Deuterium Discharges Research Articles

Development on JET of advanced tokamak operations for ITER

Recent research on advanced tokamak in JET has focused on scenarios with both monotonic and reversed shear q-profiles having plasma parameters as relevant as possible for extrapolation to ITER. Wide internal transport barriers (ITBs), r/a ∼ 0.7, are formed at ITER relevant triangularity δ ∼ 0.45 and moderate plasma current, IP = 1.5–2.5 MA, with ne/nG ∼ 60% when ELMs are moderated by Ne injection. At higher current (IP ≤ 3.5 MA, δ ∼ 0.25) wide ITBs sitting at r/a ≥ 0.5, in the positive shear region, have been developed. Generally MHD events terminate these barriers otherwise limited in strength by power availability. ITBs with core density close to Greenwald value, Te ∼ Ti and low toroidal rotation (4 times lower than standard ITBs) are obtained in plasma target preformed by opportune timing of lower hybrid current drive (LHCD), pellet injection and a small amount of NBI power. Wide ITBs, r/a ∼ 0.6, of moderate strength, can be sustained without impurities accumulation for a time close to neoclassical resistive time in 3 T/1.8 MA discharges that exhibit reversed magnetic shear profiles and type-III ELMy edge. These discharges have been extended to the maximum duration allowed by JET subsystems (20 s) bringing to the record of injected energy in a JET discharge: E ∼ 330 MJ. Portability of ITB physics has been addressed through dedicated similarity experiments. The ITB is identified as a layer of reduced diffusivity studying the propagation of the heat wave generated by modulating the ICRF mode conversion (MC) electron heating. Impressive results, QDT ∼ 0.25, are obtained in these deuterium discharges with 3He minority when the MC layer is located in the core. The ion behaviour has been investigated in pure LHCD electron ITBs optimizing the 3He minority concentration for direct ion heating. Preliminary results of particle transport, studied via injection of a trace of tritium and an Ar–Ne mixture, will be presented.

Diagnostics of the cesium amount in an RF negative ion source and the correlation with the extracted current density

The formation of negative ions in plasma sources by the surface process requires covering the extraction grid with a material of low work function. This can be achieved by cesium evaporation but for operational reasons the consumption of cesium should be minimised. In order to quantify the amount of cesium in the RF discharge optical emission spectroscopy is used as diagnostic tool. Suitable diagnostic lines and their analysis to obtain particle densities and particle fluxes are described. Influences of argon admixtures to hydrogen and deuterium discharges on cesium emission are shown. A correlation of the cesium emission with the extracted negative ion current density is discussed.

Release Behavior of Hydrogen Isotopes from JT-60U Graphite Tiles

Release behavior of hydrogen isotopes from the graphite tiles used in JT-60U was observed using the thermal desorption method where temperature was stepwise elevated to 300, 600 and 1000 °C. When first wall tile was left under helium atmosphere at 600 °C for 8 hours, about 40 % of total amount of hydrogen and deuterium retained in the tile was released, although only a small amount of hydrogen isotopes was released at 300 °C, which is the base temperature of inner wall of JT-60U. This indicates that a higher temperature of inner wall causes hydrogen retention to reduce considerably. When the graphite tiles were exposed to hydrogen at 1000 °C, the release of deuterium and tritium was enhanced. It is considered that the deuterium and tritium left in the graphite tile was released by the isotope exchange reaction. In order to remove almost all deuterium or tritium from the graphite tile without combustion of graphite, isotope exchange method at high temperature is effective. It was found that the amount of hydrogen retained in the graphite tile was much larger than that of deuterium. This indicates that a large amount of deuterium trapped in the tiles during deuterium discharge experiments was replaced with hydrogen during hydrogen discharge experiments. Additionally, depth profiles of hydrogen isotope are discussed from the obtained release curves.

Characterization of the H-mode edge barrier at ASDEX Upgrade

The scaling of the edge transport barrier (ETB) that sustains H-mode plasmas is crucial for the performance of next step tokamaks. At ASDEX Upgrade, the suite of edge diagnostics has been significantly improved to address this issue. High spatial resolution profiles of most of the key edge plasma parameters necessary to determine the magneto hydrodynamic (MHD) stability are now available. New high temporal resolution measurements give clear indications of the nonlinear evolution of the ELM crash. The correlation lengths of edge turbulence have been shown to be correlated with the edge radial electric field shear using a new correlation Doppler reflectometer system. The measured pressure gradient in the ETB is found to be consistent with ideal MHD stability limits, both for Type I and II ELMs. In addition, the edge electron temperature and density gradient lengths are found to be strongly correlated, leaving only the ETB width as a free parameter. In ASDEX Upgrade, the ETB width does not vary significantly over the entire H-mode edge database. Modelling of the transport of comparison discharges in hydrogen and deuterium shows that the expected mass effect on neutral penetration is largely compensated by more efficient heating of deuterium neutrals but requires a transport barrier in both the energy and particle channels in order to reproduce the measured edge temperature and density profiles.

