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Aug 2008

Volume 79, Issue 8, Articles (08xxxx)

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Use of the absolute phase in frequency modulated continuous wave plasma reflectometry

G. Cunningham

Rev. Sci. Instrum. 79, 083501 (2008); http://dx.doi.org/10.1063/1.2964233 (5 pages)

Online Publication Date: 4 August 2008

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In frequency modulated continuous wave reflectometry, used for density profile measurement in fusion plasmas, it is usual to measure the beat frequency between the launched wave and the reflected wave, and from this to calculate the position of the reflecting layer in the plasma. The absolute phase of the beat signal is usually neglected. The reason is that the phase shift between sweeps is usually comparable with or more than 2π, leading to an ambiguity that is impossible to resolve. However, recent observations on the MAST tokamak have shown that, under quiet plasma conditions (this term has to be defined), the phase shift between sweeps is small compared with 2π and the phase ambiguity can be readily resolved. The reflectometer signal is then being analyzed as an interferometer signal would normally be, and there is a substantial improvement in spatial resolution. The method is illustrated by application to small edge localized mode precursor and allows what is believed to be the first quantitative measurement of the displacement of the plasma boundary by such a precursor mode. The errors in both the absolute phase measurement and the more conventional frequency measurement are also estimated.
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52.70.-m Plasma diagnostic techniques and instrumentation
52.55.Fa Tokamaks, spherical tokamaks
52.40.Hf Plasma-material interactions; boundary layer effects
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

A new diagnostic of the plasma density profile near the edge transport barrier from measurements of the radial profile of the Hα line intensity

G. S. Voronov

Rev. Sci. Instrum. 79, 083502 (2008); http://dx.doi.org/10.1063/1.2964925 (5 pages)

Online Publication Date: 6 August 2008

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An edge transport barrier is now one of the most important subjects of controlled fusion research. The edge transport barrier is located in the plasma region where hydrogen atoms readily penetrate, so the intensity of the Hα (Dα) line is high enough. A new diagnostic method uses the well-known property of hydrogen atoms that the ratio of the ionization rate Si to the excitation rate Sv for the Hα line is nearly constant over a wide range of plasma temperatures and densities. An expression has been derived that relates the radial profiles of the plasma density and Hα intensity. The use of charge coupled device detectors makes it possible to measure the radial profile of Hα line intensity with a resolution ∼ 0.1 cm; a high intensity of the Hα line ensures a high time resolution ∼ 1 ms. A high resolution is thus achieved for the density profile calculated from the Hα intensity profile. The method was tested when studying the plasma density profile in the region of edge transport barrier in the L-2M stellarator. It has been shown that the density gradient varies during the barrier formation and that a fine structure of the density profile correlates with a character of the plasma transport near resonance magnetic flux surfaces.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.25.Fi Transport properties

Development and research of a coaxial microwave plasma thruster

Juan Yang, Yingqiao Xu, Jinlan Tang, Genwang Mao, Tielian Yang, and Xiaoquen Tan

Rev. Sci. Instrum. 79, 083503 (2008); http://dx.doi.org/10.1063/1.2967340 (6 pages)

Online Publication Date: 7 August 2008

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An overview of the research on a coaxial microwave plasma thruster at Northwestern Polytechnic University is presented. Emphasis is put on the development and research on key components of the thruster system, a microthrust balance, plasma plume diagnostics, and a numerical simulation of the plasma flow field inside the thruster cavity. The developed thruster cavity is chosen from a coaxial resonant cavity with concentrated capacitance, which can operate well in atmosphere and vacuum conditions. The development of a microwave source shows that a magnetron powered by a switch power supply has advantages in the power level and efficiency, but a solid state microwave source synthesized from the arsenide field effect transistor is superior in weight and volume. Through elimination of the effect of large gravity and resistance force induced by a gas pipe line and a microwave transmitting line on the microthrust, 15 mN and 340 s in the performance of the microwave plasma thruster at 70 W and with helium gas are measured. Diagnosing experiment shows that the plasma plume density is in the range of (1–7.2)×1016/m3. Numerical simulation of the plasma flow field inside the coaxial thruster cavity shows that there is a good match between the microwave power and gas flow rate.
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52.75.Di Ion and plasma propulsion
52.50.Sw Plasma heating by microwaves; ECR, LH, collisional heating
52.30.-q Plasma dynamics and flow
52.65.-y Plasma simulation
52.70.-m Plasma diagnostic techniques and instrumentation
84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)

In-vessel visible inspection system on KSTAR

Jinil Chung and D. C. Seo

Rev. Sci. Instrum. 79, 083504 (2008); http://dx.doi.org/10.1063/1.2969654 (4 pages) | Cited 6 times

