RFX/RFP mode control issues Piero Martin & Sergio Ortolani Consorzio RFX Associazione Euratom-ENEA sulla fusione Padova, Italy Presented by P. Martin at the 2003 workshop on “active control of MHD stability: extension to the burning plasma regime” University of Texas-Austin Nov. 3-5, 2003

CONTRIBUTORS L. Marrelli, G. Spizzo, P. Franz, P. Piovesan, I. Predebon, T.Bolzonella, S. Cappello, A. Cravotta, D.F. Escande, L. Frassinetti, S. Martini, R. Paccagnella, D. Terranova and the RFX team And fruitful collaborations with the MST team: B.E. Chapman, D. Craig, S.C. Prager, J.S. Sarff EXTRAP T2R team: P. Brunsell, J.-A. Malmberg, J. Drake TPE-RX team: Y. Yagi, H. Koguchi, Y. Hirano

Hopefully this is the last workshop… …. without RFX ! …. without RFX ! …. RFX reconstruction is in the final phase and first plasmas are expected in Sept …. RFX reconstruction is in the final phase and first plasmas are expected in Sept The new RFX will be a “state of the art” MHD MODE CONTROL FACILITY: The new RFX will be a “state of the art” MHD MODE CONTROL FACILITY: 192 ACTIVE COILS, INDEPENDENTLY DRIVEN, COVERING THE WHOLE PLASMA SURFACE 192 ACTIVE COILS, INDEPENDENTLY DRIVEN, COVERING THE WHOLE PLASMA SURFACE

Outline of the talk 1. Status of the RFX reconstruction 2. RFP mode control issues a. Mode rotation b. The monochromatic dynamo c. Suppression of dynamo modes d. RWMs 3. RFX operational scenarios

1. Status of the RFX reconstruction RFP mode control issues a. a. Mode rotation b. b. The monochromatic dynamo c. c. Suppression of dynamo modes d. d. RWMs RFX operational scenarios

The new RFX device Main new components: saddle coils, covering the whole plasma boundary, each independently powered and feedback controlled 2.a smoother and thinner shell 3.the first wall with higher power handling capabilities 4.an in-vessel system of magnetic and electrostatic probes 5.the toroidal field power supply R /a = 2.0 / 0.46 (m) - I up to 2 MA

Overview of the magnetic boundary - 4 coils in the poloidal direction: 90° spaced – 48 coils in the toroidal direction: 7.5° spaced shell vessel Active saddle coils

Saddle coil performance each independently powered 24 kAt: 400 A x 60 turns Wide spectrum of Fourier components can be produced: m=1,2 n ≤ 24 DC < f < 100 Hz Significant amplitude available. For example: edge br for (1,8) mode: 20 Hz 1.3 Hz

The new RFX shell 3 mm Cu layer3 mm Cu layer 50 ms time constant (450 ms before)50 ms time constant (450 ms before) High reduction of gap field errorHigh reduction of gap field error 1 overlapped poloidal gap 1 toroidal gap on high field side.

Magnetic probe 97 Electrostatic (Langmuir) probes 139 Magnetic pick up probes 8 Calorimetric probes Langmuir probe Integrated System of the Internal Sensors (ISIS)

Test of RFX mode control equipment in T2R Some RFX digital controllers have been moved to Stockholm to be tested in an “intelligent shell” experiment, which will be done in the EXTRAP T2R device as a joint T2R-RFX collaboration. Some RFX digital controllers have been moved to Stockholm to be tested in an “intelligent shell” experiment, which will be done in the EXTRAP T2R device as a joint T2R-RFX collaboration. Preliminary results with analog controllers developed by KTH positive! Preliminary results with analog controllers developed by KTH positive! MORE IN JIM DRAKE’S TALK WEDNESDAY! MORE IN JIM DRAKE’S TALK WEDNESDAY! 16 coils Partial coverage