DD reaction enhancement and X-ray generation in a high-current pulsed glow discharge in deuterium with titanium cathode at 0.8–2.45 kV

The DD reaction yield (3-MeV protons) and the soft X-ray emission from a titanium (Ti) cathode surface in a periodic pulsed glow discharge in deuterium were studied at a discharge voltage of 0.8–2.45 kV and a discharge current density of 300–600 mA/cm2. The electron screening potential Ue = 610 ± 150 eV was estimated in the range of deuteron energies 0.8 keV < Ed < 2.45 keV from an analysis of the DD reaction yield as a function of the accelerating voltage. The obtained data show evidence for a significant enhancement of the DD reaction yield in Ti in comparison to both theoretical estimates (based on the extrapolation of the known DD reaction cross section for Ed ≥ 5 keV to low deuteron energies in the Bosch-Halle approximation) and the results of experiments using accelerators at the deuteron energies Elab ≥ 2.5 keV and current densities 50–500 μ A/cm2. Intense emission of soft X-ray quanta (1013–1014 s−1 cm−2) was observed at an average energy of 1.2–1.5 keV. The X-ray emission intensity and the DD reaction yield enhancement strongly depend on the rate of deuterium diffusion in a thin subsurface layer of Ti cathode.

Masterless Soft Lithography: Patterning UV/Ozone−Induced Adhesion on Poly(dimethylsiloxane) Surfaces

A novel microreactor-based photomask capable of effecting high resolution, large area patterning of UV/ozone (UVO) treatments of poly(dimethylsiloxane) (PDMS) surfaces is described. This tool forms the basis of two new soft lithographic patterning techniques that significantly extend the design rules of decal transfer lithography (DTL). The first technique, photodefined cohesive mechanical failure, fuses the design rules of photolithography with the contact-based adhesive transfer of PDMS in DTL. In a second powerful variation, the UVO masks described in this work enable a masterless soft lithographic patterning process. This latter method, UVO-patterned adhesive transfer, allows the direct transfer of PDMS-based polymer microstructures from a slab of polymer to silicon and other material surfaces. Both methods exploit the improved process qualities that result from the use of a deuterium discharge lamp to affect the UVO treatment to pattern complex, large area PDMS patterns with limiting feature sizes extending well below 1 microm (> or = 0.3 microm). The use of these structures as resists is demonstrated for the patterning of metal thin films. A time-of-flight secondary ion mass spectroscopy study of the process provides new insights into the mechanisms that contribute to the chemistry responsible for the interfacial adhesion of DTL transfers.

Comparison of deuterium and hydrogen experiments in the Sustained Spheromak Physics Experiment

In this paper we report on the results of deuterium and hydrogen experiments in the Sustained Spheromak Physics Experiment (SSPX). We have compared ∼500 deuterium discharges with similar discharges in hydrogen. Typically, we produce H2 plasmas with peak toroidal currents in the range of 0.6MA, electron temperatures (Te) of ∼200eV and energy confinement times (τE) of ∼200μs. The D2 fueled discharges show similar results to those with H2 fueling with no obvious differences in confinement time. Electron temperatures of ∼200 eV with similar electron densities were observed. Both the deuterium and hydrogen fueled discharges have a calculated thermal diffusivity below χe<10m2/s. However, the D2 fueled discharges had a factor of ∼5 increase in high-Z (titanium) impurity content suggesting an increase of physical sputtering. We find no significant mass scaling effects.

Neutral Beam Injection System for the SHIP Experiment

The injector ion source is based on an arcdischarge plasma box. The plasma emitter is produced by a 1 kA arc discharge in deuterium. A multipole magnetic field produced with permanent magnets at the periphery of the plasma box is used to increase its efficiency and improve homogeneity of the plasma emitter. The ion beam is extracted by a 4-electrodes ion optical system (IOS). Initial beam diameter is 200 mm. The grids of the IOS have a spherical curvature for geometrical focusing of the beam. The optimal IOS geometry and grid potentials were found by means of numerical simulation to provide precise beam formation. The measured angular divergence of the beam is 0.025 rad, which corresponds to a 4.7 cm Gaussian radius of the beam profile measured at focal point.

