T. Windisch

Radial propagation of structures in drift wave turbulence

The formation and propagation of spatiotemporal fluctuation structures in weakly developed drift-wave turbulence in a linearly magnetized helicon device is investigated. Turbulent density fluctuations in the far edge plasma display an intermittent character with large-amplitude positive density bursts. Their peak amplitudes correspond to the time-averaged density in the maximum radial plasma pressure gradient. The conditional average technique is applied to reconstruct the dynamics of turbulent coherent structures in the azimuthal plane. The formation of turbulent structures is closely linked to a quasicoherent m=1 drift wave mode, which is generally observed in the radial density gradient region in the weakly developed turbulent state. It is demonstrated that every positive high amplitude density burst in the plasma edge is due to the radial propagation of a turbulent structure. The typical scale size of the turbulent structures is 4ρs and their lifetime exceeds the eddy turnover time by orders of magnit...

Laser-heated emissive plasma probe

Emissive probes are standard tools in laboratory plasmas for the direct determination of the plasma potential. Usually they consist of a loop of refractory wire heated by an electric current until sufficient electron emission. Recently emissive probes were used also for measuring the radial fluctuation-induced particle flux and other essential parameters of edge turbulence in magnetized toroidal hot plasmas [R. Schrittwieser et al., Plasma Phys. Controlled Fusion 50, 055004 (2008)]. We have developed and investigated various types of emissive probes, which were heated by a focused infrared laser beam. Such a probe has several advantages: higher probe temperature without evaporation or melting and thus higher emissivity and longer lifetime, no deformation of the probe in a magnetic field, no potential drop along the probe wire, and faster time response. The probes are heated by an infrared diode laser with 808 nm wavelength and an output power up to 50 W. One probe was mounted together with the lens system on a radially movable probe shaft, and radial profiles of the plasma potential and of its oscillations were measured in a linear helicon discharge.

Three-dimensional global fluid simulations of cylindrical magnetized plasmas

Plasma dynamics in cylindrical geometry, with many well diagnosed experiments in operation worldwide, is of fundamental interest. These linear machines can provide an unique testing ground for direct and detailed comparisons of numerical simulations of nonlinear plasma dynamics with experiments. Thus, it is possible to assess the reproductive and predictive capabilities of plasma simulations in unprecedented detail. Here, three-dimensional global fluid simulations of a cylindrical magnetized plasma are presented. This plasma is characterized by the existence of spatially localized sources and sinks. The traditional scale separation paradigm is not applied in the simulation model to account for the important evolution of the background profiles due to the dynamics of turbulent fluctuations. Furthermore, the fluid modeling of sheath boundary conditions, which determine the plasma conditions, are an important ingredient to the code presented here. The linear properties of the model equations are studied and ...

Experimental evidence of mode coupling in drift wave intermittent turbulence using a wave number bicoherence analysis

Spatiotemporal mode coupling is studied experimentally in a cylindrical plasma device. For that purpose, a bicoherence analysis is applied to spatially resolved measurements of drift wave fluctuations in order to study nonlinear coupling in the wave number spectrum. The use of the k bicoherence is shown to be much more accurate and straightforward than one of the ω bicoherence, revealing bicoherence bursts with a characteristic duration shorter than the characteristic period of the signals. It is demonstrated that intermittent structures can be produced during these events.

Performance and properties of the first plasmas of Wendelstein 7-X

The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction and one year commissioning. Errors in the magnet assembly were confirmend to be small. Plasma operation was started with 5 MW electron cyclotron resonance heating (ECRH) power and five inboard limiters. Core plasma values of keV, keV at line-integrated densities were achieved, exceeding the original expectations by about a factor of two. Indications for a core-electron-root were found. The energy confinement times are in line with the international stellarator scaling, despite unfavourable wall conditions, i.e. large areas of metal surfaces and particle sources from the limiter close to the plasma volume. Well controlled shorter hydrogen discharges at higher power (4 MW ECRH power for 1 s) and longer discharges at lower power (0.7 MW ECRH power for 6 s) could be routinely established after proper wall conditioning. The fairly large set of diagnostic systems running in the end of the 10 weeks operation campaign provided first insights into expected and unexpected physics of optimized stellarators.

Intermittent transport events in a cylindrical plasma device: experiment and simulation

The origin of intermittent fluctuations in the plasma edge of a linearly magnetized plasma column is investigated and closely compared with three-dimensional global numerical simulations. The intermittent character is caused by radially propagating turbulent structures. The radial propagation of the structures is due to the self-consistent potential perturbation associated with them. Their formation is closely linked to transport events caused by nonlinear drift-wave fluctuations. In the formation region a sheared azimuthal flow velocity is observed, which is driven by the Reynolds stress.

Laboratory studies of drift waves: nonlinear mode interaction and structure formation in turbulence

Coherent drift modes and drift wave turbulence are studied in a collisionality dominated high-density helicon plasma. The drift wave instability is identified by comparison of the drift mode density fluctuation characteristics with a linear dispersion relation, which reproduces the experimentally observed bent eigenmode structures due to the radial gradients of the collisionality profile. Attention is especially paid to the parallel currents associated with the drift mode density fluctuations, which are in agreement with the basic drift wave instability mechanism. A coupling of the drift mode to the externally excited Alfv?n waves is found, which results in the frequency changes of the drift mode. The parallel currents associated with radially propagating density fluctuation structures in drift wave turbulence are similar to the drift mode case, which strongly suggests that polarization of the structures is a result of the parallel electron dynamics.

Results of direct measurements of the plasma potential using a laser-heated emissive probe

Reliable diagnostics of the plasma potential is one of the most important challenges in context with the production, control and confinement of a plasma. Emissive probes are readily available as direct diagnostic tools for the plasma potential with a good temporal and spatial resolution in many plasmas, even up to middle-sized fusion experiments. We present the results of investigations on the heating of lanthanum hexaboride and graphite with an infrared diode laser and on the development of a laser-heated emissive probe. Such a probe has a higher electron emission, much longer life time and better time response than a conventional emissive wire probe. We have observed that from both materials electron emission current can be achieved sufficiently strongly even for dense laboratory and experimental fusion plasmas.

Status of the diagnostics development for the first operation phase of the stellarator Wendelstein 7-X

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described.

Study of a scalable large-area radio-frequency helicon plasma source

A large-area planar spiral antenna has been installed on the linear laboratory device VINETA. For low radio-frequency (RF) powers the induction coil is operated in the inductive discharge mode with typical plasma densities of n ? 1 ? 1016?m?3 and electron temperatures of Te ? 5?eV. At higher RF powers a transition to the helicon wave sustained discharge mode is observed, in which the density increases by a factor of 200 and Te drops by a factor of 2. Detailed measurements of the helicon wavefield show that a helicon wave with azimuthal mode number m = 0 is launched. Due to the larger wave excitation region of the m = 0 discharges the width of the plasma density profile in the helicon mode increases by a factor of 1.5 compared with the standard VINETA helicon operation with a helical m = +1 antenna.