Outline of the talk 1. Status of the RFX reconstruction 2. RFP mode control issues a. a. Mode rotation b. b. The monochromatic dynamo c. c. Suppression of dynamo modes d. d. RWMs RFX operational scenarios

What shall we use the new RFX for ? To explore the RFP physics and to optimize the RFP confinement performance in a steady fashion in the MA current range To explore the RFP physics and to optimize the RFP confinement performance in a steady fashion in the MA current range To contribute to the worldwide program on MHD modes control in fusion devices To contribute to the worldwide program on MHD modes control in fusion devices

What do we use the new RFX for ? To explore the RFP physics and to optimize the RFP confinement performance in a steady fashion in the MA current range To explore the RFP physics and to optimize the RFP confinement performance in a steady fashion in the MA current range Control of MHD modes: Control of MHD modes: m=1 “dynamo” modes (resonant inside the Bt reversal surface) m=1 “dynamo” modes (resonant inside the Bt reversal surface) m=0 non-linearly generated and/or linearly unstable m=0 non-linearly generated and/or linearly unstable The standard RFP has many of these modes with 1% B amplitude simultaneously present, and they spoil confinement! RWM when the shell is resistive (reactor relevance) RWM when the shell is resistive (reactor relevance)

The modes we have to deal with m=0 m=0 various n: resonant at the B t reversal surface various n: resonant at the B t reversal surface m=1 m=1 |n| ≥ 2R/a, resonant inside the B t reversal surface (resistive kink, “dynamo modes”) |n| ≥ 2R/a, resonant inside the B t reversal surface (resistive kink, “dynamo modes”) |n| ≤ 2R/a, internally non resonant from above (RWM, with the same helicity as the “dynamo” modes and the same handedness as the core B) |n| ≤ 2R/a, internally non resonant from above (RWM, with the same helicity as the “dynamo” modes and the same handedness as the core B) |n| ≤ R/a, externally non resonant (RWM, with opposite helicity) |n| ≤ R/a, externally non resonant (RWM, with opposite helicity)

The RFP dynamo issue The electrical currents flowing in a RFP can not be directly driven by the inductive electric field E o The electrical currents flowing in a RFP can not be directly driven by the inductive electric field E o ….but RFP plasmas last for times much longer than the resistive diffusion time ! (actually, as long as E o is applied) ….but RFP plasmas last for times much longer than the resistive diffusion time ! (actually, as long as E o is applied)

The RFP dynamo: E + vxB =  J An additional electric field, besides that externally applied, is necessary to sustain and amplify the toroidal magnetic flux. An additional electric field, besides that externally applied, is necessary to sustain and amplify the toroidal magnetic flux. A Lorentz contribution vxB is necessary, which implies the existence of a self-organized velocity field in the plasma. A Lorentz contribution vxB is necessary, which implies the existence of a self-organized velocity field in the plasma. The origin of this contribution is the classical RFP dynamo problem The origin of this contribution is the classical RFP dynamo problem

Turbulent dynamo: remarkable self-organization A wide experimental and numerical database supports the MHD turbulent dynamo theory: A wide experimental and numerical database supports the MHD turbulent dynamo theory: the dynamo electric field is produced by the coherent (and non-linear) interaction of a large number of MHD modes: Multiple Helicity (MH) dynamo

The standard Multiple Helicity (MH) RFP m=1 “dynamo” modes (resonant inside the Bt reversal surface) m=1 “dynamo” modes (resonant inside the Bt reversal surface) m=0 non-linearly generated and/or linearly unstable m=0 non-linearly generated and/or linearly unstable Magnetic stochasticity allover the plasma ! … and severe plasma-wall interaction if the modes lock in phase and to the wall !

