Nikolai Marushchenko - Academia.edu (original) (raw)
Papers by Nikolai Marushchenko
AIP Conference Proceedings
ABSTRACT The last experimental campaign at W7-AS took advantage of the full designed ECRH power. ... more ABSTRACT The last experimental campaign at W7-AS took advantage of the full designed ECRH power. Together with the high flexibility of the experimental setup, this allows for new sophisticated experimental scenarios which will be described. With central ECRH deposition of 2.1 MW power a record electron temperature of 6.5 keV has been reached with a strongly peaked temperature profile. It has been shown that the confinement improves significantly due to the radial electric field (``electron root'') induced by ECRH driven particle losses. The limits of the linear theory were demonstrated in high power ECRH experiments. The power transmitted through the plasma is reduced by an order of magnitude due to a suprathermal population generated from HFS launch. The dynamic behavior of the EC driven current was investigated in order to develop an adaptive feedback current control for ECCD. .
Nuclear Fusion
The effect of electron cyclotron heating (ECH) and current drive (ECCD) on energetic-particle (EP... more The effect of electron cyclotron heating (ECH) and current drive (ECCD) on energetic-particle (EP)-driven magnetohydrodynamic (MHD) modes is studied in the helical devices LHD, TJ-II and Heliotron J. We demonstrate that EP-driven MHD modes, including Alfvén eigenmodes (AEs) and energetic particle modes (EPMs), can be controlled by ECH/ECCD. In the LHD device, which has a moderate rotational transform and a high magnetic shear, co-ECCD enhances toroidal AEs (TAEs) and global AEs (GAEs), while counter-ECCD stabilizes them, which improves the neutron rate compared with the co-ECCD case. Counter-ECCD decreases the core rotational transform and increases the magnetic shear, strengthening the continuum damping on the shear Alfvén continua (SAC). In the TJ-II device, which has a high rotational transform, moderate magnetic shear and low toroidal field period, helical AEs (HAEs) appear when the HAE frequency gap of the SAC is changed by counter-ECCD combined with a bootstrap current and NB-driven current. On the other hand, both co-and counter-ECCD are effective in stabilizing GAEs and EPMs in the Heliotron J device, which
Computer Physics Communications
A new 3D full-wave code, named CUWA, is developed to investigate the physics of RF wave propagati... more A new 3D full-wave code, named CUWA, is developed to investigate the physics of RF wave propagation in the electron cyclotron frequency range in magnetised plasmas. The code utilises the Finite Difference Time Domain (FDTD) technique and takes advantage of massive parallel computations with Graphics Processing Units (GPU), which allows for a significant acceleration of the computations. As examples of code application we show 3D calculations of the linear transformation of ordinary to extraordinary electron-cyclotron waves and mode coupling in a sheared magnetic field. Thanks to its speed, the GPU-capable code allows for efficient and large parametric scans over a broad range of parameters.
EPJ Web of Conferences
The ECE diagnostic at W7-X in its standard mode of operation measures in X2 mode polarization wit... more The ECE diagnostic at W7-X in its standard mode of operation measures in X2 mode polarization with a 32 channel radiometer in the frequency band around 140 GHz for central magnetic field 2.5T. The radiometer is calibrated by a noise source and the overall system absolutely calibrated by means of a hot-cold source placed outside the torus in front of a Gaussian telescope optics with identical geometry and transmission line as it is installed for the measurements in the plasma vessel. The system is supplemented with a 16 channel zoom device with 4 GHz span for higher frequency resolution at a suitable radial range and a Michelson interferometer for the characterization of higher harmonics sharing the same line of sight.
EPJ Web of Conferences
A radial correlation ECE radiometer diagnostic has been developed for electron temperature fluctu... more A radial correlation ECE radiometer diagnostic has been developed for electron temperature fluctuation measurements in the helical-axis heliotron device, Heliotron J. The radiometer consists of two heterodyne detection systems. One system scans the frequency of a local oscillator from 52 to 64 GHz with a single intermediate frequency filter, and the second system has a fixed frequency, 56 GHz local oscillator with four intermediate frequency filters. This frequency range covers measurement positions spanning from the plasma core to the half radius. Laboratory tests indicate that each system has narrow intermediate frequency bandwidth and high-sensitivity over a large dynamic range. During plasma experiments with NBI heating, radiation temperature fluctuation measured by the CECE radiometer decrease with increasing ECCD commensurate with previous measurements of energetic particle driven modes on Heliotron J.
