Impurity Line Radiation Research Articles

A Nodal Model for Tokamak Burning Plasma Space-Time Dynamics

Fusion alpha heating introduces new phenomena into plasma dynamics and control. On the worrisome side is the well-known fact that the dependence of the predominantly central fusion heating mechanism, coupled with the less appreciated fact of the predominantly plasma edge location of bremsstrahlung and impurity line and recombination radiation cooling and of ion orbit loss cooling, suggests the possibility of a thermal runaway fusion power excursion in the plasma core. On the encouraging side is the fact that the fusion alpha energy is transferred first to heat the core electrons and produce electron cyclotron radiation that is transferred instantaneously, predominantly to outer plasma regions and the surrounding material wall, reducing its availability for further heating of core plasma ions. This paper discusses the temporal and spatial dependence of the various heating and cooling mechanisms involved in the burn dynamics of a fusion plasma, introduces a spatially coarse nodal space-time calculation model (suitable for dynamics and controller calculations) for the analysis of burning plasmas, and identifies the research needed to fully evaluate the parameters of such a model.

ITER breakdown and plasma initiation revisited

This paper revisits a number of key aspects of plasma initiation, aiming to clarify concepts, provide definitions and improve understanding, in view of ITER First Plasma operation. It shows that size matters and that breakdown and plasma initiation differ in larger devices. The large thick and conductive ITER vessel slows down the prefill process, the development of the toroidal electric field and affects the dynamics of the poloidal magnetic field at breakdown. The large vacuum vessel requires more than 1 s for the prefill gas to spread over the vessel. It slows down the development of the toroidal electric field, applied to ionize the prefill gas, by about 1 s and complicates the control of the magnetic configuration required for plasma initiation. It is shown that the avalanche process that provides the initial ionization slows down towards the end, if the cross-section of the toroidal discharge is larger. On the other hand, a larger plasma volume is beneficial for the subsequent burn-through of main-species ionization and impurity line-radiation. ITER First Plasma operation aims to reach a minimum plasma current of 100 kA, which will require a full avalanche and a partial burn-through to reach electron temperatures of at least 10–20 eV. The burn-through limits the ITER prefill pressure upper range of about 1 mPa. These predictions are based on 0D models of plasma initiation that furthermore assume the plasma volume remains constant. As burn-through is a critical process, models that consider radial profiles and/or volume dynamics, could provide further insight in the constraints of ITER plasma initiation. Electron cyclotron heating (ECH) high-power microwaves can be used to assist the ITER plasma initiation process. Although the ECH burn-through assist is reasonably well understood, no model exist that can predict the behaviour of ECH pre-ionization on ITER plasma initiation. ECH burn-through assist is shown to be ineffective for ITER First Plasma operations. Low prefill pressures are thought to increase the likelihood to develop a highly energetic supra-thermal electron discharge during the plasma initiation process. Considering supra-thermal electrons could affect the assumptions for the plasma resistance or even the ionization rate coefficient in plasma initiation models, hence altering the expected dynamics. Furthermore, only a qualitative picture of the processes that cause such supra-thermal discharges exists and a dedicated model is needed to improve ITER predictions.

Novel approach of pulsed-glow discharge wall conditioning in the ADITYA Upgrade tokamak

In the ADITYA Upgrade tokamak, glow discharge wall conditioning (GDC) is performed regularly during the high-temperature plasma operation cycle using hydrogen (H) and helium (He) gases. H GDC is carried out after long durations (few hours) of plasma operations on every plasma operation day in automatic mode to control oxygen (O) and carbon (C) containing impurities. This leads to high retention of H gas on graphite limiter plates and stainless steel (SS) vessel walls. Subsequently, the high outgassing of H requires a prolonged pumping time and high H recycling during plasma discharges affects the plasma performance in respect to H fueling control of the plasma. To overcome the above-mentioned issues with continuous H GDC for longer durations, a new approach involving pulsed glow discharge wall conditioning (P-GDC) has been introduced in the ADITYA-U tokamak to reduce the residual H and He concentration in SS vessel walls and graphite limiter plates. To facilitate the fast initiation of a discharge in the case of P-GDC, a source of free electrons from a hot filament has been introduced in the vessel. A fast feedback controlled pulsed-gas-fueling system has been developed to initiate a glow discharge in each gas-feed pulse at various operating pressures from 1 × 10−4 Torr to 10−3 Torr in the presence of an applied DC voltage. The different P-GDC experiments have been carried out with H, He and argon as the working gases and the results are compared with traditional continuous GDC. The P-GDC experiments have been optimized to provide beneficial wall conditioning for plasma operations. In this paper, the design, development and operation of P-GDC has been described along with the preliminary studies of its effect on the measured impurity line radiation during a plasma discharge.

