Showing papers by "R. C. Wolf published in 2018"

Real-time dispersion interferometry for density feedback in fusion devices

Abstract: Interferometry as one of the most common core fusion diagnostics has traditionally suffered from incomplete vibration compensation Dispersion interferometry promises a more complete compensation of vibrations For this reason it is being employed in an increasing number of experiments However, thus far none of them have shown reliable real-time low-latency processing of dispersion interferometry data Nonetheless this is a necessity for most machines when trying to do density feedback control, most notably in long discharges like the ones planned at the W7-X stellarator and ITER In this paper we report the development of a new phase extraction method specifically developed for real-time evaluation using field programmable gate arrays (FPGA) It has been shown to operate reliably during the operation phase OP12a at W7-X and is now routinely being used by the W7-X density feedback system up to very high densities above 14×1020 m-2 without showing 2π-wraps and exhibits increased wrap stability by double-data-rate sampling A rigorous error analysis has been conducted shedding insights into the signal composition of a dispersion interferometer have been gained This includes the environmental effects, most notably air humidity, on the phase measurement and the correction thereof

Global energy confinement in the initial limiter configuration of Wendelstein 7-X

Abstract: Global confinement properties of the limiter plasmas of the first operational campaign of W7-X are investigated with special focus on the energy confinement and possible operational limits. The energy confinement time was found to be close to expectations from the empirical ISS04 scaling for stellarators. Absolute values up to 160 ms were achieved. This can be considered as a great success for the initial operation of the device. However, a clear degradation of the performance was observed when radiative losses became significant, which was typically the case at low heating power. While a significant improvement of the plasma purity is expected during divertor operation, the presented results underline the importance of the impurity dynamics for the development of high-performance steady-state scenarios in W7-X. Furthermore, comparisons of the global performance properties with neoclassical transport modeling are presented. These studies are not yet fully conclusive, which could indicate that anomalous transport may have played an essential role in the low-density OP1.1 plasmas.

Bayesian uncertainty calculation in neural network inference of ion and electron temperature profiles at W7-X

Abstract: We make use of a Bayesian description of the neural network (NN) training for the calculation of the uncertainties in the NN prediction. Having uncertainties on the NN prediction allows having a quantitative measure for trusting the NN outcome and comparing it with other methods. Within the Bayesian framework, the uncertainties can be calculated under different approximations. The NN has been trained with the purpose of inferring ion and electron temperature profile from measurements of a X-ray imaging diagnostic at W7-X. The NN has been trained in such a way that it constitutes an approximation of a full Bayesian model of the diagnostic, implemented within the Minerva framework. The network has been evaluated using measured data and the uncertainties calculated under different approximations have been compared with each other, finding that neglecting the noise on the NN input can lead to an underestimation of the error bar magnitude in the range of 10%-30%.

High density and high performance operation with pellet injection in W7-X

Abstract: S.A. Bozhenkov1, Y. Kazakov2, J. Baldzuhn1, H.P. Laqua1, J.A. Alonso3, M.N.A. Beurskens1, C. Brandt1, K.J. Brunner1, H. Damm1, G. Fuchert1, M. Hirsch1, U. Höfel1, M.W. Jakubowski1, J. Knauer1, G. Kocsis4, R. König1, A. Langenberg1, S. Lazerson5, N.B. Marushchenko1, K.J. McCarthy3, E. Pasch1, N. Pablant5, N. Panadero Alvarez3, K. Rahbarnia1, J. Schmitt6, H. Thomsen1, Y. Turkin1, F. Warmer1, G. Wurden7, D. Zhang1, T.S. Pedersen1, R.C. Wolf1 and W7-X team1

