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Dec 2007

Volume 78, Issue 12, Articles (12xxxx)

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Rev. Sci. Instrum. 78, 121301 (2007); http://dx.doi.org/10.1063/1.2821148 (15 pages)

A. Westphalen, M.-S. Lee, A. Remhof, and H. Zabel
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Fine positioning of a poloidal probe array

T. Yamada, Y. Nagashima, S. Inagaki, Y. Kawai, M. Yagi, S.-I. Itoh, T. Maruta, S. Shinohara, K. Terasaka, M. Kawaguchi, M. Fukao, A. Fujisawa, and K. Itoh

Rev. Sci. Instrum. 78, 123501 (2007); http://dx.doi.org/10.1063/1.2818796 (5 pages) | Cited 18 times

Online Publication Date: 4 December 2007

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Multipoint detection is an essential requirement for investigating plasma turbulence which is a highly nonlinear phenomenon in space and time. We have fabricated an array of 64-channel poloidal probes surrounding the linear cylindrical plasma named LMD-U in order to study turbulence properties, particularly the nonlinear mode couplings, in the domain of poloidal wave number and frequency. However, misalignments of probe tips produce spurious modes, which do not exist in the real plasma, to distort the precise wave number measurements. The paper presents the description of the 64-channel poloidal probe array with means to adjust the probe positions, with discussion on the effects of the misalignments on the wave number measurements.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Ra Plasma turbulence
52.70.-m Plasma diagnostic techniques and instrumentation

Two-dimensional signal reconstruction: The correlation sampling method

H. E. Roman

Rev. Sci. Instrum. 78, 123502 (2007); http://dx.doi.org/10.1063/1.2821142 (5 pages) | Cited 1 time

Online Publication Date: 6 December 2007

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An accurate approach for reconstructing a time-dependent two-dimensional signal from non-synchronized time series recorded at points located on a grid is discussed. The method, denoted as correlation sampling, improves the standard conditional sampling approach commonly employed in the study of turbulence in magnetoplasma devices. Its implementation is illustrated in the case of an artificial time-dependent signal constructed using a fractal algorithm that simulates a fluctuating surface. A statistical method is also discussed for distinguishing coherent (i.e., collective) from purely random (noisy) behavior for such two-dimensional fluctuating phenomena.
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52.35.Ra Plasma turbulence
52.30.-q Plasma dynamics and flow
02.50.-r Probability theory, stochastic processes, and statistics
05.40.Ca Noise
05.45.Df Fractals

A new emissive-probe method for electron temperature measurement in radio-frequency plasmas

Kouta Kusaba and Haruo Shindo

Rev. Sci. Instrum. 78, 123503 (2007); http://dx.doi.org/10.1063/1.2821200 (8 pages) | Cited 3 times

Online Publication Date: 13 December 2007

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A new method to measure electron temperature by an emissive probe has been proposed. The method is based on measurement of the functional relationship between the floating potential and the heating voltage of emissive probe. From the measured data of the floating potential change as a function of the heating voltage, the electron temperature could be determined by comparing with the theoretical curve obtained under the assumption of Maxwellian distribution. The overall characteristic of the floating potential change could be explained as a function of the heating voltage. The electron temperatures obtained by the present method were consistent with those measured by the rf-compensated Langmuir probe within the error. These experimental verifications were made in the electron density range of 2.6×1011–2.8×1012 cm−3. It was stressed that the present method is advantageous in that the probe is operated in a floating condition, hence applicable to plasmas produced in an insulated container.
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52.80.Pi High-frequency and RF discharges
52.50.Qt Plasma heating by radio-frequency fields; ICR, ICP, helicons
52.70.Ds Electric and magnetic measurements
52.25.-b Plasma properties

Neutron production from feedback controlled thermal cycling of a pyroelectric crystal

V. Tang, G. Meyer, J. Morse, G. Schmid, C. Spadaccini, P. Kerr, B. Rusnak, S. Sampayan, B. Naranjo, and S. Putterman

Rev. Sci. Instrum. 78, 123504 (2007); http://dx.doi.org/10.1063/1.2823973 (4 pages) | Cited 8 times

Online Publication Date: 26 December 2007

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The LLNL Crystal Driven Neutron Source is operational and has produced record ion currents of ∼ 10 nA and neutron output of 1.9(±0.3)×105 per thermal cycle using a crystal heating rate of 0.2 °C/s from 10 to 110 °C. A 3 cm diameter by 1 cm thick LiTaO3 crystal with a socket secured field emitter tip is thermally cycled with feedback control for ionization and acceleration of deuterons onto a deuterated target to produce D–D fusion neutrons. The entire crystal and temperature system is mounted on a bellows which allows movement of the crystal along the beam axis and is completely contained on a single small vacuum flange. The modular crystal assembly permitted experimental flexibility. Operationally, flashover breakdowns along the side of the crystal and poor emitter tip characteristics can limit the neutron source. The experimental neutron results extend earlier published work by increasing the ion current and pulse length significantly to achieve a factor-of-two higher neutron output per thermal cycle. These findings are reviewed along with details of the instrument.
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29.25.Dz Neutron sources
34.50.Fa Electronic excitation and ionization of atoms (including beam-foil excitation and ionization)
25.60.Pj Fusion reactions

Compact cantilever force probe for plasma pressure measurements

I. S. Nedzelskiy, C. Silva, H. Fernandes, P. Duarte, and C. A. F. Varandas

Rev. Sci. Instrum. 78, 123505 (2007); http://dx.doi.org/10.1063/1.2813897 (6 pages) | Cited 5 times

Online Publication Date: 28 December 2007

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A simple, compact cantilever force probe (CFP) has been developed for plasma pressure measurements. It is based on the pull-in phenomenon well known in microelectromechanical-system electrostatic actuators. The probe consists of a thin (25 μm) titanium foil cantilever (38 mm of length and 14 mm of width) and a fixed electrode separated by a 0.75 mm gap. The probe is shielded by brass box and enclosed into boron nitride housing with a 9 mm diameter window for exposing part of cantilever surface to the plasma. When the voltage is applied between the cantilever and the electrode, an attractive electrostatic force is counterbalanced by cantilever restoring spring force. At some threshold (pull-in) voltage the system becomes unstable and the cantilever abruptly pulls toward the fixed electrode until breakdown occurs between them. The threshold voltage is sensitive to an additional externally applied force, while a simple detection of breakdown occurrence can be used to measure that threshold voltage value. The sensitivity to externally applied forces obtained during calibration is 0.28 V/μN (17.8 V/Pa for pressure). However, the resolution of the measurements is ±0.014 mN (±0.22 Pa) due to the statistical scattering in measured pull-in voltages. The diagnostic temporal resolution is ∼ 10 ms, being determined by the dynamics of pull-in process. The probe has been tested in the tokamak ISTTOK edge plasma, and a plasma force of ∼ 0.07 mN (plasma pressure ∼ 1.1 Pa) has been obtained near the leading edge of the limiter. This value is in a reasonable agreement with the estimations using local plasma parameters measured by electrical probes. The use of the described CFP is limited by a heat flux of Q ∼ 106 W/m2 due to uncontrollable rise of the cantilever temperature (ΔT ∼ 20 °C) during CFP response time.
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52.25.-b Plasma properties
52.70.Ds Electric and magnetic measurements
52.55.Fa Tokamaks, spherical tokamaks
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