Investigation of Zeff and impurity behaviour in lithium coating experiments with full metallic first wall in HT-7 tokamak

The control of the impurity level in magnetically confined plasmas is a critical issue for future fusion devices. All the graphite tiles have been replaced by molybdenum tiles as limiter materials in the 2011 spring campaign in order to further reduce the recycling and hydrogen content of the plasma. A lithium coating technique has been applied as an important wall conditioning method to the HT-7 tokamak. The effective ion charge Zeff and impurity behavior with full metallic first walls of high-Z materials and lower hydrogen recycling have been investigated in a series of lithium coating experiments in this paper. Plasma performance and impurity behavior without wall coatings are studied in the early stage of the campaign. Comparison of Zeff with different plasma-facing components has been made. A typical lithium coating experiment has been analyzed in order to understand the effect of lithium coating. The evolution of main impurity line radiation, Zeff and the H/(H + D) ratio is analyzed in detail as lithium coating is repeated, indicating that lithium coating is a very effective tool to control impurity level and reduce hydrogen recycling. Furthermore, a boronization is conducted at the end of this campaign in order to make comparison with lithium coating. Experimental results show that lithium coating has much more advantages in edge recycling control, though it does not reduce impurity level as effectively as boronization.

A reconsideration of the noise equivalent power and the data analysis procedure for the infrared imaging video bolometers.

The infrared imaging video bolometer (IRVB) used for measurement of the two-dimensional (2D) radiation profiles from the Large Helical Device has been significantly upgraded recently to improve its signal to noise ratio, sensitivity, and calibration, which ultimately provides quantitative measurements of the radiation from the plasma. The reliability of the quantified data needs to be established by various checks. The noise estimates also need to be revised and more realistic values need to be established. It is shown that the 2D heat diffusion equation can be used for estimating the power falling on the IRVB foil, even with a significant amount of spatial variation in the thermal diffusivity across the area of the platinum foil found experimentally during foil calibration. The equation for the noise equivalent power density (NEPD) is re-derived to include the errors in the measurement of the thermophysical and the optical properties of the IRVB foil. The theoretical value estimated using this newly derived equation matches closely, within 5.5%, with the mean experimental value. The change in the contribution of each error term of the NEPD equation with rising foil temperature is also studied and the blackbody term is found to dominate the other terms at elevated operating temperatures. The IRVB foil is also sensitive to the charge exchange (CX) neutrals escaping from the plasma. The CX neutral contribution is estimated to be marginally higher than the noise equivalent power (NEP) of the IRVB. It is also established that the radiation measured by the IRVB originates from the impurity line radiation from the plasma and not from the heated divertor tiles. The change in the power density due to noise reduction measures such as data smoothing and averaging is found to be comparable to the IRVB NEPD. The precautions that need to be considered during background subtraction are also discussed with experimental illustrations. Finally, the analysis algorithm with all the improvements is validated and found to reproduce the input power well within 10% accuracy. This article answers many fundamental questions relevant to the IRVB and illustrates the care to be exercised while processing the IRVB data.

Installation of soft X-ray array diagnostics and its application to tomography reconstruction using synthetic KSTAR X-ray images.

Four-array system of soft X-ray diagnostics was installed on KSTAR tokamak. Each array has 32 viewing chords of two photo-diode array detectors with spatial resolution of 2 cm. To estimate signals from the soft X-ray radiation power, typical ne, Te, and argon impurity line radiation profiles in KSTAR are chosen. The photo-diodes were absolutely calibrated as a function of the incident photon energy in 2-40 keV range with a portable X-ray tube. Two-dimensional Te image properties by multi-energy method were simulated and visualized with six combinations of beryllium filter sets within the dynamic range of signal ratio.

Design and initial operation of a two-color soft x-ray camera system on the Compact Toroidal Hybrid experiment.

A multi-camera soft x-ray diagnostic has been developed to measure the equilibrium electron temperature profile and temperature fluctuations due to magnetohydrodynamic activity on the Compact Toroidal Hybrid experiment. The diagnostic consists of three separate cameras each employing two 20-channel diode arrays that view the same plasma region through different beryllium filter thicknesses of 1.8 μm and 3.0 μm allowing electron temperature measurements between 50 eV and 200 eV. The Compact Toroidal Hybrid is a five-field period current-carrying stellarator, in which the presence of plasma current strongly modifies the rotational transform and degree of asymmetry of the equilibrium. Details of the soft x-ray emission, effects of plasma asymmetry, and impurity line radiation on the design and measurement of the two-color diagnostic are discussed. Preliminary estimates of the temperature perturbation due to sawtooth oscillations observed in these hybrid discharges are given.