EPJ Web of Conferences
In the present work, an ECRH scenario with reduced magnetic field 1.75 T is considered. For 140 GH... more In the present work, an ECRH scenario with reduced magnetic field 1.75 T is considered. For 140 GHz, this field corresponds to X3 heating. The high mirror-ratio magnetic configuration, B01/B00 ≃ 0.24, was considered as one from most attractive for long-pulse operation with low bootstrap current. Since X3 wave mode can be effectively absorbed only in sufficiently hot plasmas, a preheating stage is necessary, and the requirements for target plasmas suitable for starting X3 have been studied. Different ways to establish target plasmas are also discussed, in particular, augmenting X3 heating with X2 beams at 105 GHz.
EPJ Web of Conferences
The wave physics of OX conversion in overdense W7-X plasma is discussed. For this study, a new 3D... more The wave physics of OX conversion in overdense W7-X plasma is discussed. For this study, a new 3D, cold plasma full-wave code has been developed. The code takes advantage of massive parallel computations with Graphics Processing Units (GPU), which allows for up to 100 times faster calculations than on a single-CPU. A 3D calculation of the OX conversion is demonstrated. We discuss limitations of the mode conversion scenario within the capabilities of the existing ECRH system in W7-X, and demonstrate an optimised conversion scenario in which the launching antenna location is altered. The conversion efficiency of the optimised scenario is predicted to be >85%. 3 Perfectly matched layer Minimization of the computation domain plays a critical role in efficient 3D computations. A physical absorber layer with a non-zero collisionality (ν > 0) is often used in plasma physics computations to truncate the computation
EPJ Web of Conferences
During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonanc... more During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3•10 19 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2•10 20 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2.
Plasma and Fusion Research
Electron Bernstein Emission (EBE) diagnostic system using Ordinary (O)-eXtraordinary (X)-Bernstei... more Electron Bernstein Emission (EBE) diagnostic system using Ordinary (O)-eXtraordinary (X)-Bernstein (B) mode conversion process has been developed for determining electron temperature profiles at overdense plasmas in a helical-axis heliotron device, Heliotron J. Ray-tracing calculation results show that the OX mode conversion window is accessible with a current ECH/ECCD launcher and transmission system. The EBW radiation from the core region is expected to be measurable by adjusting the magnetic field strength. Effect of density fluctuation on OX mode conversion efficiency is also discussed. Based on the calculations, a multi-channel radiometer of Ka-band range has been designed, assembled and tested.
EPJ Web of Conferences
Microwaves, launched from a vacuum, are reflected from their corresponding cutoff layers. In W7-X... more Microwaves, launched from a vacuum, are reflected from their corresponding cutoff layers. In W7-X, the X2 and O2-modes are used in a wide range of operation scenarios. Whereas X2 has a cutoff at 1.2 × 10 20 m-3 , the O2-mode can be used in plasmas with densities up to 2 × 10 20 m-3. The propagation of these modes is well tested experimentally and studied theoretically within geometrical optics (WKB). Possible operation at higher densities would involve mode-conversion (from O-to X-to Bernstein-mode, i.e. "OXB"). The conversion of modes is outside of the applicability of WKB-theory and usually considered with the "full-wave" approach. Full-wave models are also used to study processes which involve scales comparable to the wavelength (~1mm), for example, effects of the density fluctuations. This work reports on the development of a new 3D cold plasma full-wave code. The code utilizes the Finite Difference Time Domain (FDTD) technique [1] and has an interface with the ray-tracing code TRAVIS [2]. The computation domain is "minimized" around the WKBtrajectory obtained from the ray-tracing code; the background magnetic field is recovered from the precomputed 3D equilibrium data. The code takes advantage of massive parallel computations with Graphics Processing Units (GPUs), which allows for up to ×100 faster calculations than on a single-CPU. The minimization of the computation domain plays a critical role in efficient 3D computations. A so-called CPML boundary layer [3] is implemented in our fullwave code to truncate the computation domain. Note that is it common to discretize the magnetized plasma response current field (J) in such a way that its J x , J y and J z components are co-located in space [1, 4, 5]. This facili
Physics of Plasmas
The Electron Cyclotron Current Drive (ECCD) efficiency is usually modeled in collisionless limits... more The Electron Cyclotron Current Drive (ECCD) efficiency is usually modeled in collisionless limits. While such models are sufficient for plasmas with rather low collisionality, they might underestimate the current drive in plasmas at low temperatures as they occur at an initial phase of device operation. In this paper, the impact of finite collisionality effects on the wave-induced current drive is studied for a highmirror configuration of Wendelstein 7-X using a combination of the drift kinetic equation solver NEO-2 and the ray-tracing code TRAVIS for a realistic set of plasma parameter profiles. The generalized Spitzer function, which describes the ECCD efficiency in phase space, is modeled with help of NEO-2, which uses the full linearized Coulomb collision operator including energy and momentum conservation. Within this approach the linearized drift kinetic equation is solved by means of the field line integration technique without any simplifications on device geometry. Results of the ray-tracing code TRAVIS using the ECCD efficiency from NEO-2 within the adjoint approach show a significant difference of the driven current as compared to commonly used collisionless models for the ordinary as well as the extraordinary second harmonic mode.
