Helium charge exchange recombination spectroscopy on Alcator C-Mod Tokamak
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The Wide-View Charge Exchange Recombination Spectroscopy (CXRS) diagnostic at Alcator C-Mod, originally designed for measurement of boron, has been modified to fit several different roles. By measuring the He¹⁺ (n = 4 [rightwards arrow] 3) emission line at 4686Å and surrounding spectra, we can measure ⁴He and ³He density, temperature, and velocity profiles and use this information to study turbulent impurity transport. The transport is characterized using a standard ansatz for the radial particle flux: [mathematical equation]. This effort is designated He CXRS. Also, direct measurement of ³He are used to test models of Ion Cyclotron Resonance Heating (ICRH). We look for evidence of fast ion production and the effect of the minority ion profile on fast wave heating. Several modifications were made to the hardware. Light is collected via two optical arrays: poloidal and toroidal. The toroidal array has been upgraded to increase throughput and spatial resolution, increasing the number of toroidal channels from 10 to 22. A new protective shroud was installed on the poloidal array. Additional diagnostics (a 11 channel beam duct view, neutralizer view, duct pressure monitor) were added to the Diagnostic Neutral Beam to improve DNB modeling for CXRS. This work includes investigation of plasmas where helium is at low concentration (<1%), acting passively, as well as scenarios with a large fraction (>~20%). Using the STRAHL code, time-dependent helium density profiles are used to obtain anomalous transport parameters. Thermodiffusion and curvature pinch terms are also estimated from experimental scaling studies. Results are compared with neoclassical results from the NCLASS code and calculations by the GENE gyrokinetic code. Another focus is verification of power deposition models which are crucially dependent on minority ion density, for which ³He is used. At low ³He fraction, direct absorption by ³He generates fast ions with anisotropic velocity-space distribution functions. At high ³He fraction, mode conversion heating of electrons is dominant. The minority distribution function and predicted wave deposition are simulated using AORSA and CQL3D. This work provides the first measurements of helium transport on C-Mod and expands our understanding of helium transport and fast wave heating.