Silicon drift detector based X-ray spectroscopy diagnostic system for the study of non-thermal electrons at Aditya tokamak

Silicon drift detector based X-ray spectrometer diagnostic was developed to study the non-thermal electron for Aditya tokamak plasma. The diagnostic was mounted on a radial mid plane port at the Aditya. The objective of diagnostic includes the estimation of the non-thermal electron temperature for the ohmically heated plasma. Bi-Maxwellian plasma model was adopted for the temperature estimation. Along with that the study of high Z impurity line radiation from the ECR pre-ionization experiments was also aimed. The performance and first experimental results from the new X-ray spectrometer system are presented.

An ion Doppler spectrometer instrument for ion temperature and flow measurements on SSPX

A high-resolution ion Doppler spectrometer (IDS) has been installed on the sustained spheromak plasma experiment to measure ion temperatures and plasma flow. The system is composed of a 1 m focal length Czerny-Turner spectrometer with a diffraction grating line density of 2400 lines/mm, which allows for first order spectra between 300 and 600 nm. A 16-channel photomultiplier tube detection assembly combined with output coupling optics provides a spectral resolution of 0.0126 nm/channel. We calculate in some detail the mapping of curved slit images onto the linear detector array elements. This is important in determining the wavelength resolution and setting the optimum vertical extent of the slit. Also, because of the small wavelength window of the IDS, a miniature fiber-optic survey spectrometer sensitive to a wavelength range 200-1100 nm and having a resolution of 0.2 nm is used to obtain a time-integrated spectrum for each shot to verify specific impurity line radiation. Several measurements validate the systems operation. Doppler broadening of C III 464.72 nm line in the plasma shows time-resolved ion temperatures up to 250 eV for hydrogen discharges, which is consistent with neutral particle energy analyzer measurements. Flow measurements show a sub-Alfvenic plasma flow ranging from 5 to 45 kms for helium discharges.

Physics of island divertors as highlighted by the example of W7-AS

Based on theoretical analysis, numerical simulations and experimental results, the paper outlines a self-consistent physics picture of the island divertor transport in W7-AS, as it emerges from the present understanding, documented over the past several years of theoretical and experimental research on the subject. Key function elements of a divertor, such as particle flux enhancement, neutral screening, impurity retention, thermal power removal via impurity line radiation and detachment, are examined for the island divertor and assessed with respect to tokamak divertors. The paper focuses on describing the global scrape-off layer (SOL) transport behaviour associated with the specific island topology and aims at illustrating the elementary differences and similarities in divertor physics between a tokamak and a typical helical device. Shown and analysed are also the correlation between the SOL and core plasma and the role of the island divertor for improving the global plasma performance. Discussion is mainly based on simple models and estimations, while three-dimensional modelling calculations serve only for control of self-consistency and for determining basic functional dependences not accessible otherwise. The island divertor physics is presented within a theoretical frame with most key issues, however, being related to experimental results.

Modelling of Ohmic discharges in ADITYA tokamak using the Tokamak Simulation Code

Several Ohmic discharges of the ADITYA tokamak are simulated using the Tokamak Simulation Code (TSC), similar to that done earlier for the TFTR tokamak. Unlike TFTR, the dominant radiation process in ADITYA is through impurity line radiation. TSC can follow the experimental plasma current and position to very good accuracy. The thermal transport model of TSC including impurity line radiation gives a good match of the simulated results with experimental data for the Ohmic flux consumption, electron temperature and Zeff. Even the simulated magnetic probe signals are in reasonably good agreement with the experimental values.

Measurement and modelling of neon radiation profiles in radiating boundary discharges in ASDEX Upgrade

The radiation and transport characteristics of ASDEX Upgrade discharges with a neon-driven radiative mantle are modelled using a one-dimensional radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The code is well suited to describing the measured impurity line radiation, total, soft x-ray and bremsstrahlung radiation in regions of the plasma that are not dominated by two-dimensional effects. The recycling and pumping behaviour of neon as well as the bulk transport of neon for radiative boundary scenarios are discussed.

Collisional-radiative modeling of the L-shell emission of Mo30+ to Mo33+ emitted from a high-temperature-low-density tokamak plasma.

