DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 101: Measurements of Neutral Beam Excited State Lifetime | ||
| Name: | George R. McKee ( |
Affiliation: | University of Wisconsin, Madison |
| Research Area: | Transport | Presentation time: | Requested | Co-Author(s): | M. Shafer, D. Schlossberg |
| Description: | Indirectly measure the effective lifetime in plasma of the n=3 excited state of beam atoms as a function of density. This is to provide necessary atomic physics information to assist with calculations of the spatial spot size for beam emission spectroscopy measurements. This information will feed into calculations of measured turbulence characteristics (wavenumber spectra, amplitudes) and point-spread-functions/spatial transfer functions for use in synthetic diagnostics for simulation comparisons. | ||
| Experimental Approach/Plan: | Run low power, high field, low-temperature L-mode plasmas and measure the turbulence eddy structure with the 2D configuration of BES. Locate the BES array at the radial position with best optical resolution (near r/a=0.75), and at a safety factor profile that most closely aligns the local pitch angle with the sightline angle. Minimize the turbulence structure size, which has been shown to be proportional to ion gyroradius size, requiring low ion temperature and high field. Vary the 150L neutral beam acceleration voltage over as wide a range as feasible (e.g, 45-85 keV) and then separately vary the density over as wide a range as feasible, while maintaining the ion temperature (and gyroradius) nearly constant (power/density scan), staying in L-mode throughout. The goal is to minimize the turbulence structure size and optimize spatial resolution so that these so-called "beam smearing" effects will be most accute.
Measure turbulence radial (and poloidal) correlation lengths as a function of beam voltage (velocity) and density. Radial correlation length is of key importance here. The excited state lifetime will affect the measured radial correlation and variation should be discernible over the range tested. Basically, the variation of the radial correlations will be related to a spatial smearing that arises from the finite lifetime effects of the beam atoms. | ||
| Background: | The spatial resolution of BES plays an important role in the wavenumber sensitivity of the diagnostic and for discerning spatial characteristics of turbulence. This resolution is calculated from viewing optics, neutral beam geometry, magnetic field pitch angle as well as on the finite lifetime of the excited state atoms in the n=3 state (BES views emission from the n=3-2 D-alpha transition). This natural (in vacuum) excited state lifetime is about tau_l=10 ns, but this is significantly reduced to an effective lifetime of tau = 2-5 ns in a plasma as a result of collisional excitation/ionization processes [I. Hutchinson, PPCF (2002), fig. 2(c)]. This lifetime is calculated from theoretical atomic physics excitation rate calculations and therefore subject to uncertainties in those calculations. An 80 keV Deuterium beam atom has a line-of-sight velocity of v_b=2.8x10^6 m/s, so the spatial spread from from this finite lifetime is about L=v_b * tau=1 cm., comparable to the optical resolution and turbulence correlation lengths, so has a non-negligible impact on the effective spatial resolution of the diagnostic.
There is a sense from the turbulence imaging measurements obtained with BES that the actual spatial resolution is better than that calculated using these theoretical rates. One possible explanation is that the effective lifetimes are shorter than calculated. Given the importance of these lifetime for calculating the effective point spread function (spatial transfer function), for use in unfolding the turbulence characteristics and performing experimental validation of simulation codes via synthetic diagnostics, an experimental test is warranted. | ||
| Resource Requirements: | 150LT neutral beam and the 30/330L neutral beams; USN or IWL plasma | ||
| Diagnostic Requirements: | BES, CER, correlation reflectometry | ||
| Analysis Requirements: | BES and CR spatial correlation turbulence analysis | ||
| Other Requirements: | -- | ||
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