Hard x-ray diagnostic for lower hybrid experiments on Alcator C-Mod

Liptac, J.; Parker, R.; Tang, V.; Peysson, Y.; Decker, J.

doi:10.1063/1.2214695

Cited by 50 publications

(31 citation statements)

References 12 publications

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“…Alcator C-Mod hard x-ray camera C-Mod has an outboard mid-plane HXR camera with 32 spatial chords, designed to detect bremsstrahlung emission from LH-driven fast electrons [13,14]. The camera has a spatial resolution of ∼2 cm.…”

Section: Methodsmentioning

confidence: 99%

Investigation of lower hybrid physics through power modulation experiments on Alcator C-Mod

et al. 2011

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Lower hybrid current drive (LHCD) is an attractive tool for off-axis current profile control in magneticly confined tokamak plasmas and burning plasmas (ITER), because of its high current drive efficiency. The LHCD system on Alcator C-Mod operates at 4.6 GHz, with ∼ 1 MW of coupled power, and can produce a wide range of launched parallel refractive index (n || ) spectra. A 32 chord, perpendicularly viewing hard x-ray camera has been used to measure the spatial and energy distribution of fast electrons generated by LH waves. Square-wave modulation of LH power on a time scale much faster than the current relaxation time does not significantly alter the poloidal magnetic field inside the plasma and thus allows for realistic modeling and consistent plasma conditions for different n || spectra. Inverted hard x-ray profiles show clear changes in LH-driven fast electron location with differing n || . Boxcar binning of hard x-rays during LH power modulation allows for ∼ 1 ms time resolution, which is sufficient to resolve the build-up, steady-state, and slowing-down phases of fast electrons. Ray-tracing/Fokker-Planck modeling in combination with a synthetic hard x-ray diagnostic show quantitative agreement with the x-ray data for high n || cases. The time histories of hollow x-ray profiles have been used to measure off-axis fast electron transport in the outer half of the plasma, which is found to be small on a slowing down time scale.

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Section: Methodsmentioning

confidence: 99%

Investigation of lower hybrid physics through power modulation experiments on Alcator C-Mod

et al. 2011

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“…The Hard X-Ray (HXR) diagnostic on C-Mod [12] is used both for localizing the fast electron bremsstrahlung emission, and for comparing the relative magnitude of the X-ray count rate. The count rate can be taken as a proxy for the density of the fast electron tail generated by LHCD.…”

Section: A Hard X-ray Bremsstrahlungmentioning

confidence: 99%

Absorption of lower hybrid waves in the scrape off layer of a diverted tokamak

Wallace¹,

Parker²,

Bonoli³

et al. 2010

Physics of Plasmas

133

126

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The goal of the Lower Hybrid Current Drive (LHCD) system on the Alcator C-Mod tokamak is to investigate current profile control under plasma conditions relevant to future devices such as ITER and DEMO. Experimental observations of an LHCD "density limit" for C-Mod are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line averaged densities of 10 20 m −3 (ω/ω LH ∼3-4), well below the density limit previously observed on other experiments (ω/ω LH ∼ 2). Electric currents flowing through the scrape off layer (SOL) between the inner and outer divertors increase dramatically across the same density range that the core bremsstrahlung emission drops precipitously. These experimental x-ray data are compared to both conventional modeling, which gives poor agreement with experiment above the density limit, and a model including collisional absorption in the SOL, which dramatically improves agreement with experiment above the observed density limit. These results show that strong absorption of LH waves in the SOL is possible on a high density tokamak and the SOL must be included in simulations of LHCD at high density.

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“…Bremsstralung radiation from the resulting fast electrons was measured with Alcator C-Mod's 32 spatial chord hard x-ray (HXR) camera. 5 The camera's pulse trains are digitized and post-processed for flexible time and energy binning. The resulting HXR emission was boxcar binned in time for high signal-to-noise, high timeresolution x-ray time traces.…”

Section: Lower Hybrid Power Modulation Experimentsmentioning

confidence: 99%

Measurement of Fast Electron Transport by Lower Hybrid Modulation Experiments in Alcator C-Mod

Schmidt¹,

Bonoli²,

Parker³

et al. 2009

AIP Conference Proceedings

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Abstract. The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod can produce spectra with a wide range of peak parallel refractive index (n || ). An experiment in which LH power is square-wave modulated on a time scale much faster than the current relaxation time does not significantly alter the poloidal magnetic field inside the plasma and thus allows for realistic modeling and consistent plasma conditions for different n || spectra. Boxcar binning of hard x-rays during LH power modulation allows for time resolution sufficient to resolve the build-up, steady-state, and slowing-down of fast electrons. A transport model built in Matlab has been used to determine a fast electron pinch velocity for a high-n || case of 1-2 m/s. Keywords LOWER HYBRID POWER MODULATION EXPERIMENTSIn order to control the location of LH-driven current, it is important to understand where in the plasma the LH power is deposited as well as the transport properties of the current-carrying fast electrons. To this end, there have been several studies of fast electron diffusion in LHCD tokamak plasmas 1,2 which have determined an upper bound on fast electron diffusion in the range of 0.2-2 m 2 /s. Recent experiments on Alcator C-Mod have revealed the build-up of a radial electric field in the core of the plasma during LHCD operation, 3,4 speculated to be caused by an inward pinch of LHdriven fast electrons.In this experiment, designed to measure transport properties of the LH-driven fast electrons, LH power was square-wave modulated with a 25 ms period and a 50% duty cycle. Bremsstralung radiation from the resulting fast electrons was measured with Alcator C-Mod's 32 spatial chord hard x-ray (HXR) camera.5 The camera's pulse trains are digitized and post-processed for flexible time and energy binning. The resulting HXR emission was boxcar binned in time for high signal-to-noise, high timeresolution x-ray time traces. This signal was Fourier decomposed, spatially inverted, and fit to a transport model to determine the magnitude of fast electron pinch velocity. Peak coupled LH power was approximately 400 kW. In the case analyzed in this paper, n || =3.1, n e =8.3×10 19 m -3 , T e0 =2.3 keV, and I p =800 kA.

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Hard x-ray diagnostic for lower hybrid experiments on Alcator C-Mod

Cited by 50 publications

References 12 publications

Investigation of lower hybrid physics through power modulation experiments on Alcator C-Mod

Investigation of lower hybrid physics through power modulation experiments on Alcator C-Mod

Absorption of lower hybrid waves in the scrape off layer of a diverted tokamak

Measurement of Fast Electron Transport by Lower Hybrid Modulation Experiments in Alcator C-Mod

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