O. Grulke

Radially propagating fluctuation structures in the scrape-off layer of Alcator C-Mod

Radially propagating spatiotemporal fluctuation structures are observed in the scrape-off layer of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, (1994)] using the combination of electric probes, a radial array of views measuring Dα emission, and two-dimensional imaging of Dα emission. For a specific magnetic-field configuration the electric probe and the Dα array measured plasma density and potential fluctuations along the same magnetic-flux tube. Calculations of the cross-correlation functions of Dα intensity fluctuations with ion saturation current fluctuations and floating potential fluctuations, respectively, reveal that the potential associated with fluctuation structures is of dipole type, consistent with fundamental models for radial blob propagation. Radial and poloidal velocities of fluctuation structures are obtained by two-dimensional spatiotemporal turbulence imaging using an ultrafast framing camera observing the Dα emission intensity in the poloidal plane. In the poloidal directio...

Proton-driven plasma wakefield acceleration: a path to the future of high-energy particle physics

New acceleration technology is mandatory for the future elucidation of fundamental particles and their interactions. A promising approach is to exploit the properties of plasmas. Past research has focused on creating large-amplitude plasma waves by injecting an intense laser pulse or an electron bunch into the plasma. However, the maximum energy gain of electrons accelerated in a single plasma stage is limited by the energy of the driver. Proton bunches are the most promising drivers of wakefields to accelerate electrons to the TeV energy scale in a single stage. An experimental program at CERN—the AWAKE experiment—has been launched to study in detail the important physical processes and to demonstrate the power of proton-driven plasma wakefield acceleration. Here we review the physical principles and some experimental considerations for a future proton-driven plasma wakefield accelerator.

Mode Transitions in Helicon Discharges

High resolution density measurements in a conventional helicon source show sudden jumps during a rf power ramp. The downwards power ramp enters all three known discharge modes, capacitive, inductive, and helicon wave sustained mode. For an upwards power ramp the discharge jumps directly from the capacitive into the helicon mode. This observation stands in contrast to models that are based on the plasma density as the critical parameter for the transition to the helicon mode.

Drift waves in a high-density cylindrical helicon discharge

A low-frequency instability is investigated in a helicon plasma, which is characterized by comparably high plasma-β and high collision frequencies. Single movable Langmuir probes and a poloidal probe array are used for studies of spatiotemporal dynamics and for characterization of the background plasma parameters. All experimentally observed features of the instability are found to be consistent with drift waves. A linear nonlocal numerical model for drift modes, based on the two-fluid description of a plasma, is used for comparison between the experimental observations and theory. Comparing numerical and experimental frequencies, it is found that the experimentally observed frequencies are consistent with drift waves. The numerical results show that the high electron collision frequencies provide the strongest destabilization mechanism in the helicon plasma.

Transition from unbounded to bounded plasma whistler wave dispersion

Whistler wave dispersion measurements are done in a linear magnetized helicon plasma experiment. The waves are excited by an induction loop and detected by movable magnetic probes for a frequency range of 100–800 MHz, corresponding to 0.05–0.9 ωce. The dispersion of whistler waves is measured for various plasma densities and magnetic field strengths. A key issue is to study the transition from an unbounded to bounded plasma wave dispersion. A comparison with theoretically derived dispersion relations is made. For small wavelengths, the dispersion can be described with whistler wave theory for unbounded plasmas whereas for larger wavelengths, the bounded geometry must be taken into consideration. The experimental results agree with theoretical dispersion relations derived for the bounded and the unbounded situation.

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...

Spatial mode structures of electrostatic drift waves in a collisional cylindrical helicon plasma

In a cylindrical helicon plasma, mode structures of coherent drift waves are studied in the poloidal plane, the plane perpendicular to the ambient magnetic field. The mode structures rotate with a constant angular velocity in the direction of the electron diamagnetic drift and show significant radial bending. The experimental observations are compared with numerical solutions of a linear nonlocal cylindrical model for drift waves [ Ellis et al., Plasma Phys. 22, 113 (1980) ]. In the numerical model, a transition to bended mode structures is found if the plasma collisionality is increased. This finding proves that the experimentally observed bended mode structures are the result of high electron collisionality.

Magnetic fluctuation probe design and capacitive pickup rejection

In this article the capacitive pickup of magnetic fluctuation probes for plasma applications is studied. The nine most commonly used probe designs are compared with respect to their capacitive pickup rejection, magnetic sensitivity, and minimum plasma disturbance. For absolute calibration, well defined electric and magnetic field fluctuations are produced separately in a Faraday cup and in a Helmholtz magnetic field coil configuration, respectively. A sample measurement in a radio frequency helicon plasma demonstrates that the optimum probe design is well suited for measuring magnetic fluctuations in a plasma environment.

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world׳s first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected into the sample wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

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.