Fusion Science and Technology
Fusion Science and Technology
The W 7-X Stellarator (R = 5.5 m, a = 0.55 m, B<3.0 T), which is presently being built at IPP-Gre... more The W 7-X Stellarator (R = 5.5 m, a = 0.55 m, B<3.0 T), which is presently being built at IPP-Greifswald, aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters. A 10 MW ECRH plant with cwcapability is under construction to meet the scientific objectives. The physics background of the different heating-and current drive scenarios is presented. The expected plasma parameters are calculated for different transport assumptions. A newly developed ray tracing code is used to calculate selected reference scenarios and optimize the EC-launcher and in-vessel structure. Examples are discussed and the technological solutions for optimum wave coupling are presented. The ECRH plant consists of 10 RFmodules with 1 MW power each at 140 GHz. The RF-beams are transmitted to the W7-X torus (typically 60 m) via two open multi-beam mirror lines with a power handling capability, which would already satisfy the ITER requirements (24 MW). Integrated full power, cw tests of two RF-modules (gyrotrons and the related transmission line sections) are reported and the key features of the gyrotron-and transmission line technology are presented. As the physics and technology of ECRH for both W7-X and ITER has many similarities, test results from the W7-X ECRH may provide valuable input for the ITER-ECRH plant.
Nature communications, Jan 30, 2016
Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but rec... more Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy.
Physics of Plasmas, 2014
ABSTRACT Enhancement of the output power per gyrotron has been planned in the Large Helical Devic... more ABSTRACT Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5 keV with the line-averaged electron density n e_fir = 1 × 1019 m−3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of n e_fir. The plasma stored energy of 530 kJ with n e_fir = 3.2 × 1019 m−3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.
AIP Conference Proceedings
ABSTRACT The last experimental campaign at W7-AS took advantage of the full designed ECRH power. ... more ABSTRACT The last experimental campaign at W7-AS took advantage of the full designed ECRH power. Together with the high flexibility of the experimental setup, this allows for new sophisticated experimental scenarios which will be described. With central ECRH deposition of 2.1 MW power a record electron temperature of 6.5 keV has been reached with a strongly peaked temperature profile. It has been shown that the confinement improves significantly due to the radial electric field (``electron root'') induced by ECRH driven particle losses. The limits of the linear theory were demonstrated in high power ECRH experiments. The power transmitted through the plasma is reduced by an order of magnitude due to a suprathermal population generated from HFS launch. The dynamic behavior of the EC driven current was investigated in order to develop an adaptive feedback current control for ECCD. .
Nuclear Fusion
The effect of electron cyclotron heating (ECH) and current drive (ECCD) on energetic-particle (EP... more The effect of electron cyclotron heating (ECH) and current drive (ECCD) on energetic-particle (EP)-driven magnetohydrodynamic (MHD) modes is studied in the helical devices LHD, TJ-II and Heliotron J. We demonstrate that EP-driven MHD modes, including Alfvén eigenmodes (AEs) and energetic particle modes (EPMs), can be controlled by ECH/ECCD. In the LHD device, which has a moderate rotational transform and a high magnetic shear, co-ECCD enhances toroidal AEs (TAEs) and global AEs (GAEs), while counter-ECCD stabilizes them, which improves the neutron rate compared with the co-ECCD case. Counter-ECCD decreases the core rotational transform and increases the magnetic shear, strengthening the continuum damping on the shear Alfvén continua (SAC). In the TJ-II device, which has a high rotational transform, moderate magnetic shear and low toroidal field period, helical AEs (HAEs) appear when the HAE frequency gap of the SAC is changed by counter-ECCD combined with a bootstrap current and NB-driven current. On the other hand, both co-and counter-ECCD are effective in stabilizing GAEs and EPMs in the Heliotron J device, which
Computer Physics Communications
A new 3D full-wave code, named CUWA, is developed to investigate the physics of RF wave propagati... more A new 3D full-wave code, named CUWA, is developed to investigate the physics of RF wave propagation in the electron cyclotron frequency range in magnetised plasmas. The code utilises the Finite Difference Time Domain (FDTD) technique and takes advantage of massive parallel computations with Graphics Processing Units (GPU), which allows for a significant acceleration of the computations. As examples of code application we show 3D calculations of the linear transformation of ordinary to extraordinary electron-cyclotron waves and mode coupling in a sheared magnetic field. Thanks to its speed, the GPU-capable code allows for efficient and large parametric scans over a broad range of parameters.
