 10, which is similar to the plasma edge conditions produced by Edge Localized Mode activity. A detailed comparison with the existing 4.6 GHz FAM launcher is also performed.

First experimental results of the PAM LHCD launcher at 2.45 GHz on EAST

This paper presents the first results of a passive active multijunction (PAM) launcher at 2.45 GHz during the lower hybrid current drive (LHCD) experiments on EAST. Good coupling performance with a power reflection coefficient (RC) ∼3% has been achieved at the plasma–antenna distance up to ∼11 cm in L-mode edge plasmas without local gas puffing near the PAM launcher. Reliable power coupling of this PAM during the edge perturbations induced by type I edge localized modes (ELMs) has been successfully demonstrated. Compared with the old full active multijunction (FAM) launcher, the new PAM can be placed ∼2 cm further away from the plasma in normal operations, which is in good agreement with the previous prediction (Li et al 2019 Fusion Eng. Des. 147 111 250), by the linear wave–plasma coupling code ALOHA (Hillairet et al 2010 Nucl. Fusion 50 125 010). The flexibility of the power spectrum by changing the phase difference between adjacent modules was validated and ray-tracing/Fokker–Planck simulations can reproduce the experimental features. The achievable power handling is as high as 25 MW m−2, although with a shot pulse length of ∼10 s. The first experiment successfully demonstrated the coupling performance of a PAM launcher at low density and this launcher construction provides helpful engineering experience for the 4.6 GHz PAM development in the near future on EAST.

New method for reshaping of the WEST Lower Hybrid launcher to reach long pulse operation

The aim of WEST (tungsten-W Environment in Steady-state Tokamak) experiments is to master long pulses (1000s) and submit the ITER-grade tungsten divertor to power fluxes of 10 MW/m2. The lower hybrid current drive (LHCD) system, composed of a fully active multijunction (FAM-LH1) launcher and a passive active multijunction (PAM-LH2) launcher, with a total capability of 7 MW/1000s, is essential to meet these goals. The LH1 launcher was reshaped before the start of WEST to match the new plasma shape and thus improve its coupling capability. During the 2019–2021 experimental campaign, long pulse operation (∼ 55 s) was successfully and routinely performed with PLH = 3 MW and good LH coupling properties. Thanks to infrared monitoring during long pulses, the new toroidal shape of the LH1 launcher was found to avoid overheating on the front face edges. It also highlighted the existence of hot spots on the LH2 launcher's front face edges, limiting its power capability in high power and long pulse scenarios. A reshaping inside the vacuum vessel of the LH2 launcher was therefore performed during the shutdown in 2021. This paper describes the reshaping methods used for the two launchers and the tools developed for the reshaping of LH2 inside the tokamak.

Conceptual design of multichannel fast electron bremsstrahlung detection system to study fast electron dynamics during lower hybrid current drive in ADITYA-U tokamak

To drive plasma current non inductively, a Lower Hybrid Current Drive (LHCD) system with a passive-active multijunction (PAM) antenna for injecting lower hybrid wave (LHWs) has been designed, fabricated and to be integrated with ADITYA-U Tokamak. A fast electron population in the energy range of a few keV to several hundreds of keV is generated with the injection of the LHWs. These fast electrons interacting mainly with ions and electrons result in Hard X-Ray (HXR) emission called Fast Electron Bremsstrahlung acronym as FEB emission. A single channel FEB detection system is being used in the earlier experiment of LHCD in ADITYA-U tokamak. However, the single channel detection system can't provide a radial emissivity profile of HXR intensity distribution. In order to determine a radial emissivity profile, for the first time, a multichannel FEB detection system is designed for ADITYA-U tokamak. This paper describes the detailed conceptual design for the selection of suitable detectors, optimization of the collimators, shielding geometry, and low energy filtering cut-off. In the present design analysis, the soller collimator concept is considered over a simple pinhole camera system due to its several advantages. In order to optimize the multichannel FEB system parameters for the best possible performance, a forwarded modelling code has been used. The signal to background ratio at each detector location has been estimated for a few system parameters and reported herein.