Online Publication Date: 18 August 2008

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To monitor the global formation of the initial plasma and damage to the internal structures of the vacuum vessel, an in-vessel visible inspection system has been installed and operated on the Korean superconducting tokamak advanced research (KSTAR) device. It consists of four inspection illuminators and two visible/H-alpha TV cameras. Each illuminator uses four 150 W metal-halide lamps with separate lamp controllers, and programmable progressive scan charge-coupled device cameras with 1004×1004 resolution at 48 frames/s and a resolution of 640×480 at 210 frames/s are used to capture images. In order to provide vessel inspection capability under any operation condition, the lamps and cameras are fully controlled from the main control room and protected by shutters from deposits during plasma operation. In this paper, we describe the design and operation results of the visible inspection system with the images of the KSTAR Ohmic discharges during the first plasma campaign.
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52.55.Fa Tokamaks, spherical tokamaks
28.52.Av Theory, design, and computerized simulation
84.71.Ba Superconducting magnets; magnetic levitation devices
52.80.-s Electric discharges
28.52.Fa Materials

Two-dimensional micron-step probe drive for laboratory plasma measurement

A. Collette and W. Gekelman

Rev. Sci. Instrum. 79, 083505 (2008); http://dx.doi.org/10.1063/1.2972150 (4 pages) | Cited 1 time

Online Publication Date: 25 August 2008

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Laboratory measurement of small-scale ( ∼ 1 mm) magnetic phenomena over an extended area is a challenge requiring precise diagnostics. We present a novel two dimensional magnetic probe platform capable of directly measuring the magnetic field over a 36 cm2 region at spatial resolutions better than 1 mm. The platform is discussed in the context of an experiment at the Large Plasma Device facility at UCLA, designed to measure the magnetic interaction between two counterpropagating laser-produced plasmas. The use of a precise, repeatable positioning platform enables the recovery of information about the interaction using cross-correlation techniques.
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52.70.Ds Electric and magnetic measurements
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Controlled reproducible alignment of cone targets and mitigation of preplasma in high intensity laser interactions

Nathalie Renard-Le Galloudec, Byoung-Ick Cho, Jens Osterholz, and Todd Ditmire

Rev. Sci. Instrum. 79, 083506 (2008); http://dx.doi.org/10.1063/1.2972152 (4 pages) | Cited 3 times

Online Publication Date: 25 August 2008

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The use of cone targets in high intensity laser-plasma experiments has been of recent interest because of their potential use in integrated fast ignition experiments. Simpler experiments provide a good avenue for understanding the underlying physics, however precise control of the alignment along with good pointing accuracy is of crucial importance. While on big laser facilities target alignment is done precisely with several microscopes, it is not always the case on smaller facilities. This can have a detrimental effect on the quality of the results. We have developed and characterized a method for accurate alignment of intense laser pulses into a cone target. This, along with optimal positioning of the focus compared to the tip, efficiently uses the shape of the target to microfocus the laser light and concentrates the hot electrons in the tip, and can mitigate preplasma issues.
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52.38.-r Laser-plasma interactions

Magnetically and optoelectronically isolated trigger for pulse-power applications

Yi Yu, Yi-Zhi Wen, Chang-Xuan Yu, Shu-De Wan, and Wan-Dong Liu

Rev. Sci. Instrum. 79, 083507 (2008); http://dx.doi.org/10.1063/1.2972173 (4 pages)

Online Publication Date: 25 August 2008

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In this article the design of a magnetically and optically isolated trigger is discussed. Critical issues for trigger design are presented together with some experimental usages. In this trigger, an optical coupler is used to cut off the ground loop between the circuits of the preceding control system and the power supplies of the double functional device KT-5D (as a simple magnetic torus or a tokamak). A magnetic coupler is used to provide a pulse-power output for the silicon controlled rectifier. The output is a 230 μs transistor-transistor logic (TTL) with an amplitude of 3.0 V. The rising time and the trailing time are no more than 4.0 μs. The delay time between the input and the output of the trigger is 6.8±0.2 μs. A resistance-capacity branch is integrated into the trigger to provide an adjustable delay time of up to 72 ms. The zero quiescent dissipation character endows the trigger with a long lifetime of years dispensing with any charging or replacing batteries. It is observed that the trigger has a good stability even in a high electromagnetic circumstance (at the order of 1 T). Using it as a trigger for the silicon controlled rectifier, we realized the compatible operation of the steady state mode and the pulse mode in KT-5D.
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42.79.Gn Optical waveguides and couplers
84.30.Jc Power electronics; power supply circuits
42.82.Et Waveguides, couplers, and arrays
84.30.Sk Pulse and digital circuits
85.60.-q Optoelectronic devices

Laser-heated emissive plasma probe

Roman Schrittwieser, Codrina Ionita, Petru Balan, Ramona Gstrein, Olaf Grulke, Thomas Windisch, Christian Brandt, Thomas Klinger, Ramin Madani, George Amarandei, and Arun K. Sarma

Rev. Sci. Instrum. 79, 083508 (2008); http://dx.doi.org/10.1063/1.2968114 (9 pages) | Cited 15 times

Online Publication Date: 28 August 2008

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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.
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52.70.Ds Electric and magnetic measurements
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
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