Density Normalization

SNAPIN reads all of the 10 chord-integrated electron density measurements provided by the vertically-viewing MIRI diagnostic. A proper normalization of the Thomson scattering profile measurement of n_e(R) to one of these chords would iteratively

• Calculate the n_e(r) by smoothing the original Thomson n_e(R) and mapping to minor radius according to the chosen algorithm;
• Integrate n_e(r) vertically along a selected MIRI viewing chord (i.e. a given major radius) to determine the Thomson value of ;
• Normalize the n_e(r) profile to give the value of measured by MIRI at this radius.

However, up through release 6.0 of SNAP this algorithm is not used. Instead, irrespective of which MIRI chord is selected as the one to be used for normalization, it is treated as a horizontal line integral measurement of . The Thomson scattering measurement is then normalized so that its horizontal integral of equals the MIRI value. Note that the Thomson measurement of n_e(R) is performed in the horizontal midplane, so mapping to minor radius is not required to evaluate its in this algorithm. The practical consequences of using this more rudimentary algorithm are:

• You must choose the MIRI chord whose sightline most closely passes through the plasma center as the one to normalize the Thomson profile against (or, if available, use the waveform FM-YI-NE which represents the horizontal line-integral electron density inferred by Abel inverting the MIRI data).
• The Abel-inversion algorithm for FM-YI-NE makes an approximate -dependent correction for the plasma ellipticity, which is reasonably accurate for most beam-heated plasmas. Its accuracy is not assured for the very high regime plasmas produced by our collaborators from Columbia University.

  Using one of the individual MIRI chords for normalization (i.e. not FM-YI-NE) will effectively overestimate the plasma density by its elongation , which for typical plasmas is an error of a few percent.