Conceptual design of the LHCD system on CFETR

With the capability of saving flux consumption in the current ramp up phase, controlling the safety factor (q) profile, providing the required off-axis current drive (CD), a lower hybrid current drive (LHCD) system was eventually determined to be used on the China Fusion Engineering Test Reactor (CFETR). In this paper, a 20 MW/4.6 GHz system composed of 40 units of 500 kW continuous wave (CW) klystron amplifier is preliminarily designed. The feasibility of the frequency choice of 4.6 GHz in the physical aspect is discussed. In order to minimize the transmission loss, a TE01 over-mode of circular waveguide will be used in the transmission system. Although previous calculations indicates that high field side (HFS) launcher location results in waves damping at inner region than low field side (LFS) launcher, which is favorable for plasma stability, HFS launcher will be extremely difficult in engineering due to the limited space. Calculations show that the tritium breeding ratio (TBR) will be decreased by ∼ 1.5% with HFS launcher, while it will be decreased by ∼ 0.22% only with LFS launcher. As a result, the LH power will be coupled to plasma from the top port at LFS. The designed passive active multi-junction (PAM) launcher is arranged in an array of 5 rows and 8 columns with the height of 1181 mm and the width of 794 mm. A shielding block with a thickness of 20 cm will be equipped around the feeding waveguides to protect the feeding waveguides and to prevent neutron leakage. A preliminary scheme of the remote maintenance for the LFS launcher is given. The modeling results shows that the density for optimum coupling is ∼ 1.8 × 1017/m³, lower than the 4.6 GHz cut-off density ne_co = 2.6 × 1017/m³. Around this density, the power directivity (Dp) can be high as 71%.

Recent EAST Experimental Results and Systems Upgrade in Support of Long-Pulse Steady-State Plasma Operation

In recent years, EAST has achieved a 60-s scale steady-state plasma operation (SSO), which is electron dominant heated by the electron cyclotron wave and low hybrid wave. Substantial progresses have been made in the development of long-pulse operation, such as active control of radiative divertor, edge localized mode (ELM) suppression, and He plasma experiment. To further develop the steady-state operation scenario and study the related critical physical issues, the upgrade of EAST has been completed in 2020. The auxiliary heating systems were rearranged and upgraded with a total heating power capability of 33 MW. The 2.45-GHz lower hybrid current drive (LHCD) launcher has been updated from full active multijunction (FAM) to passive active multijunction (PAM) to avoid damage caused by plasma–material interaction. The injection power capability of electron cyclotron resonance heating (ECRH) system is upgraded to 1.4 MW. The upgraded ion cyclotron resonance frequency (ICRF) antenna injects low parallel wavenumber ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{\vert \vert }$ </tex-math></inline-formula> ) spectra with good single pass absorption. The neutral beam injection (NBI) system is moved from F-port to D-port and is reoriented from counterclockwise to clockwise injection. The armor material of lower divertor has been upgraded from graphite to tungsten improving the steady-state heat handling capability from 2 to 10 MW/m2. Based on the upgraded pumping system and improved conductance, the effective pumping speed for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{2}$ </tex-math></inline-formula> of the EAST lower divertor increased twice. With all these upgrades, 100-s scale steady-state operation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{e0} &gt;10$ </tex-math></inline-formula> keV) and 1056-s high-performance operation with 1.73-GJ injection energy were achieved in 2021 campaigns. The experience in high-performance long-pulse operation obtained on EAST will lay a foundation for the ITER and Chinese Fusion Engineering Testing Reactor (CFETR) operation.

