Electron temperature barriers in the RFX-mod experiment A. Scaggion Consorzio RFX, Padova, Italy Tuesday 5 th October 2010 ADVANCED PHYSICS LESSONS 27/09/2010 07/10/2010 IPP GARCHING JOINT EUROPEAN RESEARCH DOCTORATE IN FUSION SCIENCE ENGINEERING
Summary 1. Overview of RFX-mod experiment 2. Evidence on electron-internal Temperature Barriers 3. e-itbs = TRANSPORT BARRIERS 4. Observation of electron-external Temperature Barriers 5. Recent investigations on e-etbs 6. Conclusions 1/25
RFX-mod: experiment and mission Major radius Minur radius Plasma current Applied toroidal field Stationary phase duration 2 m 0.459 m 2 MA 0.7 T 500 ms RFX-mod is the largest operating RFP and has the most advanced feedback coil system ever realized in a fusion device ( 4 48=192 feedback saddle coils independently controlled ) This lets the physics programme to aim at : - understanding the RFP physics and optimize confinement at high current - contributing to the solution of ITER-relevant physics and technology problems, in particular improving feedback control of magnetic stability 2/25
RFP significant parameters - 1 Reversal parameter: quantifies how reversed is the magnetic field at the edge F = B ϕ B ( a) RFX convention on MHD modes: negative n stands for modes which resonate internally respect to reversal surface ϕ 3/25
RFP significant parameters - 2 Greenwald density: density limit due to radiative collapse or strong discharge deterioration ( the same limit affecting tokamak devices but without disruptions ) n G [10 20 m 3 ] = I p [ MA] πa[ m] n e (10 20 /m 3 ) 4/25
Evidence of a self organized helical plasma The advanced feedback control system opens the possibility of reliable high current operation. Dominant mode When the current increases, the amplitude of the innermost resonant mode ( m=1, n =-7 ) increases and eventually saturates while the secondary modes decrease. [P. Piovesan et al., NF 49, 085036 (2009)] Secondary modes Spontaneous transition to Quasy Single Helicity state with the periodicity of the dominant mode ( m=1, n=-7 ) During a discharge plasma dithers between QSH state and MH (Multiple Helicity) state, but QSH phases become more frequent, longer and purer increasing I p 5/25
Helical state: a new magnetic topology Theory and 3D MHD codes describe a helical ohmic equilibrium self-sustained by a single mode. This is the chaos-free Single Helicity (SH) state. [S. Cappello et al., PPCF 46 B313 (2004)] When the ratio between the dominant and the secondary m=1 modes amplitude exceeds a threshold the plasma undergoes a change of the magnetic topology. Plasma develops a single magnetic axis (SHAx) state characterized by reduced stochastic transport. [Lorenzini et al., Nature Physics 2009] Lorenzini et al., Nature Physics 2009 When b dom / b sec increases Double Axis state (DAx) Single Helical Axis state (SHAx) 6/25
SHAx states trigger the onset of e-itbs Due to the reduced magnetic chaos, during SHAx states we observe an improved confinement with the onset of an electron internal transport barrier (e-itbs) surrounding a large fraction of the plasma volume. [R. Lorenzini et al., PRL 101, 025005 (2008)] L Te ~ 20 cm, χ e ~ 10 m 2 s -1 7/25
e-itbs and magnetic shear e-itbs are correlated to the q profile: a null magnetic shear, corresponding to the maximum of q, is located around the barrier foot In the nex figures ρ stands for the helicoidal flux coordinate ( isobaric, isothermal surfaces ) Gobbin, submitted to PRL 8/25
e-itbs and thermal transport At the barrier the electron thermal conduction is reduced, χ e 5-10 m 2 /s ( T e /T i increases with n/n G, from passive measurement, T e /T i ~ 0.7 @ n/n G < 0.3 ) 9/25
Thermal transport and MHD mode amplitude Thermal transport increases with stochasticity level L Te never found below 0.1m, this indicates that some different gradient-driven mechanism adds to MHD instabilities and limits Te. L Te limit? Lorenzini, to be submitted 10/25
Main features of Internal Transport Barrier They develop when helical equilibrium is set ( regimes of reduced magnetic chaos in high current operation ) and in low collisionality conditions Even in stellarators ITBs onset only at low collisionality The e-barrier develops where q has a maximum ( zero shear ). Even in tokamaks ITBs are related to a flattening of the q profile At the barrier thermal conduction is reduced. Thermal conduction improves with decreasing stochasticity In tokamak and stellarator they are related to a reduction of turbulence Moreover ( see references for more details ): Inside the barrier also particle transport improves ( pellet injection analysis ) [ D. Terranova et al., NF 50, 035006 (2010) ] Barrier acts as an effective barrier for impurities influx [ Menmuir, PPCF 2010 ] Lower limit to L Te suggests the presence of a gradient driven transport mechanism ( triggering of different instabilities) [ S. C. Guo, PoP 15,122510 (2008), I. Predebon et al., PoP 17, 012304 (2010), F. Sattin et al., submitted ] 11/25
