Study of B +1, B +4 and B +5 impurity poloidal rotation in Alcator C-Mod plasmas for 0.75 ρ 1.0.

Transcription

1 Study of B +1, B +4 and B +5 impurity poloidal rotation in Alcator C-Mod plasmas for 0.75 ρ 1.0. Igor Bespamyatnov, William Rowan, Ronald Bravenec, and Kenneth Gentle The University of Texas at Austin, Fusion Research Center Robert Granetz and Dexter Beals MIT Plasma Science and Fusion Center Session QP1: Poster Session VI, Abstract: QP :00pm-5:00pm Wednesday, November 1, 2006 Philadelphia Marriott Downtown - Franklin Hall AB 48th Annual Meeting of the Division of Plasma Physics of the American Physical Society October 30 November ; Philadelphia, Pennsylvania

2 Abstract Poloidal velocities of B +1, B +4 and B +5 impurity ions are measured in Alcator C-Mod tokamak plasmas using charge exchange recombination spectroscopy (CXRS) for B +5 and ambient emission for B +1 and B +4. The set of 25 poloidal optical channels, 10 toroidal optical channels, modulated diagnostic neutral beam and fast Roper CCD camera allow 2mm poloidal spatial resolution in the region 0.75 < r/a < 1.0 and 10 mm toroidal spatial resolution with 13 m temporal resolution at all times during the 1.5 plasma pulse. The variation in the poloidal rotation as the plasma transitions from ohmic to L- to H- mode will be described. Implications for E r will be discussed. Data for EDA H-modes and ITB discharges will also be presented. The emphasis of this work is on comparing the poloidal rotation of B +1, B +4 and B +5 impurities, cataloging t h e effects o f different p lasma modes a n d f i nally o n a ttempting to understand the poloidal rotation based on neoclassical theories. Supported by USDOE grant DE-FG03-96ER54373 and Coop. Agreement DE-FC02-99-ER54512.

3 Motivations Poloidal rotation has been shown to deviate significantly from neoclassical expressions in DIII-D, TFTR and in JET - R. E. Bell, Physical Review Letters 81, 7, 1429 (1998) - K. Crombe, Physical Review Letters 95, (2005) - K. Crombe, EPS conference on Contr. Fusion and Plasma Phys. St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P W.M. Solomon, Physics of Plasmas 13, (2006) - G. M. Staebler, Nuclear Fusion, Vol. 41, 7, 891 (2001) JET proposes that large V p deviation may be due to an effect of heating beams. JET observed the largest excursions of V p from neoclassical values temporally coincide with the reduction in turbulence fluctuations which occurred ms before the ITB formation. TFTR observed a vast V p overshoot (precursor triggering the transition) during the enhanced reversed shear (ERS) discharges. TFTR also found a sudden change in the V p during Li pellet injection which relaxed to the initial value within τ ~ 0.5. DIII-D made a comparison of V p with the neoclassical theory in the steady state portion of the discharge assuming that it is in equilibrium with no consideration of possible V p relaxation from previous transition. JET observed ta V p relaxation (τ ~ ) after the transition to ITB It has been conjectured that drift waves may be responsible for sudden changes of the V p during the plasma transitions.

4 Summary and conclusions In C-Mod, The impurity density must be inferred from CXRS measurements using cross tions which take into account high density and multiple beam components Cross tion effects on poloidal rotation measurements are small Poloidal rotation compares favorably to neoclassical predictions of Kim. After H-mode transitions, the rotation is apparently driven away from the neoclassical value and then relaxes to the value on a long time scale. There is no heating beam to complicate analysis Transiently, the V p can be significantly larger (one order of magnitude) than the neoclassical theory predicts but V p settles to the neoclassical value in a quiescent discharge. The approach to neoclassical equilibrium is examined with an empirical model in which the time derivative of the density is employed as a surrogate for the other physics. The time scale of the V p relaxation (τ ~ 0.4 ) is consistent with TFTR and JET results. The V p plays an important role in fusion plasmas confinement, including the E r B shear suppression of turbulence, stabilization of the plasma instabilities and the formation of the internal transport barrier (ITB). For more details on E r please see the poster GP (R. Bravenec, Gyrokinetic Microstability Analysis of the Inner Boundary of the H-mode Pedestal) and invited talk GI (A. Hubbard, H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod)

5 Analysis Physics The analysis employs a complete set of fine structure components for the CXRS transition. Cross tion effects due to view components parallel to the beam are negligible as the views are essentially perpendicular to the beam. Cross tion effects due to finite lifetime of the excited states are negligible due to the low ion temperature. Ion temperature and measured impurity temperatures are the same due to collisionality. The density of B +5 is based on detailed analysis of excitation cross tions and on a combination of in-situ and post campaign calibration procedures.

6 Charge Exchange Recombination Spectroscopy (CXRS) H 0 + q + ( q 1) + ( n ) + + A A H i A + ( q 1 ( n ) A ( n ) + hν + ( q 1) ) i f CXRS emission (active charge exchange), hydrogen- like B +4 (n=7 6) (visible spectrum) for measurements of n B, T B, V B Blended with a line from a lower ionization stage , (2s3d 1 D 2 2s4f 1 F 3 ) B +1 Blended with (n=7 6) B +4 excited by thermal charge exchange and electron collisions Accompanied by line (n=11 8) B +4 Red spectrum: acquired during DNB pulse Blue spectrum: acquired just after the DNB was turned off Red - Blue = CXRS enhancement ( ) 2s3d 1 D 2 2s4f 1 F 3, B +1 ( ) (n=7 6) B +4 R=0.847 m ( ) (n=11 8) B +4 E B =50 kev