Kinetic neutral transport effects in the pedestal of H-mode discharges in the DIII-D tokamak

A series of hydrogen and deuterium discharges are analyzed with fluid plasma and Monte Carlo neutrals codes. Comparison of poloidally averaged radial distributions of core neutral density and ionization with analytic solutions of 1-D plasma and neutrals continuity equations support the hypothesis that the width of the density pedestal is largely determined by the neutral source. The increased neutral penetration depth that arises from multiple charge exchange can be included in the analytic model with radially dependent scale lengths. The scale length in the analytic model depends on the neutral fluid velocity which increases across the divertor and pedestal as the neutral atoms charge exchange with the higher temperature background ions. The neutral penetration depth and corresponding density pedestal width depend sensitively on the neutral temperature and the degree of ion-neutral temperature equilibration.

Toroidal rotation in DIII-D in electron cyclotron heating and Ohmic H-mode discharges

Spatially and temporally resolved toroidal rotation measurements have been made in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] discharges with no externally applied torque. The velocity measurements are made using the charge exchange recombination (CER) technique viewing emission from the intrinsic carbon impurity in deuterium discharges. Three cases have been studied: L mode and H mode with Ohmic heating and H mode with electron cyclotron heating (ECH). The ECH H mode has carbon counter-rotation in the center of the plasma, and co-rotation outside, where co- and counter- are relative to the direction of the toroidal plasma current. The Ohmic H mode has carbon rotation everywhere in the co-direction. Neoclassical theory is applied to compute the deuterium toroidal velocity and it is found that the counter-rotation measured for carbon in the core of the ECH H mode is also thus predicted for the bulk deuterium species. Short blips of neutral beams (NB) must be used for the CER technique and these blips do apply a toroidal torque. Care is taken to verify that a nonperturbative measurement is made; data from the first 2 ms of NB injection in each discharge are used for this measurement.

Study of anomalous inward drift in tokamaks by transport analysis and simulations

The origin of the anomalous inward drift is explored by transport analysis of Ohmic, L- and H-mode discharges in ASDEX Upgrade using a special version of the 1.5-D BALDUR transport code. It is shown that the anomalous particle pinch significantly affects the density profile, in contrast to the Ware pinch. The measured density profiles can be modelled by the anomalous inward drift velocity with Cv equal to 0.2 for H-mode and 1.1 for Ohmic plasmas and by a strongly rising vin/D near the edge. Here, x = ρ/ρw and xs = ρs/ρw with effective radii ρ, ρw and ρs of flux surface, wall contour and separatrix contour, respectively. At low densities, beam fuelling alone yields peaked density profiles. With increasing density the beam fuelling is shifted to the edge which causes the observed density flattening. Evaluation of measured electron density and temperature profiles in deuterium and hydrogen discharges with various heating schemes yields with the electron temperature gradient length. A semi-empirical scaling is set up and validated against ASDEX Upgrade, DIII-D, JET and ASDEX discharges. It is shown to work in the core and edge regions of Ohmic, L- and H-mode plasmas. The ratio vin/D is independent of density, plasma current, toroidal magnetic field, hydrogenic atomic mass number, collisionality and Zeff. The anomalous inward flux is driven by the square of the electron temperature gradient. Simulations of ITER using the new vin scaling predict peaked density profiles for gas puffed scenarios because of central heating due to alpha particles.

Plasma Modelling for the PSI Linear Plasma Device

AbstractDeuterium discharges in the PSI device have been modelled using the coupled package of the B2 hydrodynamic plasma code and the Eirene Monte‐Carlo neutral code. Radial and axial plasma profiles have been calculated for different magnetic field configurations, various radial diffusion laws and for different values of the flux limiter in the parallel electron heat conduction law used in the B2 code. The results are compared with experimental findings. The axial variation of the magnetic field strength is found to have an important influence on the plasma state via the axial plasma flow which closely resembles the neutral gas streaming through a series of laval nozzles. For particular magnetic field configurations, an appropriate ansatz for the parallel electron heat conduction turns out to be a crucial point for the applicability of hydrodynamic models to linear devices like the plasma generator PSI. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Analysis of Langmuir probe data in high density plasmas

Analysis of Langmuir probe data by using the parametrization of Laframboise's numerical results was performed to characterize high density plasmas (argon or deuterium discharges) in terms of plasma parameters such as plasma density and electron temperature. The use of parameterizing Laframboise's results was found to readily extract the plasma parameters for arbitrary ratios of probe radius to Debye length in high density plasmas. It was observed that the electron temperature increased with decreasing gas pressure, and was nearly constant with power in both argon and deuterium plasmas. The plasma density increased with both power and pressure in both argon and deuterium plasmas. Over the power and pressure ranges used in this work, the plasma density in deuterium plasmas was found to be an order of magnitude lower than that in argon plasmas. A simulation study showed good agreement of predicted electron temperature and plasma density with experimental results for argon plasmas.