Overview of W7-X ECRH Results in OP1.2a

Abstract: The stellarator Wendelstein7-X was exclusively heated by ECRH in the operation phase OP1.2a. The ECRH system consists of 10 gyrotrons, with an output power of 0.6-1.0 MW each, a quasi optical transmission line and flexible microwave launcher inside the plasma vessel [1]. The over-all transmission efficiency was estimated to be ~ 94%. The ECRH system is commissioned for 1800 s operation at full power. It already demonstrated all requirements, which are necessary for a high-performance steady state operation at reactor relevant parameters at W7X. The 140 GHz ECRH uses the second harmonic resonance at 2.5T with an absorbed power of up to 7 MW. Besides the reliable plasma start-up und routine ECRH wall conditioning, stationary discharges up to 30 s have been achieved, which were only limited by the maximum test divertor energy load. Furthermore, the novel remote steering launcher, an important concept for the future fusion reactor, was tested for the first time for plasma heating and current drive. The long discharges were used to demonstrate current control and bootstrap current compensation by ECCD. Localized ECCD strongly changed the rotation transform iota and thus the confinement and stability. In particular by dedicated ECCD strong MHD activity with repetitive central temperature collapse could be driven, which in the worst case lead to a total loss of the plasma confinement. In combination with pellet injection (PI), highest performance with a plasma energy above 1 MJ has been achieved with the X2mode ECRH at a plasmas density of 0.8 1020 m-3 and an ion temperature of 3.8 keV, which demonstrates the already good collisional coupling between electrons and ions at that density. Even higher densities have been achieved using the O2-mode combined with PI as shown in Fig.1. Here highperformance plasmas at densities above the X2-cutoff and up to 1.4 1020 m-3 have been demonstrated, which is already close to the envisaged future steady state high-performance plasma scenario. Achieving efficient plasma heating with the O2 mode requires establishing a sufficiently dense and hot target plasma for the O2 mode to take over. This was accomplished using the X2-mode for the plasma start-up, and then during the initial two seconds of plasma operation the polarization was changed to O2 before the start of pellet fuelling. The O2-mode heating efficiency of > 90% was achieved with the help of individually shaped reflector tiles for the 10 ECRH beams enabling at least 3 passes through the EC-resonance in the plasma center. The reliable and efficient operation was guaranteed by a set of ECRH protective diagnostics, which were specially established for the ECRH requirements. In particular the ECRH stray radiation measurement turned out to be a reliable interlock signal to prohibit ECRH operation with insufficient plasma absorption. Furthermore the position and launch direction of each beam was measured by infrared cameras observing its thermal footprint at the heat shield tiles opposite of the antennas. Finally selected 140 GHz beams was also used as a source for the collective Thomson scattering (CTS), which was also commissioned [2]. Here a local ion temperature measurement was demonstrated (see Fig.2). For the next operational campaign op1.2b new ECRH scenarios have been prepared. In particular the operation at a reduced magnetic field strength of 1.75 T requires the use of the third harmonic xmode (X3). Here ray-tracing calculation predicted sufficiently high absorption for plasma build-up and sustain if the electron temperature is above 200 eV. The plasma start-up with X3 needs probably an assistance by another heating method, like X2 ECRH at 105 GHz or ICRH. It is also planed to test a new method to excite waves at the ion cyclotron (IC) frequency with ECRH. The ECRH power will be modulated with the IC-frequency using the beat wave of two crossing ECRH beams. For the conversion into IC-oscillation the magnetic field strength variation along the magnetic axis will be used. The electron perpendicular magnetic momentum M will be increased by the EC-interaction and the grad B∙M force will drive the oscillation.

Increasing the Density in W7-X: Benefits and Limitations

Abstract: In stellarators, increasing the density is beneficial for the energy confinement. While there is probably not one single reason for this observation, it is still very robust across different devices. This is, for example, reflected in the empirical energy confinement time scaling for stellarators, ISS04. In order to study whether this is also true for Wendelstein 7-X, the energy confinement time scaling for the first divertor experiments is analyzed and compared to ISS04. When the density is increased too much, however, radiative collapses are frequently observed. Existing analytical models for the critical density are revisited to assess whether they can predict the accessible density range. Furthermore, since close to the collapse the radiation losses increase substantially, the impact of the global energy confinement is investigated. It is found that energy confinement starts to become affected as soon as the radiation losses reach 50 % of the heating power. In the second half of the first divertor campaign, boronization has been applied to W7-X for the first time. This broadened the operational window, allowing for operation at higher density and, hence, higher stored energy.