The strategy towards dynamo modes Keep them rotating in the lab frame Keep them rotating in the lab frame Reduces amplitude b r Reduces amplitude b r Optimizes the basic standard target plasma Optimizes the basic standard target plasma Make their amplitude lower Make their amplitude lower …but you must provide dynamo electric field from outside …but you must provide dynamo electric field from outside Run the plasma in a regime where resort to dynamo is reduced Run the plasma in a regime where resort to dynamo is reduced Work in a regime where their spatial spectrum is monochromatic, i.e. dynamo is driven only by ONE INDIVIDUAL SATURATED MODE Work in a regime where their spatial spectrum is monochromatic, i.e. dynamo is driven only by ONE INDIVIDUAL SATURATED MODE ALL THESE TOPICS MIGHT BE INFLUENCED BY ACTIVE CONTROL !

Outline of the talk Status of the RFX reconstruction 2. RFP mode control issues a. Mode rotation b. b. The monochromatic dynamo c. c. Suppression of dynamo modes d. d. RWMs RFX operational scenarios

Mode dynamics in RFPs Previous experimental evidence in several different devices shows that the evolution of MHD modes, including the dynamo modes, depends on the magnetic boundary, and in particular on the shell: Previous experimental evidence in several different devices shows that the evolution of MHD modes, including the dynamo modes, depends on the magnetic boundary, and in particular on the shell: thickness thickness proximity proximity geometry geometry

Experiment R/a (m) b/a  shell   ms)  pulse (ms)  pulse/  shel RFX922/ /3 RFX new 2/ ~ 150 ≥ 3 MST1.5/ ¼ TPE RX 1.72/ /5 T2R1.24/ >3 The RFP synopsis

TPE-RX spontaneous mode rotation b 1 /a = 1.08 thin shell, b 2 /a = 1.16 thick shell t shell =10 ms t shell =330 ms

Spontaneous Rotation in EXTRAP T2R RWM’s Tearing modes rotate From Malmberg Brunsell PoP 2002

MST modes spontaneous rotation ….listen Brett Chapman’s invited talk ! ….listen Brett Chapman’s invited talk !

Conclusions on mode rotations Mode rotation is beneficial and depends on magnetic boundary Mode rotation is beneficial and depends on magnetic boundary Modes were locked to the wall in the old RFX and this lead to a serious deterioration of performance Modes were locked to the wall in the old RFX and this lead to a serious deterioration of performance Slow rotation of modes was actively driven in RFX ( Bartiromo et al, PRL 99) Slow rotation of modes was actively driven in RFX ( Bartiromo et al, PRL 99) Dynamo modes are spontaneously rotating in RFP devices with boundary conditions similar to the new RFX. Dynamo modes are spontaneously rotating in RFP devices with boundary conditions similar to the new RFX. There is a reasonable basis to hope for spontaneous mode rotation in the new RFX (even if a threshold in current might exist) There is a reasonable basis to hope for spontaneous mode rotation in the new RFX (even if a threshold in current might exist)

Outline of the talk Status of the RFX reconstruction 2. RFP mode control issues a. a. Mode rotation b. The monochromatic dynamo c. c. Suppression of dynamo modes d. d. RWMs RFX operational scenarios

Magnetic chaos is not intrinsic to RFP DYNAMO CAN BE PRODUCED BY A SINGLE MHD MODE

The Single Helicity (SH) dynamo a theoretically predicted state with a unique m = 1 saturated resistive kink (a pure helix wound on a torus), a theoretically predicted state with a unique m = 1 saturated resistive kink (a pure helix wound on a torus), Stationary LAMINAR dynamo mechanism with good helical flux surfaces Stationary LAMINAR dynamo mechanism with good helical flux surfaces Escande et al., PRL 85 (2000)

Magnetic order with SH dynamo Good magnetic flux surfaces in SH SH Turbulent (MH) Overlapping of many modes !