7 Neoclassical prediction of V p of B +5 impurity The neoclassical theory employed here is described in Neoclassical poloidal and toroidal rotation in tokamaks, Y. B. Kim, P. H. Diamond, and R. J. Groebner, Physics of Fluids B 3, 2050 (1991). The poloidal component of the velocity is given by v I p = T Z i i K 1 + 3K Z Z i I 1 L TI 1 L ni + Z Z i I 1 L ni B B t 2 L T T n TI i I i, T, n L I I ni, v, L high density I p ni CXRS measurements TS measurements of K1 and K2 calculated inverse scale lengths plasma, high collisionality n e and from VB array from T, n, B profiles i T I and rotation velocity is derived from width and shift of CXRS spectral lines in the usual way The absolute density for B +5 is described here in some detail as it is a recent development Z eff i of T I, n, n i I profiles

8 Finding the absolute boron density Absolute density optical measurements are always complicated and usually require invessel calibration of the full optical path. The more frequent the calibration is made the more suitable the calibration result for the following experiments. During the plasma discharges the optical parts can be displaced and darkened which makes the preceding calibration obsolete. This is a complex procedure for large tokamaks which are usually opened for in-vessel work once or twice a year. Another way is proposed here for shot to shot calibration measurements. It employs the bremsstrahlung background of each optical channel to calculate the given calibration coefficient. The method uses the Thompson scattering n e, t e and VB array data to predict the bremsstrahlung background for each spatial channel. The validity of this method was confirmed by absolute in-vessel calibration made on 08/10/06. In order to acquire the density of B +5 one may need to know the CXRS effective emission rate coefficients for proper transition. This non-trivial task includes the calculation of the neutral beam propagation and excitation, atomic model of photon emission and correction of CXRS cross tions. I.O. Bespamyatnov, W.L. Rowan, Effects of neutral-beam excited states on charge-exchange emission cross tions, Review of Scientific Instruments 77, 10F

9 The bremsstrahlung calibration # (1.1 ) B +5 density

10 The bremsstrahlung calibration # (1.0 ) B +4 density

11 The scope of the plasma shot #

12 Neoclassical comparison, # time

13 Local n e comparison and V p fits, # n e V eq = 1 km V p - V p fit V 0 = 0 km α = 0.4 τ = 0.4 V eq = V eq = 1 km 1 km V eq = 1 km

14 B +1 and B +4 poloidal velocities (chord averaged), # internal chord internal chord edge chord edge chord

15 B +1 and B +4 poloidal velocities # and internal chord edge chord Conclusions: B +4 poloidal velocity undergoes the similar increase as B +5 poloidal velocity B +1 poloidal velocity also increases during the transition and relaxes to the initial level rapidly during the final part of the transition. The formation of the edge pedestal may control the B +1 velocity evolution since B +1 emits mainly from the region outside the separatrix and edge pedestal position will determine the behavior of the edge, cold, low-charge-state impurities.

16 The scope of the plasma shot #

17 Neoclassical comparison, # time

18 Local n e comparison and V p fits, # V 0 - n e V p - V p fit = 0 km α = 0.4 τ = 0.4 V eq = V eq = V eq = 1 km 1 km 0 km V eq = 2 km

19 The scope of the plasma shot #

20 Neoclassical comparison, # time

21 Model for temporal evolution of V p It was found that poloidal velocity doesn t follow the neoclassical equilibrium solution but strongly depends on temporal transients of the local electron density. It may well be that it is not electron density transients that drive the velocity but some different process which drives both density and poloidal velocity similar in ways. In addition it was observed that poloidal velocity undergoes an equilibration process which pushes the velocity to the neoclassical equilibrium solution. The first attempt was made to produce the simple poloidal velocity model which implies all preceding arguments. dv dt p dn = α dt { V ( 0) = V } 0 e V p V τ eq α driving equilibrium velocity τ equilibration time Only 4 parameters (V 0, V eq, α, τ) describe the model. Some of them may be similar for typical plasma discharges. V V 0 eq initial velocity coefficient

22 The scope of the plasma shot #

23 The scope of the plasma shot # (reversed B)

24 Neoclassical comparison, shots # , Conclusions: There is noticeable correlation between the V p and the local n e. Equilibration of the V p in the absence of the n e transitions is also observed. Multiple transitions on shot do not allow for V p to equilibrate. There is no such vivid correlation for reversed field plasma discharges.

25 Local n e and V p comparison, # V 0 - n e V p - V p fit = 0 km α = 0.4 τ = 0.4 V eq = V eq = V eq = 2 km 1 km 0 km V eq = 2 km

26 Summary and conclusions In C-Mod, The impurity density must be inferred from CXRS measurements using cross tions which take into account high density and multiple beam components Cross tion effects on poloidal rotation measurements are small Poloidal rotation compares favorably to neoclassical predictions of Kim. After H-mode transitions, the rotation is apparently driven away from the neoclassical value and then relaxes to the value on a long time scale. There is no heating beam to complicate analysis Transiently, the V p can be significantly larger (one order of magnitude) than the neoclassical theory predicts but V p settles to the neoclassical value in a quiescent discharge. The approach to neoclassical equilibrium is examined with an empirical model in which the time derivative of the density is employed as a surrogate for the other physics. The time scale of the V p relaxation (τ ~ 0.4 ) is consistent with TFTR and JET results. The V p plays an important role in fusion plasmas confinement, including the E r B shear suppression of turbulence, stabilization of the plasma instabilities and the formation of the internal transport barrier (ITB). For more details on E r please see the poster GP (R. Bravenec, Gyrokinetic Microstability Analysis of the Inner Boundary of the H-mode Pedestal) and invited talk GI (A. Hubbard, H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod)