K. Nagasaki

S. Sakakibara

Y. Suzuki

National Institute for Fusion Science (NIFS-Repository)

 The ratio of the alpha ash confinement time to the energy confinement time (τα*/τE) is one of important parameters in designing the FFHR helical reactor.  To study the alpha ash confinement time, He exhaust experiments have been performed. However, it is difficult to evaluate its value accurately due to complicated observation geometry in the LHD CXS system.  For experimental data analysis we are preparing the numerical tools to obtain the profile of charge exchanged excited He ions by NBI from the line-integrated signal.  In the previous report we assumed that plume He+ ions drifts to infinite distance.  In this report the drifting distance is assumed to be limited to ±3.75 m ( ±60 degree) in both the toroidal directions from the NBI injected area.      He++ ion density in the plasma surface by gas puffing is assumed by Bi-Fermi profiles as n(r)n(0)!"#$%&He++= exp 40'(a() rEDao*+,-./2) 0a(1234356373!"##$%&&+1!"##$%&&)1+                                    (1)exp 40'(a(+rEDao*+,-./2) 0a(1234356373!"##$%&&+1!"##$%&&)1    where rED is the peak distance from the plasma center Ro=3.6mΔaφ is its widthCXS observation geometry is shown in Fig, 1.    The neutral density profile in NBI with the simplified ionization cross section for the parabolic density profile is given by  nNBI (r)nNBI (0)!"#$%& = exp 'a( )ANBIZeff5.5 *10'3ENBI [keV ])n(0)102023' x + x33+,-./0+,-./0     (2)  with x=ρφ/aφ, ENBI=40keV, Zeff=2, ANBI=1, and the peak electron density n(0).  Thus, excited He+ ions by charge exchange is proportional to the product of Eqs.(1) and (2).    The integrated signal along the sight line is calculated by I (y)I (0)=n(r)n(0)!"#$%&'(max( max)He+*1c2+1xob 'yobc*+,-./cos( ' 1csin(*+,-./2d(                                (3)    In Fig. 2 and 3 are shown the cases of the peak density of 0.5x1019 m-3 and 2x1019 m-3.  Left figure shows the signal (excited He+ ions by NBI) on the sight line through the point of R=3.6m in the NBI injected area.  As the plume is assumed to be from -24 to 96 degree in Fig. 1 and Fig. 2-(Left), the signal from the inboard side is reduced.  Right figure shows the integrated signal (solid red line) along the sight line for various minor radii and excited He+ ion profile by NBI (dashed line).  In the low-density regime of 5x1012 m-3 (Fig. 2) NBI penetration is good, therefore signals come from the inboard and outboard sides. However, in the higher density (Fig. 3) the signal comes only from the outboard side due to poor NBI penetration.   The 3D magnetic field line through the NBI injected area is shown by Mathematica in Fig. 3, which was calculated by Prof emeritus T. Watanabe.  The rotation of the magnetic field line indicates importance of the finite NBI beam geometry to take the plume effect into account.    This work is performed with the support and under the  auspices of the NIFS Collaborative Research Program  Fig. 3. Horizontal view from the outboard side of the 3D magnetic field lines through the NBI (right circular column) injected area and CXS sight line (horizontal black line) for He ion plume analysis.      Fig. 1. Approximated magnetic field line on the equatorial plane in the LHD, and the observing lines of the CXS.  02468102.8 3.2 3.6 4.0 4.4NRHEPE1IXTANI(Rmeas)/IX0Rmeas [m]  02468102.8 3.2 3.6 4.0 4.4NRHEPE1IXTANI(Rmeas)/IX0Rmeas [m]  Fig. 2. (Left) Distributed signal along the tangential sight line (Black). (Right) The observed integrated intensity (solid red line) along the tangential sight line for the excited He+ ion distribution (dashed line) by NBI. (Upper: n(0)=0.5x1019 m-3 and Lower: n(0)=2x1019 m-3)   In the LHD experiments, the volume beta value <> achieved to about 5%, where disruptive events did not appear. This suggests the plasma beta does not achieve to the equilibrium beta limit. To show the equilibrium beta limit, the magnetic field structure for high- plasma was studied by nonlinear MHD simulation codes. In that study, the beta limit is higher than the achieved beta in the experiment then experimental and theoretical consideration are consistent up to now. However, in that study, stochastic field lines of the equilibrium response are appeared due to increased  and the confinement is degraded due to increased stochastization. Since the high- experiment is done for the low filed, the plasma is low temperature and high density, so-called the collisional plasma. If the plasma changes toward the collisionless regime, the degradation of the confinement due to stochastic field lines will be expected. To aim the production of high- plasma in the reactor-relevant regime, studying the impact of stochastic field lines to the confinement are urgent and critical issues. However, we also observed fluctuations due to MHD instabilities. Thus, we need further studies how much impact fluctuations have. For the MHD instability, if the dominant instability is the resistive interchange mode, the resistive interchange mode will be decreased due to decreased the resistivity. Thus, producing higher temperature plasma, we can distinguish flattening profiles made by stochastization of the MHD equilibrium response or instability. Such a study is also an important issue.     To produce high- plasma in more collisionless regeme, the magnetic field is increased to 1.5T. Figure 1 shows the achieved beta value as the function of the magnetic field in operations. In the 15th experiment campaign, achieved beta values were to 4.1% and 3.4% for B=0.75 and 1.0T. This was a record in the LHD experiment.    To see the impact of high temperature to the operation regeme, in fig.2, magnetic fluctuations are plotted as functions of the magnetic Reynolds number and achieved beta value. In fig. 2, points below a dashed line indicate fluctuation date before 14th campaign. If the beta is high, the fluctuation becomes strong. Red circles above the dashed line indicate data in 15th campaign. Tha parameter range is improved than previous campaign. In addition, the fluctuation level decreases due to increased Reynolds number. This means it is a possibility to distinguish the flatten profile driven by changing of the MHD equilibrium or instability. Detailed comparison is a future subject. Now it is ongoing.    Fig.1 The achieved beta is plotted as the function of the magnetic field. In the 15th campaign, the diamagnetic beta value is achieved to 4.1% and 3.4% for B=0.75 and 1T. For a comparison, the achieved beta is plotted before 14th campaign.   Fig.2 The magnetic Reynolds number is plotted as the function of the volume averaged beta. Plots below a dashed line are obtained data before 14th experiment campaign. Data indicated by red circles is new data in the 15th campaign. Magnetic fluctuations are decreased because the Reynolds number is increased. 17§12. Production of High-Beta PlasmaSuzuki, Y., Sakakibara, S.,  Nagasaki, K. (Kyoto Univ.)

Production of High-Beta Plasma

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Production of High-Beta Plasma

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