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Momentum losses by charge exchange friction with neutral particles

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					37th EPS Conference on Plasma Physics                                                                     P1.1100



Momentum losses by charge exchange friction with neutral particles in JET
T.W. Versloot1, P.C. de Vries1, C. Giroud2, M. Brix2 , M.G. von Hellermann1, M. O’ Mullane3,
     A. Salmi4, T. Tala5, I. Voitsekhovich2, K.D. Zastrow2 and JET EFDA contributors§
                      JET-EFDA Culham Science Centre, Abingdon, OX14 3DB, UK
 1FOM    Institute for Plasma Physics Rijnhuizen, Ass. EURATOM-FOM, Nieuwegein, The Netherlands
          2EURATOM/CCFE Fusion Ass., Culham Science Centre, Abingdon, OX14 3DB, UK
    3Department of Physics and Applied Physics, University of Strathclyde, Glasgow, G4 0NG, UK
                4Association EURAROM-Tekes, HUT, P.O. Box 4100, 02015 TKK, Finland.
                  5Association EURATOM-Tekes VTT, P.P. Box 1000, 02044 VTT, Finland
  § See the Appendix of F. Romanelli et al., Proceedings of the 22nd IAEA Fusion Energy Conference,

                                       Geneva, Switzerland, 2008
                                   E-mail: Thijs.Versloot@ccfe.ac.uk
The effect of a neutral density background on the angular momentum and kinetic energy
profiles has been studied in JET. It is well known that the fraction of momentum and energy
stored in the H-mode pedestal can contribute significantly to the global confinement. The
penetration of neutral particles towards the pedestal from the plasma boundary, can cause
losses in both energy and momentum by charge-exchange interactions with the main plasma
ions. Unfortunately, the exact shape and magnitude of the neutral density profile remains
difficult to measure directly. In order to qualitatively capture the atomic physics related to
multiple neutral-ion interactions, a simple 1D fluid approximation is used to model the neutral
transport response in the plasma. To increase confidence, the obtained neutral density profile
is then used to forward model the passive charge exchange emission and compared to
measurements of the Charge Exchange Recombination Spectroscopy (CXRS) diagnostic [1].
This method allows for a correction on the neutral density and a rough estimate of the
momentum and energy losses at the plasma edge.
	

     In experiments under equivalent conditions but with increased gas fuelling, it is
observed that both the edge rotation and temperature decrease with a reduction in core and
edge thermal mach number. As expected, an increase in edge electron density is seen, but was
not sufficient to compensate the rotation and temperature loss such that both energy and
momentum confinement time are significantly reduced. The ratio of confinement times (τE/τφ)
is seen to increase with gas fuelling due to a larger reduction in global angular momentum in
comparison to the kinetic energy, suggesting a larger momentum loss at the plasma boundary.
	

     Both in the non-fuelled and highest fuelling case (Γg=4x1022 elec/s) a neutral density
in the order of 1017 m-3 at the last closed flux surface was required to match the carbon passive
light emission profile from CXRS. The neutral density reduces steeply over the edge gradient
but multiple charge exchange interactions allow for a penetration up to the pedestal. This
effect has an impact on the modelled profiles of the charge exchange losses. The momentum
sink was seen to increase with neutral flux up to 15% of the input torque while energy losses
shifted outwards. This paper will present and discuss the obtained neutral density profiles in
order to quantify the effect of momentum and energy losses at the plasma edge. These results
are then compared to the confinement properties of a series of gas fuelled discharges at JET.
 This work, supported by the European Communities under the contract of Association between EURATOM and FOM and
  CCFE, was carried out within the framework of the European Fusion Development Agreement. The views and opinions
                      expressed herein do not necessarily reflect those of the European Commission.
[1] M. Tunklev, et. al., Plasma Phys. Control. Fusion, 41, 1999, 985-1004

				
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posted:10/21/2011
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