Helical states in the experiment Quasi Single Helicity (QSH) spectra have been observed in all RFP devices, under a variety of boundary conditions (Martin, NF 2003). Quasi Single Helicity (QSH) spectra have been observed in all RFP devices, under a variety of boundary conditions (Martin, NF 2003). The mode spectrum is dominated by one geometrical helicity The mode spectrum is dominated by one geometrical helicity The other modes have still non-zero amplitude The other modes have still non-zero amplitude

Stationary Quasi-Single Helicity Stationary QSH spectra have been observed in the RFX device Stationary QSH spectra have been observed in the RFX device with a helical coherent structure emerging from magnetic chaos in the plasma core. with a helical coherent structure emerging from magnetic chaos in the plasma core. Toroidal mode number n spectrum vs. time RFX pulse length

Flow velocities measurements in QSH plasmas Plasma flow velocity fluctuations measured in MST with Doppler spectroscopy (Den Hartog et al., Phys Plasmas 99) In QSH not only the spectrum of magnetic fluctuation spectra become narrower in comparison with MH, but also that of flow velocity fluctuations In QSH not only the spectrum of magnetic fluctuation spectra become narrower in comparison with MH, but also that of flow velocity fluctuations Remember: Work due to D. Craig, L. Marrelli, P. Piovesan in MST

Magnetic and Flow velocity fluctuations toroidal spectra

Dynamo electric field in QSH Dynamo in QSH becomes more concentrated in one mode than in standard MHD plasmas! Dynamo in QSH becomes more concentrated in one mode than in standard MHD plasmas!

MH vs. QSH vs. SH

QSH and mode wall locking The access to QSH regime is beneficial for the problem of modes wall locking. The access to QSH regime is beneficial for the problem of modes wall locking. The dominant (big) mode might be more prone to lock to the wall (see Brett Chapman’s talk), but… The dominant (big) mode might be more prone to lock to the wall (see Brett Chapman’s talk), but… Non-linear interaction between modes decreases Non-linear interaction between modes decreases The “strength” of mode locking decreases The “strength” of mode locking decreases Easier rotation for secondary modes Easier rotation for secondary modes

Spontaneous mode rotation in RFX during QSH Dominant and Rotating Mode amplitude Phase of Rotating Mode From Bolzonella,Terranova, PPCF 2002 This is consistent with theoretical calculations (Guo-Chu, Fitzpartick,Chapman), which predict that modes rotate more easily if they are smaller

PWI in QSH is milder anyway ! Vertical displacement of the plasma column in RFX Vertical displacement of the plasma column in RFX Toroidal angle Edge fluctuating magnetic field Edge fluctuating magnetic field MH QSH

Favorable conditions for QSH active control 1. There are already plasma regimes where monochromatic spectra are more easily obtained spontaneously a. At higher plasma current b. With shallower reversal 2. We can also “select” the toroidal mode number n we wish to be the dominant one.

Mode selection Note that the pre-programming the magnetic equilibrium allows to select efficiently the mode that will dominate the spectrum! Note that the pre-programming the magnetic equilibrium allows to select efficiently the mode that will dominate the spectrum!

Shallow reversal, m=0 modes and QSH Shallow reversal brings outwards the reversal surface, where q=0. Shallow reversal brings outwards the reversal surface, where q=0. Narrower stochastic region produced by m =0 when the reversal surface is closer to the plasma edge. Narrower stochastic region produced by m =0 when the reversal surface is closer to the plasma edge. This provides a smoother plasma boundary, which helps the onset of QSH This provides a smoother plasma boundary, which helps the onset of QSH In the new RFX m=0 modes are not any more LINEARLY unstable, as they were in the OLD device. In the new RFX m=0 modes are not any more LINEARLY unstable, as they were in the OLD device. Positive feedback, since in QSH non- linear generation of m=0 modes is strongly reduced. Positive feedback, since in QSH non- linear generation of m=0 modes is strongly reduced.