EPJ Web of Conferences
The ECE diagnostic at W7-X in its standard mode of operation measures in X2 mode polarization wit... more The ECE diagnostic at W7-X in its standard mode of operation measures in X2 mode polarization with a 32 channel radiometer in the frequency band around 140 GHz for central magnetic field 2.5T. The radiometer is calibrated by a noise source and the overall system absolutely calibrated by means of a hot-cold source placed outside the torus in front of a Gaussian telescope optics with identical geometry and transmission line as it is installed for the measurements in the plasma vessel. The system is supplemented with a 16 channel zoom device with 4 GHz span for higher frequency resolution at a suitable radial range and a Michelson interferometer for the characterization of higher harmonics sharing the same line of sight.
EPJ Web of Conferences
A radial correlation ECE radiometer diagnostic has been developed for electron temperature fluctu... more A radial correlation ECE radiometer diagnostic has been developed for electron temperature fluctuation measurements in the helical-axis heliotron device, Heliotron J. The radiometer consists of two heterodyne detection systems. One system scans the frequency of a local oscillator from 52 to 64 GHz with a single intermediate frequency filter, and the second system has a fixed frequency, 56 GHz local oscillator with four intermediate frequency filters. This frequency range covers measurement positions spanning from the plasma core to the half radius. Laboratory tests indicate that each system has narrow intermediate frequency bandwidth and high-sensitivity over a large dynamic range. During plasma experiments with NBI heating, radiation temperature fluctuation measured by the CECE radiometer decrease with increasing ECCD commensurate with previous measurements of energetic particle driven modes on Heliotron J.
EPJ Web of Conferences
In the present work, an ECRH scenario with reduced magnetic field 1.75 T is considered. For 140 GH... more In the present work, an ECRH scenario with reduced magnetic field 1.75 T is considered. For 140 GHz, this field corresponds to X3 heating. The high mirror-ratio magnetic configuration, B01/B00 ≃ 0.24, was considered as one from most attractive for long-pulse operation with low bootstrap current. Since X3 wave mode can be effectively absorbed only in sufficiently hot plasmas, a preheating stage is necessary, and the requirements for target plasmas suitable for starting X3 have been studied. Different ways to establish target plasmas are also discussed, in particular, augmenting X3 heating with X2 beams at 105 GHz.
EPJ Web of Conferences
The wave physics of OX conversion in overdense W7-X plasma is discussed. For this study, a new 3D... more The wave physics of OX conversion in overdense W7-X plasma is discussed. For this study, a new 3D, cold plasma full-wave code has been developed. The code takes advantage of massive parallel computations with Graphics Processing Units (GPU), which allows for up to 100 times faster calculations than on a single-CPU. A 3D calculation of the OX conversion is demonstrated. We discuss limitations of the mode conversion scenario within the capabilities of the existing ECRH system in W7-X, and demonstrate an optimised conversion scenario in which the launching antenna location is altered. The conversion efficiency of the optimised scenario is predicted to be >85%. 3 Perfectly matched layer Minimization of the computation domain plays a critical role in efficient 3D computations. A physical absorber layer with a non-zero collisionality (ν > 0) is often used in plasma physics computations to truncate the computation
EPJ Web of Conferences
During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonanc... more During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3•10 19 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2•10 20 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2.
Plasma and Fusion Research
Electron Bernstein Emission (EBE) diagnostic system using Ordinary (O)-eXtraordinary (X)-Bernstei... more Electron Bernstein Emission (EBE) diagnostic system using Ordinary (O)-eXtraordinary (X)-Bernstein (B) mode conversion process has been developed for determining electron temperature profiles at overdense plasmas in a helical-axis heliotron device, Heliotron J. Ray-tracing calculation results show that the OX mode conversion window is accessible with a current ECH/ECCD launcher and transmission system. The EBW radiation from the core region is expected to be measurable by adjusting the magnetic field strength. Effect of density fluctuation on OX mode conversion efficiency is also discussed. Based on the calculations, a multi-channel radiometer of Ka-band range has been designed, assembled and tested.