The x-ray spectra of several highly stripped molybdenum ions have been recorded between 0.6 and 5.5 A in the Frascati tokamak upgrade with a rotating crystal spectrometer. Detailed, quasi-steady-state collisional-radiative models have been used to interpret emission features from inner shell, electron impact excitations in molybdenum ions near the neonlike charge state and to characterize the charge state distribution in the plasma. Processes such as resonant excitation, excitation autoionization, and dielectronic recombination have been included in the models of the molybdenum ions{close_quote} emission features. Introducing the excitation-autoionization process into ionization equilibrium calculations brings agreement between observations and calculations of the relative ionization equilibrium fractions of highly stripped molybdenum ions. Absolutely calibrated spectra and detailed models for the excitation processes in these molybdenum ions allow us to calculate crucial plasma parameters, such as the concentration of impurity ions in the plasma and the amount of power lost from the plasma through impurity line radiation. {copyright} {ital 1996 The American Physical Society.}

Effects of particle fueling and plasma wall interactions on DIII-D discharges

DIII-D has successfully operated with an all-graphite first wall, including the first observations of VH-mode without boronization. A major goal of this, and other recent upgrades, was to control impurity influxes and hydrogenic fueling. Graphite tiles were carefully preconditioned, first by ex situ preparation and then by baking and helium glow conditioning. No deuterium or hydrogen was used until tokamak operation commenced. With the all graphite wall, both impurity and deuterium influxes during tokamak discharges were lower than previous boronized discharges; central nickel impurity line radiation, NiXXV and NiXXVI, was an order of magnitude lower than previous discharges during the ELM free beam heated phase. The effect of reduced particle fueling on plasma performance, particularly H- and VH-mode discharges, will be presented.

Energy balance in the CSSU device

An analysis of the energy balance in the Coaxial Slow Source Upgrade (CSSU) device is reported. The CSSU consists of two concentric coils carrying pulsed azimuthal currents only, which form an elongated plasma (an 'annular field reversed configuration (FRC)') in the space between the coils. The plasma contains no toroidal field, and is confined by poloidal fields only, resulting in a very high average beta. The CSSU, which operates at loop voltages of 2 kV or less and with risetimes of the order of 70 mu s, was developed to provide a low voltage, slow formation alternative to conventional FRC generation techniques that are based on fast theta pinch technology. It is found that the CSSU device does form annular FRCs, which persist for the duration of the inductive current drive, apparently free of MHD instability. Temperatures are low, however, and the transport is correspondingly poor. To analyse the energy balance, the power input to the plasma is calculated directly from external and internal magnetic field measurements. No assumptions about the resistivity profile have been made. A triple Langmuir probe located at the device end region was used to calculate the energy lost due to escaping particles. Electron temperature measurements from Thomson scattering and impurity estimates from doping studies are used in a time dependent corona model calculation to show that the CSSU plasma is impurity line radiation dominated. Time dependent coronal calculation simply that, with operation over much longer formation times (>100 to 200 mu s) at lower density (1014 cm-3), it may be possible to burn through the carbon and oxygen impurity radiation barriers and attain plasma conditions closer to those produced in conventional FRCs

Plasma heating and dynamics in the Coaxial Slow Source

Simple semi-analytical models are presented to calculate the temporal evolution of the plasma temperature and length or thickness in the Coaxial Slow Source (Nucl. Fusion 27 (1987) 1478) for both tearing formation and programmed formation. It is assumed that energy is delivered to the plasma via Ohmic heating and compressional work and is lost through impurity line radiation. The plasma is considered to be always fully ionized and in pressure balance; the external magnetic field is taken to be a known function of time, and particle losses are neglected. In tearing formation, a long and thin plasma sheet is initially formed. This can be studied using a 1-D model; it is shown that the higher the external field and the smaller the line integrated density, the faster the temperature increases. In programmed formation, an axial equilibrium is quickly established and a 2-D model is required. It is shown that when the external magnetic field exceeds a critical value, which depends on the temperature, the impurity fraction and the effective plasma thickness, the radiated power overcomes Ohmic heating and a radiative collapse occurs. Various time histories of the external magnetic field are analysed in order to determine the conditions that result in the fastest increase in plasma temperature

Experimental investigation of magnetic compression of a spheromak plasma

Magnetic compression of S-1 spheromak plasmas [Nucl. Fusion 22, 1407 (1982)] was performed to extend the investigation of confinement properties of the spheromak to plasmas with larger current density and higher plasma pressure than obtained before. Evolution of the magnetic configuration was mapped using internal-probe-based flux plots, and it was found that the plasma compression was approximately self-similar. The Taylor equilibrium state was maintained without flux conversion or relaxation phenomena. With a compression factor of up to 1.6, a significant increase of plasma pressure was observed. The peak electron temperature rose from approximately 40 eV before compression to about 100 eV, and the peak density increased somewhat less than the adiabaticity based scaling, which suggests the presence of an enhanced particle loss mechanism. In general, the electron’s beta value was approximately constant during the compression; that is, ne0Te0∝B20. This result is consistent with the S-1 scaling obtained earlier without compression. The ion temperature measured by Doppler broadening of low Z impurity line radiation was observed to rise as high as 0.5 keV.

Carbonization of the DIII-D tokamak

The DIII-D tokamak has been carbonized by the application of a carbon film covering all plasma facing surfaces. Carbonization was done in order to reduce the metal impurity influx and central metal accumulation especially during beam heated D 0 → D + H-mode discharges. After carbonization, nickel impurity line radiation was reduced by a factor of 10 during the ohmic phase of the discharge and up to a factor of 30 during the H-mode phase. The reduction of metal impurities also produced a reduction of total radiated power and allowed high current operation. After carbonization, the highest plasma current in a double null divertor, and the highest stored energy ever achieved on DIII-D were observed, 3 MA and 3.6 MJ respectively. A reactor relevant toroidal beta, β T = 5.1%, at full field, 2.1 T, was obtained. On the first day after carbonization, H-mode density profiles were more peaked than pre-carbonization discharges.

Observation of soft x-ray radiation from Heliotron E plasmas by the absorption method for the measurement of electron temperatures

An absorption method of soft x ray is applied to Heliotron E plasmas for measurement of the electron temperature. Nitrogen gas is used as an absorber for convenience, owing to its accurate, uniform, and easily controllable density. The general feature of the absorption method for measurement of the electron temperature is discussed using a model with two parameters: the generalized thickness of the absorber and the electron temperature. The energy resolution of this method is not sufficient as a general method for spectral analysis. Hence, it is necessary to assume in advance such a model spectrum as consists of bremsstrahlung, recombination radiation, and impurity line radiation. Since the spectrum is always assumed before the analysis, we should try to find the origins of deformation of the energy spectrum and to correct the contribution. The effect of line emission from impurity ions to the estimated electron temperature is evaluated as a function of the electron temperature and the energy of the line relative to the generalized absorber thickness used in the measurement. An actual spectrum is measured by a pulse-height analysis (PHA) of the soft x ray. The one clear line, from chlorine, is not significant in the present determination of the electron temperature by the absorption method. Another possible line from iron at energy less than 1 keV is included in the analysis. Using a convenient method for determination of local emissivity from a chord-integrated emissivity, the electron temperature is determined from the local emissivity. The observed broad electron-temperature profile might be an artifact due to recombination radiation of the highly ionized ion diffused out of the hot core of the plasma. It is confirmed that the absorption method gives absolute measurement of the electron temperature at the plasma center, when additional information on impurity lines are given by PHA. The estimated temperature is consistent with the PHA measurement. The absorption method combined with PHA provides an easy and inexpensive measurement of the electron temperature compared with a system fully constructed by PHA.

Light impurity production in tokamaks

A short summary is given on the different erosion processes of carbon materials with special emphasis on conditions relevant to plasma surface interactions. New results on the chemical erosion and radiation enhanced sublimation of boron-carbon layers are presented. The chemical hydrocarbon formation produced by the interaction of the TEXTOR scrape-off plasma with a carbon target has been investigated up to temperatures of 1500K using a Sniffer probe. The chemical interaction of the plasma with the carbon walls in TEXTOR is also analysed by measuring the hydrocarbon and CO and CO2 partial pressures built up on the surrounding walls during the discharges. The recycling of oxygen impurities in an all carbon surrounding occurs predominantly in form of CO and CO2 molecules and the analysis of both neutral pressures during the discharges has been used as an additional diagnostic for the oxygen impurity situation in TEXTOR. These data are discussed in view of spectroscopic measurements on the influx of carbon and oxygen atoms from the walls and impurity line radiation. CD-band spectroscopy in addition is employed to identify the hydrocarbon chemical carbon erosion. Our present understanding of the oxygen impurity recycling and the oxygen sources are described. Particle induced release of CO molecules from the entire first wall is believed to be the dominant influx process of oxygen in the SOL of plasmas with carbon facing materials. The influence of coating the TEXTOR first wall with a boron-carbon film (B/C approximately=1) on the light impurity behaviour is shown.