Dual-laser wavelength Thomson scattering at Wendelstein 7-X.

Abstract: This paper presents the approach of the dual-laser wavelength Thomson scattering (TS) system for the Wendelstein 7-X stellarator. The dual-laser wavelength TS method is based on two lasers with different wavelengths being fired quasi-simultaneously. This method has two advantages compared to a single laser wavelength TS system. First, the dual laser availability allows an in situ spectral calibration, and second, higher electron temperatures can be measured without any change in the spectral filter setup of the polychromators. The W7-X dual-laser wavelength TS concept is based on high power lasers: a set of standard Nd:YAG lasers with λ = 1064 nm wavelength and a Nd:YAG laser with λ = 1319 nm wavelength newly developed for this application. This laser uses a different transition line with 34% efficiency compared to the main 1064 nm Nd:YAG line. Simulations of the expected performance of the new dual-laser wavelength system show that electron temperatures up to Te = 15 keV can be measured compared to the original design parameter up to Te = 10 keV. The in situ spectral calibration can be performed using a range of temperatures from 1 keV to 10 keV using TS measurements of the 1064 nm versus 1319 nm TS simultaneously.

Studies towards an upgraded 1.5 MW gyrotron for W7-X

Abstract: Studies towards a 1.5 MW, 140 GHz CW gyrotron, with the capability of MW-class operation also at 175 GHz, are ongoing at Karlsruhe Institute of Technology in view of a possible future upgrade of the ECRH system of the stellarator W7-X. The upgrade of the existing 1.0 MW, 140 GHz European gyrotron for W7-X has been chosen as a development path. Detailed designs of the cavity, the non-linear uptaper, and the quasi-optical launcher for the upgraded gyrotron have been obtained and have been validated numerically. In parallel, a mode generator, intended for low-power tests of the quasi-optical mode converter system of the upgraded gyrotron, has been designed, manufactured, and successfully tested.

Development of Faraday-cup-based Fast Ion Loss Detector in Wendelstein 7-X

Abstract: Study of fast-ion losses due to magnetic field ripples and fast-ion-driven magnetohydrodynamic (MHD) modes is an important research regarding fusion-born alpha losses [1]. In order to understand fast-ion losses in Wendelstein 7-X (W7-X) plasmas, installation of fast-ion loss diagnostic is planned [2]. For the upcoming OP1.2b campaign in 2018, a prototype Faradaycup-based fast-ion loss detector (FILD) has been designed as a joint cooperative project between National Institute for Fusion Science (NIFS) and Max Planck Institute for Plasma Physics. The Faraday-cup-based FILD, which is relatively cost-effective in construction compared with a scintillator-based FILD, has been installed to measure the fast ion loss flux in the Compact Helical System (CHS) [3, 4], the National Spherical Torus eXperiment (NSTX) [5], the Joint European Torus (JET) [6, 7], the Doublet III-D (DIII-D) [8], and the Heliotron-J [9, 10]. The FILD is capable of measuring the flux, the pitch angle (), and the energy (E) of escaping fast ions simultaneously, thus providing a clear understanding of fast-ion losses induced by MHD modes and non-axisymmetric magnetic field ripples. An increase of co-going fast-ion losses due to energetic particle modes (EPM) and toroidal Alfvén eigenmodes (TAE) was reported in CHS [4].

Plasma Termination by Excess Fuel and Impurities in TJ-II, LHD and W7-X

Abstract: Plasma terminating events due to excess pellet fueling and impurity injection is studied in LHD, W7 -X and TJ-II. T ime scales for these events range from values of a decimal fraction of typical energy confinement times to the order of the energy confinement time. The leading mechanism of energy loss appears to be radiation revealing similarities to radiation collapses when a radiative density limit is approached. Differently to tokamaks, the capability of helical devices to provide magnetic confinement in vacuum (i.e. without large inductive plasma currents) gives rise to the observation of plasma recovery even after some energy confinement times. T ime-scales and the dynamics of recovery for close to marginal termination indicates some robustness in the response of helical devices to large unintended perturbations.