EPJ Web of Conferences
Microwaves, launched from a vacuum, are reflected from their corresponding cutoff layers. In W7-X... more Microwaves, launched from a vacuum, are reflected from their corresponding cutoff layers. In W7-X, the X2 and O2-modes are used in a wide range of operation scenarios. Whereas X2 has a cutoff at 1.2 × 10 20 m-3 , the O2-mode can be used in plasmas with densities up to 2 × 10 20 m-3. The propagation of these modes is well tested experimentally and studied theoretically within geometrical optics (WKB). Possible operation at higher densities would involve mode-conversion (from O-to X-to Bernstein-mode, i.e. "OXB"). The conversion of modes is outside of the applicability of WKB-theory and usually considered with the "full-wave" approach. Full-wave models are also used to study processes which involve scales comparable to the wavelength (~1mm), for example, effects of the density fluctuations. This work reports on the development of a new 3D cold plasma full-wave code. The code utilizes the Finite Difference Time Domain (FDTD) technique [1] and has an interface with the ray-tracing code TRAVIS [2]. The computation domain is "minimized" around the WKBtrajectory obtained from the ray-tracing code; the background magnetic field is recovered from the precomputed 3D equilibrium data. The code takes advantage of massive parallel computations with Graphics Processing Units (GPUs), which allows for up to ×100 faster calculations than on a single-CPU. The minimization of the computation domain plays a critical role in efficient 3D computations. A so-called CPML boundary layer [3] is implemented in our fullwave code to truncate the computation domain. Note that is it common to discretize the magnetized plasma response current field (J) in such a way that its J x , J y and J z components are co-located in space [1, 4, 5]. This facili
Physics of Plasmas
The Electron Cyclotron Current Drive (ECCD) efficiency is usually modeled in collisionless limits... more The Electron Cyclotron Current Drive (ECCD) efficiency is usually modeled in collisionless limits. While such models are sufficient for plasmas with rather low collisionality, they might underestimate the current drive in plasmas at low temperatures as they occur at an initial phase of device operation. In this paper, the impact of finite collisionality effects on the wave-induced current drive is studied for a highmirror configuration of Wendelstein 7-X using a combination of the drift kinetic equation solver NEO-2 and the ray-tracing code TRAVIS for a realistic set of plasma parameter profiles. The generalized Spitzer function, which describes the ECCD efficiency in phase space, is modeled with help of NEO-2, which uses the full linearized Coulomb collision operator including energy and momentum conservation. Within this approach the linearized drift kinetic equation is solved by means of the field line integration technique without any simplifications on device geometry. Results of the ray-tracing code TRAVIS using the ECCD efficiency from NEO-2 within the adjoint approach show a significant difference of the driven current as compared to commonly used collisionless models for the ordinary as well as the extraordinary second harmonic mode.
Fusion Science and Technology
Fusion Science and Technology
The W 7-X Stellarator (R = 5.5 m, a = 0.55 m, B<3.0 T), which is presently being built at IPP-Gre... more The W 7-X Stellarator (R = 5.5 m, a = 0.55 m, B<3.0 T), which is presently being built at IPP-Greifswald, aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters. A 10 MW ECRH plant with cwcapability is under construction to meet the scientific objectives. The physics background of the different heating-and current drive scenarios is presented. The expected plasma parameters are calculated for different transport assumptions. A newly developed ray tracing code is used to calculate selected reference scenarios and optimize the EC-launcher and in-vessel structure. Examples are discussed and the technological solutions for optimum wave coupling are presented. The ECRH plant consists of 10 RFmodules with 1 MW power each at 140 GHz. The RF-beams are transmitted to the W7-X torus (typically 60 m) via two open multi-beam mirror lines with a power handling capability, which would already satisfy the ITER requirements (24 MW). Integrated full power, cw tests of two RF-modules (gyrotrons and the related transmission line sections) are reported and the key features of the gyrotron-and transmission line technology are presented. As the physics and technology of ECRH for both W7-X and ITER has many similarities, test results from the W7-X ECRH may provide valuable input for the ITER-ECRH plant.
Nature communications, Jan 30, 2016
Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but rec... more Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy.
Physics of Plasmas, 2014
ABSTRACT Enhancement of the output power per gyrotron has been planned in the Large Helical Devic... more ABSTRACT Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5 keV with the line-averaged electron density n e_fir = 1 × 1019 m−3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of n e_fir. The plasma stored energy of 530 kJ with n e_fir = 3.2 × 1019 m−3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.