Improvement of lower hybrid current drive systems for high-power and long-pulse operation on EAST

Aiming at high-power and long-pulse operation up to 1000 s, some improvements have been made for both 2.45 GHz and 4.6 GHz lower hybrid (LH) systems during the recent 5 years. At first, the guard limiters of the LH antennas with graphite tiles were upgraded to tungsten, the most promising material for plasma facing components in nuclear fusion devices. These new guard limiters can operate at a peak power density of 12.9 MW/m2. Strong hot spots were usually observed on the old graphite limiters when 4.6 GHz system operated with power >2.0 MW [B. N. Wan et al., Nucl. Fusion57(2017) 102019], leading to a reduction of the maximum power capability. With the new limiters, 4.6 GHz LH system, the main current drive (CD) and electron heating tool for EAST, can be operated with power >2.5 MW routinely. Long-pulse operation up to 100 s with 4.6 GHz LH power of 2.4 MW was achieved in 2021 and the maximal temperature on the guard limiters measured by an infrared (IR) camera was about 540 °C, much below the permissible value of tungsten material (∼1200 °C). A discharge with a duration of 1056 s was achieved and the 4.6 GHz LH energy injected into the plasma was up to 1.05 GJ. Secondly, the fully-active-multijunction (FAM) launcher of 2.45 GHz system was upgraded to a passive-active-multijunction (PAM), for which the density of optimum coupling was relatively low (below the cut-off value). Good coupling with reflection coefficient ∼3% has been achieved with plasma-antenna distance up to 11 cm for the new PAM. Finally, in order to eliminate the effect of ion cyclotron range of frequencies (ICRF) wave on 4.6 GHz LH wave coupling, the location of the ICRF launcher was changed to a port that is located 157.5° toroidally from the 4.6 GHz LH system and is not magnetically connected.

Neutronics assessments of LHCD antenna system for CFETR

To maintain a long-duration plasma current and obtain steady-state operation for CFETR, a non-inductive Heating and Current Drive (H&CD) system is currently under study. Lower Hybrid Current Drive (LHCD) is an important auxiliary heating system, that has good current drive efficiency and improves plasma confinement by changing the current distribution.Neutronics calculation was conducted to study the nuclear performance and shielding effectiveness. This study was performed on a CFETR 22.5° model integrated with an upper-port LHCD antenna structure. The detailed 3D neutronics model was built using the computer-aided modeling platform cosVMPT, and particle transport was performed using MCNP5 with the nuclear data library FENDL-2.1. With the support of high performance variance reduction technique, “on-the-fly” GVR method, global neutron and photon flux distributions with reasonable relative error were obtained. The irradiation damage to components in the antenna system was assessed, including DPA, gas production and nuclear heat density. The total nuclear heat for the transmission structure was 1.192 MW, and the peak irradiation damage was located at the front end of the Passive-Active Multi-junction (PAM) launcher. Owing to the blanket opening for wave propagation, the induced nuclear responses for VV and TFC were analyzed, and the shielding design was initialized. The results showed that all the parameters were within the design limit after the shielding design. Because part of the blanket module was occupied by the antenna launcher, the TBR of CFETR was evaluated, and it decreased by only 0.22% for the whole fusion reactor.

Design of a 3.7 GHz TE10 – TE30 mode converter using stepped impedance transformer for CW applications

This paper presents the design and simulation of a novel TE10 to TE30 mode converter operating at 3.7 GHz and is subjected to high RF power of 500 kW in Continuous Wave (CW) regime. The design incorporates a stepped impedance transformer as opposed to the conventional capacitive button to improve the return loss and power handling capacity of the converter. A rigorous RF, breakdown and multiphysics analysis including thermal, fluid dynamics and stress physics is performed on the mode converter to ascertain its viability and usefulness as a power dividing network in the Passive Active Multijunction launcher for Lower Hybrid Current Drive systems of tokamaks. The proposed mode converter exhibits a return loss of ∼50 dB, mode conversion efficiency of ∼97% and a peak E-field of 685 kV/m is obtained as compared to that of 4.2 MV/m obtained in the conventional mode converter. Thus, the power handling capacity of the proposed mode converter design is found to be 97.3% higher than the conventional mode converter.