Aim of the work In RFX-mod internal transport barriers are related to the SHAX state but the region of improved confinement covers only about 20% of the total plasma volume Not a huge improvement of the global confinement, but EVIDENCES OF EXTERNAL TEMPERATURE BARRIERS on going effort for the characterization ( here lies my own contribution ) In tokamaks and stellarators external transport barriers of temperature, density and angular momentum have led to high confinement regimes (H-modes) Can we have a H-mode even in RFP configuration? Can we produce an ITB on top of an external barrier? 12/25
Observation of electron External Temperature Barrier Beside e-itb during high current operation occasional very high external temperature gradient have been observed 13/25
e-etb vs e-itb typical values Te = 49 kev/m L Te = 9 mm position = 0.8 a extention = 3.5 cm @ I p = 1.54 MA n el = 2,58 10 19 m -3 F = -0.018 e-etb case e-itb case Te = 4.5 kev/m L Te = 123 mm position = 0.45 a extention = 13 cm @ Ip = 1.38 MA nel = 3,85 10 19 m-3 F = -0.05 14/25
Characterization of e-etbs Characterization via Fermi s function f ( r) e A = r B + C 1 D Te = ΔT Δ e L Te = T e Te 15/25
Higher T e (r=0) Higher gradients, lower L Te s Gradient and characteristic length depends on average core temperature 16/25
Operational definition of e-etbs e-etbs has been defined as those cases which seem to disagree with general scaling, in particular those temperature jumps which satisfy both of Te > 40 kev/m L Te < 20 mm 17/25
e-etbs at low collisionality High gradients and short L Te s are favoured at low density e-etbs observed only at low densities ( low collisionality ) 18/25
e-etbs favoured at shallow F Te and L Te depend on the equilibium High gradients and short L Te s are observed only at the lowest values of reversal parameter 19/25
Relation with magnetic topology - 1 No clear correlation between e-etbs and the secondary modes amplitude is observed. Reconstruction of the q profile: in presence of ETBs, the reversal surface is more external ( in agreement with F scaling ) 20/25
Relation with magnetic topology - 2 Analysis with field line tracing codes ( Poincaré plots ) : e-etb are correlated to smaller volume of SOL ( In RFP the Scrape Off Layer can be defined as that volume where connection lengths are much smaller than those of main plasma ) e-etb case NO e-etb case Material wall Ordered magnetic surfaces Ordered magnetic surfaces 21/25
Evaluation of magnetic fluctuation velocity An interesting quantity is the velocity of high frequency ( 1 10 khz ) magnetic fluctuations Evaluation: Use a set of different magnetic probes placed on the wall at different toroidal angles Filter signals mantaining the desired frequency domain ( high-pass filter at 500 Hz) Evaluate the cross-correlation at different istants between each of them and a reference probe Fit the displacement of the cross-correlation maximum in time ( given the probes positions ) 22/25
Relation with magnetic fluctuation velocity Gradient and characteristic length has been found to be correlated with this velocity, anyway no clear correlation has been observed with e-etbs The measured velocity, according to various edge measurements, is opposite to the plasma current but parallel to the electronic diamagnetic drift [ V. Antoni et al, Phys.Rev.Lett. 80(19), 4185 (1998) ] 23/25
Summary on External Temperature Barriers Strong electron temperature gradients observed at r/a~0.78 0.82 with gradients up to 80 kev/m and typical lenght scales of 10 mm They have been observed in low collisionality conditions and usually in high current opration Like e-itbs They are favoured at shallow reversal and more external reversal surface They are correlated to small value of SOL volume First indications of confinement improvement ( 30% higher electron confinement time ) in presence of e-etbs. Latest observation: χ e ~ 10 m 2 s -1 like e-itbs On the other hand: They are not linked to helical equilibrium Unlike e-itbs No dependence on secondary modes amplitude No correlation with high frequency magnetic fluctuations ( 1 10 khz ) They can be related to edge phenomena (under investigation) 24/25
Future work on External Temperature Barriers Further investigation of magnetic topology at the edge ( undergoing analysis ) Improvement of statistics ( especially at I p > 1.5 MA ) An analysis of the density and pressure edge profiles is mandatory ( instrumental upgrade needed ) Evaluate the correlation with edge plasma behaviour ( edge flows, electrostatic fluctuations,... ) Sistematic study of edge plasma magnetic topology during the presence of e-etbs Evaluation of temperature profile time evolution Experimental and numerical upgrades to be performed: Operational recovery of Edge Thomson Scattering diagnostic Improvement of Main Thomson Scattering time resolution Procedure for an automatic evaluation of Scrape-Off Layer extent 25/25