Effects of ECRH power and safety factor on laser blow-off injected impurity confinement in TCV

Evidence from injection into the TCV device of laser ablated, non-recycling silicon impurities shows that the transport of impurities confinement can be remarkably different from that of energy. The ratio of impurity to energy confinement times ranges from near unity in Ohmic discharges to 5 in the presence of high power ECCD. In Ohmic discharges in deuterium, above a threshold of density and of safety factor near q95 = 4.5, the impurity confinement time increases abruptly by a factor of 2 and is sometimes accompanied by indefinite retention of non-recycling impurities within the sawtooth mixing radius.

Tritiated Dust Levitation by Beta-Induced Static Charge

Tritiated particles have been observed to spontaneously levitate under the influence of a static electric field. Tritium-containing codeposits were mechanically scraped from tiles that had been used in the Tokamak Fusion Test Reactor (TFTR) inner limiter during the deuterium-tritium campaign and were placed in a glass vial. On rubbing the plastic cap of the vial, a remarkable “fountain” of particles was seen inside the vial. Particles from an unused tile or from a TFTR codeposit that formed during deuterium discharges did not exhibit this phenomenon. It appears that tritiated particles are more mobile than other particles, and this should be considered in assessing tokamak accident scenarios and in occupational safety.

Structure of electron and ion beams generated by high-voltage nanosecond discharge in deuterium and air

Structure of electron and ion beams generated by high-voltage nanosecond discharge in deuterium and air

Power deposition measurements in deuterium and helium discharges in JET MKIIGB divertor by IR-thermography

One of the outstanding problems for a next step fusion device is the handling of the power fluxes into the divertor, in particular regarding fast transient heat deposition by edge localised modes (ELMs). For a next step fusion device, such as ITER, type-I ELMy H-Mode is the reference discharge and therefore of particular interest. At JET a thermography system with high time resolution is able to resolve temperature evolution during and between ELM periods; here, the analysis is focussed so far on type-I ELMy H-Mode discharges with ELM frequencies of 10–35 Hz. Influences from surface layers on the target tiles are investigated in special discharges and taken into account. Results about the distribution of ELM and inter-ELM power deposition on the inner and outer divertor target plates are presented. Additionally, the characteristic ELM power deposition time (or temperature rise time) and its dependence on pedestal parameters are studied. The thermal impact due to ELMs for ITER is calculated together with predictions for relative ELM midplane losses and heat flux evolution on the divertor target. The predicted values based on conservative assumptions exceed the material limits.

A plasma focus driven by a capacitor bank of tens of joules

As a first step in the design of a repetitive pulsed neutron generator, a very small plasma-focus device has been designed and constructed. The system operates at low energy (160 nF capacitor bank, 65 nH, 20–40 kV, and ∼32–128 J). The design of the electrode was assisted by a computer model of Mather plasma focus. A single-frame image converter camera (5 ns exposure) was used to obtain plasma images in the visible range. The umbrellalike current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher. The calculations indicate that yields of 104–105 neutrons per shot are expected with discharges in deuterium.

Particle recirculation studies in JET

A global particle balance analysis has been applied to a variety of L-mode and H-mode deuterium discharges of the C4 campaign of JET, performed at a lower wall temperature, in different magnetic configurations and with various divertor gas puffing location. Wall retention and divertor particle exhaust are discussed as a function of the plasma configuration, the wall temperature and the gas puffing location. The effect on ELMs frequency is also mentioned. A simple empirical law linking the divertor pressure and the Dα signal integrated on the divertor is derived, which allows one to estimate the inner and outer contribution to pumping.

A gamma-ray spectrometer system for fusion applications

A NaI scintillator spectrometer system for the measurement of gamma-ray spectra in tokamak discharges has been developed and installed on the Frascati Tokamak Upgrade. Two NaI scintillators are viewing the plasma at two different angles with respect to the equatorial plane. The main features of the spectrometer system (energy range: 0.3–23 MeV) and of the unfolding technique used to restore physical spectra from the pulse-height distributions are described: a method of solution with regularisation for matrix equations of large size, allowing to process count distributions with significant statistical noise, has been developed. A dedicated software, portable to any platform, has been written both for the acquisition and the analysis of the spectra. The typical gamma-ray spectra recorded in hydrogen and deuterium discharges, also with additional heating, are presented and discussed; two components have been observed: (a) thick-target bremsstrahlung gamma-rays produced by runaway electrons hitting the inconel poloidal limiter and/or the vessel; and (b) neutron capture gamma-rays generated in the detector shielding and tokamak structures. The maximum energy resulting from the bremsstrahlung spectra is in agreement with the runaway energy predicted by a test particle model of runaway electron dynamics.