Multiphysics Simulations of a Steady-State Lower Hybrid Current Drive Antenna for the FSNF

The Fusion Nuclear Science Facility (FNSF) is a proposed tokamak reactor with the mission to investigate operation of a fusion reactor in a nuclear environment. The high neutron fluence component of the FNSF mission requires steady-state operation for extremely long pulses (months) at full power. Plasma sustainment and current drive will be critical components of a successful FNSF. COMSOL Multiphysics® software is used for combined radiofrequency (RF) and thermal simulations of the lower hybrid current drive antenna system. These simulations consider the resistive RF losses in the antenna including realistic surface roughness and a range of potential materials. The thermal analysis adds volumetric nuclear heating, plasma heat flux on leading edges, and electromagnetic radiation from the plasma to the RF heating calculated by COMSOL. Additional neutronics calculations have been performed to determine the impact of these antenna designs on activated waste disposal for the materials considered. The simulations show that it is technically feasible to implement a fully active multijunction (FAM) rather than a passive-active multijunction (PAM) style of antenna if the septum between adjacent waveguides is sufficiently wide and the thermal conductivity of the structural material is sufficiently high. The FAM has the benefit of higher achievable power density with respect to the PAM, which results in a more compact antenna with potentially lower impact on neutron shielding and tritium breeding. These considerations point to tungsten rather than steel as the preferred structural material in constructing the antenna.

Design of a PAM launcher and comparative analysis with the old FAM launcher for 2.45 GHz LHCD system on EAST

In order to support EAST to realize the main mission of long-pulse operation lasting 1000s, the existing full active multijunction (FAM) launcher will be upgraded to a passive active multijunction (PAM) for the 2.45 GHz lower hybrid current drive (LHCD) system. The RF components were designed and optimized with consideration of the port size, power density inside the waveguides to avoid RF breakdown and power spectrum for efficient current drive. The designed PAM has a power handling capability of 4 MW CW at 2.45 GHz with matched load. The coupling properties and power spectrum were calculated using the ALOHA code and a comparative analysis with the old FAM was carried out. The main feature of this new launcher is that the density for optimum coupling and power spectrum directivity (ne ˜ 0.3 × 1017/m3) lies below the cut-off value (ne_co ˜ 0.74 × 1017/m3), which is favored for long-pulse steady-state operation on EAST. It has a capability to deliver 3.6 MW of power during real operation (ne ˜ 2 × 1017/m3) and 2 MW during large ELMs (ne = 10 ne_co). For the nominal phasing of 180 °s between modules, the power spectrum is peaked at N// = 1.8 and a maximum power directivity of 71.2% and reflection coefficient less than 1% can be obtained via simulations. Weaker hot spot phenomenon is expected with the designed PAM than with the FAM for their respective optimum densities. Finally, a thermal analysis was performed and the maximum temperature at the front of the antenna is expected to be ˜ 260 °C which is safe enough for operation. The PAM is scheduled to be installed on EAST in 2021.

Multiphysics analysis of the Passive Active Multijunction (PAM) launcher for LHCD system of ADITYA-Upgrade tokamak

A Passive Active Multijunction (PAM) launcher is designed for the ADITYA-U Tokamak. The launcher will launch 250 kW of RF power at 3.7 GHz into the tokamak. The tokamak will be operated at a toroidal magnetic field of 1.5 T and a plasma current of ∼200 kA. The performance of the launcher in terms of its temperature, thermal stress and deformations due to thermal loads and plasma current disruption is important. The paper presents the multiphysics simulation results of the proposed PAM launcher. The simulations are performed in COMSOL Multiphysics and the results are verified analytically. The maximum temperature of the launcher obtained was 38.5 °C, while the resultant thermal stress was ∼50 MPa for an input RF power of 250 kW. The peak temperature for input power of 150 kW and 350 kW was 31.5 °C and 47 °C respectively. The simulated Von-Mises stress on the launcher during plasma disruption was ∼2.21 MPa. These results are in good agreement with those obtained from analytical calculations.

Mid-plane PAM launcher study for KSTAR LHCD system

A prototype of a midplane passive active multijunction (PAM) antenna was fabricated for the lower hybrid current drive system of KSTAR. The antenna includes a PAM launcher, bidirectional couplers, power dividing components, and two RF windows. The RF characteristics of each component, such as the splitting ratio, phase shift, transmission, and reflection, were measured using a network analyzer. A taper adapter and matched load, which can be inserted into the active waveguide of the PAM mouth, were designed and fabricated for the phase shift and power coupling measurement. The maximum error and the standard deviation of measured phase shifts between adjacent active waveguides of the PAM were 6.5° and 2.28°, respectively. The design improvement of the mid-plane PAM launcher is suggested. Multiple round trips of the reflected field in a multijunction provides the advantage of lowering the reflection but, at the same time, it causes the deterioration of directivity and an increase of the electric field in the waveguide. Adding a hybrid splitter at the end of the multijunction structure reduces the reflection by redirecting the reflection to plasma as in multijunction, however, contrary to the multijunction, it does not switch the toroidal direction of phase progression and thus improves the antenna directivity. The directivity of the hybrid PAM increases by more than 10% and the reflection decreases to 1/3–1/100 in comparison with the prototype PAM.

High field side LHCD launcher study for KSTAR

Upgrading to a lower hybrid current drive (LHCD) system on the high field side (HFS) or top launcher is under consideration for efficient current drive of KSTAR. The HFS or top launcher needs to be compact because of the tight and congested space at the inboard and near the upper divertor. This paper introduces a study of compact LHCD antenna development, and includes a brief review of a traveling wave antenna cold model and comparison with the new design of a resonant type slotted waveguide array in combination with a passive active multijunction, i.e. SWA PAM. The traveling wave antenna has structural simplicity, while SWA PAM has N|| adjustment capability.

Study on rectangular-to-circular waveguide mode converter for KSTAR LHCD transmission line

A major upgrade of KSTAR LHCD system to 4 MW is planned for 2021. The system will be composed of eight 5-GHz 500-kW klystrons, low loss transmission lines with oversized circular waveguides, and a passive active multijunction launcher. Circular TE01 mode will be adopted for transmission lines longer than 50 m because it has extremely low loss in oversized waveguide, but mode converters from rectangular TE10 to circular TE01 mode are not easy to design. Mock-ups of two types of mode converters were developed. One is serpentine type, and the other is symmetric sidewall coupling type. The TE01 mode purities were demonstrated by measuring the transmission efficiencies between two identical mode converters at various angles between symmetric planes. A sidewall coupling mode converter with a refined design, which showed better circular symmetry and higher transmission efficiency than serpentine type, is under fabrication for prototype. The prototype mode converter is calculated to have 99.5% transmission efficiency, with 0.16% diffraction loss and 0.35% resistive loss, according to the simulation.

RF design of Passive Active Multijunction (PAM) launcher for LHCD system of ADITYA-Upgrade tokamak

