Charge exchange recombination spectroscopy (CXRS) is a powerful and universal technique for diagnostics of hot and dense plasma in a tokamak. It can be used to spatially and temporally resolve the principal plasma parameters such as plasma ion species temperature, density and flow velocity. It requires a source of fast atoms which is the heating or diagnostic beam which is installed on every large-scale machine for magnetic plasma confinement. As a typical optical technique, CXRS is characterized by fairly simple experimental installation employing a set of optical channels and commercially available spectrometers and CCD detectors. On the other hand, the following data interpretation and analysis is a complex, multistep analysis. In the case of CXRS the interpretative analysis is quite intricate due to the complex nature of the atomic-level interactions between the plasma and the beam. The plasma itself is known to be a complex form of matter, and the beam as well has a complex description consisting of multiple energy components each of which has several non-negligible excited states.

The purpose of this work is to apply this technique to investigate the Alcator C-Mod tokamak plasma primarily for r > 1. High-density plasmas confined by substantial magnetic fields also complicate the physical design of CXRS diagnostics and complicate the physical picture of beam-plasma interaction to a great extent. A new diagnostic neutral beam of novel design has been acquired for and installed on Alcator C-Mod for this and other diagnostics. The beam is operated in hydrogen, has a 1.5 sec pulse length, and has an accelerated ion current of 6 A at 50 kV. Boron impurity is the best choice for the observed ion by means of CXRS in C-Mod as boronization, the covering of plasma facing components by boron, is used for impurity control. For the purpose of toroidal and poloidal velocity measurement this diagnostic employs two optical systems. One looks at the emission volume in the toroidal direction, the other in the poloidal. Each system has several viewing chords to achieve spatial resolution.

So, what lies behind the name of Charge Exchange Recombination Spectroscopy? It is certainly the charge exchange process of the beam atom with the ion of the impurity by means of electron transfer to any excited impurity level n i . This process is followed by the electron deexcitation to the lower level n f and photon emission. This intensity is proportional to the density of impurity ions. The Doppler broadening and Doppler shift of the spectral line profile yield s information on the ion temperature and plasma rotation.

The high magnetic fields of C-Mod result in broad Zeeman patterns which must be taken into account for the interpretation of the line shift and broadening in terms of impurity ion velocity and temperature. The CXRS spectral emission is accompanied by the emission generated by electron impact excitation, thermal charge exchange and brehmsstrahlung. These additional processes must be accounted for. A major limit to analysis is the availability of charge exchange cross sections which relate the observed spectroscopic emission to the impurity density. To support interpretation of the measurements, the typical spectra must be simulated using measurements of the beam parameters and some plasma parameters as electron temperature and density. The effect of beam attenuation and excitation in the high neutral density outside the last closed flux surface must be included.

The ultimate goal of this work is to use the acquired impurity density, temperature and flow velocity profiles to investigate plasma transport behavior and infer the radial electric field from plasma balance equation. An intermediate goal is to make the analysis with due regard and examination of all the complexities of CXRS. This type of analysis will be shown to be essential for understanding of plasma behavior and plasma stability as a whole.

Publications and presentations on this topic:

"Effects of Neutral Beam Excited States on CXRS Emission Cross Sections," I. O. Bespamyatnov, W. L. Rowan, R. S. Granetz and D. F. Beals, 16th Topical Conference on

High-Temperature Plasma Diagnostics, 7-11 May 2006, Williamsburg , Virginia

"Effects of Neutral Beam Excited States on CXRS Emission Cross Sections," I. O. Bespamyatnov, W. L. Rowan, R. S. Granetz and D. F. Beals, accepted for publication in the Review of Scientific Instruments.

"Gyrokinetic Microstability Analysis of the Inner Boundary of the H-mode Pedestal," R. V. Bravenec, R. J. Groebner,W. L. Rowan, I. O. Bespamyatnov, M. Greenwald, G. M. Staebler, W. Dorland, DIII-D team, Alcator C-Mod Team, 19th Annual US Transport Task Force Meeting 4-7 April 2006, Myrtle Beach, SC.

"Impurity poloidal rotation for 0.75 < r/a < 1.0 in Alcator C-Mod plasmas," W. L. Rowan, I. O. Bespamyatnov, R.V. Bravenec, D.R. Ernst, D.F. Beals, R.S. Granetz, R.M. McDermott, Abstract: KP1.00010, 47th Annual Meeting of the Division of Plasma Physics, Denver, Colorado, 24–28 October 2005.

"Ion temperature in Alcator C-Mod H-mode discharges with ELM's and with the quasi-coherent (QC) mode," Igor O. Bespamyatnov, W.L. Rowan, R.V. Bravenec, D.F. Beals, R.S. Granetz, A.M. Hubbard, R.M. McDermott, and J.L. Terry, Abstract: KP1.00011, 47th Annual Meeting of the Division of Plasma Physics, Denver, Colorado, 24–28 October 2005.

"Characterization of the New Diagnostic Neutral Beam on Alcator C-Mod," R.V. Bravenec, W.L. Rowan, I. Bespamyatnov, R.S. Granetz, D.F. Beals, R. McDermott, J. Ko, and S.D. Scott, Abstract: KP1.00013, 47th Annual Meeting of the Division of Plasma Physics, Denver, Colorado, 24–28 October 2005.

"Poloidal Rotation in Alcator C-Mod," W.L. Rowan , R. V. Bravenec, I.O. Bespamyatnov, and R.M. McDermott, Abstract: PII-03, 10th Joint US-European TTF workshop and 18th Annual US TTF 2005 Transport Task Force Meeting , Napa, California, USA, 6-9 April 2005.

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