The LHCD system of the ADITYA-Upgrade tokamak is being upgraded by replacing the existing grill launcher with a Passive Active Multijunction (PAM) launcher. The PAM launcher outperforms the grill launcher with regards to lower reflections, ability to operate in the shadow of the vessel at plasma edge density corresponding to its cutoff density (f = fpe, where, f = source frequency, fpe = plasma frequency) and sustain high temperature due to a provision in its structure for efficient thermal management. This paper reports the RF design of the PAM launcher for the ADITYA-Upgrade tokamak. The launcher is designed at 3.7 GHz to deliver 250 kW of RF power into the tokamak. The plasma parameters, space and mechanical constraints along with the available port size are considered while designing the launcher. The dimensions of the passive active multijunction are optimized using the Advanced LOwer Hybrid Antenna (ALOHA) coupling code to obtain desirable performance when the launcher is loaded with plasma. The PAM launcher along with its transmission line components are analysed in COMSOL Multiphysics to determine its RF performance under matched conditions. Further, the multipaction analysis of the launcher is carried out to determine the possibility of RF breakdown. The evaluated return loss of the designed PAM launcher was ∼33.7 dB at the desired frequency of 3.7 GHz. The multipaction analysis revealed that the designed launcher can handle an input power up to 350 kW.

Overview of recent HL-2A experiments

Since the last Fusion Energy Conference, significant progress has been made in the following areas. The first high coupling efficiency low-hybrid current drive (LHCD) with a passive–active multi-junction (PAM) antenna was successfully demonstrated in the H-mode on the HL-2A tokamak. Double critical impurity gradients of electromagnetic turbulence were observed in H-mode plasmas. Various ELM mitigation techniques have been investigated, including supersonic molecular beam injection (SMBI), impurity seeding, resonant magnetic perturbation (RMP) and low-hybrid wave (LHW). The ion internal transport barrier was observed in neutral beam injection (NBI) heated plasmas. Neoclassical tearing modes (NTMs) driven by the transient perturbation of local electron temperature during non-local thermal transport events have been observed, and a new type of non-local transport triggered by the ion fishbone was found. A long-lasting runaway electron plateau was achieved after argon injection and the runaway current was successfully suppressed by SMBI. It was found that low-n Alfvénic ion temperature gradient (AITG) modes can be destabilized in ohmic plasmas, even with weak magnetic shear and low-pressure gradients. For the first time, the synchronization of geodesic acoustic mode (GAM) and magnetic fluctuations was observed in edge plasmas, revealing frequency entrainment and phase lock. The spatiotemporal features of zonal flows were also studied using multi-channel correlation Doppler reflectometers.

Design of mid-plane passive active multijunction antenna for 5-GHz KSTAR LHCD system

The major upgrade of KSTAR LHCD system to 4MW is planned in 2021. 4-MW RF power will be provided by 8×0.5MW klystrons. Highly oversized circular waveguide transmitting TE01° mode will be adopted for the low loss transmission line longer than 50m. The design of the 4-MW mid-plane Passive Active Multijunction launcher has been performed. Considering parallel refractive index N|| for the efficient current drive and the maximum power density at the launcher mouth, the launcher is composed of 256 active waveguides whose dimension is 58mm x 7mm and 18mm of spatial period for N||0=2.5. Multijunction with Emax <3.5kV/cm and high power waveguide components have been designed using HFSS code. The antenna properties were evaluated using ALOHA. Prototype PAM launcher with 32 active waveguides is under development for 0.5-MW pulsed operation.

Development and Application of 3.7GHz LHCD system on HL-2A and Development of RF Heating system on HL-2M

The first Lower Hybrid (LH) experiments were carried out with a Passive-Active Multijunction (PAM) launcher in H-mode plasmas. The experiments were performed on the HL-2A tokamak with the new 3.7 GHz LHCD system, installed and tested by SWIP in collaboration with CEA/RFM. The ELMs and local gas impact on LH power coupling was studied in the experiments. The coupled LH power in HL-2A was 200-500kW at large gap at the first experiments and reaches 900 kW now in H-mode, while it reaches 1MW in L-mode. The LH experiments on HL-2A show that the PAM launcher is a viable concept for high performance scenarios. The LH power can be coupled at large plasma-launcher gap, and assist in triggering and sustaining H-modes. Finally, an overview of the RF heating systems for the tokamak HL-2M is given. HL-2M will dispose of a 4 MW LH system and a 8 MW ECRH system, both of which are currently under installation at SWIP.