Remember we can apply with the saddle coils up to ~20 mT on a single m=1 mode  Numerical studies on active control: successful drive and sustainment of m=1 n=7 SH state starting from MH conditions Low dissipation conditionsLow dissipation conditions Thin shellThin shell Simoultaneous active control of RWMs !Simoultaneous active control of RWMs ! Paccagnella, MHD workshop 2002

Conclusions on Single Helicity There is enough theoretical understanding and experimental evidence which support the idea that this regime might lead to significant improvement of RFP performance There is enough theoretical understanding and experimental evidence which support the idea that this regime might lead to significant improvement of RFP performance We know how to produce a target plasma, which more easily could achieve a QSH spectrum. We know how to produce a target plasma, which more easily could achieve a QSH spectrum. From an active control point of view, QSH is a robust state: From an active control point of view, QSH is a robust state: One big mode, well identified, selectable in advance One big mode, well identified, selectable in advance RFX has more than enough power to deal with QSH RFX has more than enough power to deal with QSH

Outline of the talk Status of the RFX reconstruction 2. RFP mode control issues a. a. Mode rotation b. b. The monochromatic dynamo c. Suppression of dynamo modes d. d. RWMs RFX operational scenarios

Dynamo modes active reduction Pulsed Poloidal Current Drive (PPCD): Pulsed Poloidal Current Drive (PPCD): the induction of a poloidal current at the plasma edge causes a dramatic reduction of the magnetic turbulence (MST + RFX PRLs) and STRONG PLASMA HEATING the induction of a poloidal current at the plasma edge causes a dramatic reduction of the magnetic turbulence (MST + RFX PRLs) and STRONG PLASMA HEATING ROBUST TECHNIQUE (recently performed in T2R at high aspect ratio-many modes to suppress!- Cecconello, PPCF 2003) ROBUST TECHNIQUE (recently performed in T2R at high aspect ratio-many modes to suppress!- Cecconello, PPCF 2003) It is TRANSIENT, but in RFX a quasi-stationary version has been implemented

Oscillating Poloidal Current Drive (OPCD) Periodic (oscillating) applied inductive variations of the poloidal electric field allows to extend the PPCD benefits in a stationary fashion Periodic (oscillating) applied inductive variations of the poloidal electric field allows to extend the PPCD benefits in a stationary fashion stationary average improvement of confinement obtained with OPCD in RFX ( Bolzonella et al., PRL 2001 )

PPCD action strongly affects MHD in the RFP Though transient (but quasi stationary version is feasible), PPCD is an efficient tool to interact actively with MHD modes in the RFP. Though transient (but quasi stationary version is feasible), PPCD is an efficient tool to interact actively with MHD modes in the RFP. Why might be useful for future operation in the new RFX ? 1. “per se”: the new RFX coils system allow optimized, high power, quasi stationary PCD. This is a technique for improving confinement. 2. It can set-up an improved collisionless target plasma on which to work with feedback Record reduction of magnetic stochasticity Record reduction of magnetic stochasticity Change the properties of broadband magnetic turbulence Change the properties of broadband magnetic turbulence

Active control issue: PPCD triggers QSH spectra Evolution of m=1 modes in MST following the application of PPCD Evolution of m=1 modes in MST following the application of PPCD

Heat pulse propagation observed Significant transport barrier present in the plasma Significant transport barrier present in the plasma time (ms) SXR brightness (W/m 2 )

Modes decreased to very low amplitude! Beating in the SXR signals of the frequencies corresponding to TWO helical structures: Beating in the SXR signals of the frequencies corresponding to TWO helical structures: the (1,6) and the (1,7) mode the (1,6) and the (1,7) mode Beating recorded: Beating recorded: at the dominant frequency at the dominant frequency  f ~ f 1,6 -f 1,7  f ~ f 1,6 -f 1,7 and at the first harmonic and at the first harmonic  f  f

Two rotating islands Modes are so small that magnetic chaos is strongly reduced Modes are so small that magnetic chaos is strongly reduced We observe individual TINY rotating magnetic islands associated with the (1,6) and (1,7) modes: a marker of stochasticity suppression. We observe individual TINY rotating magnetic islands associated with the (1,6) and (1,7) modes: a marker of stochasticity suppression t1t1 t2t2 t3t3 t4t4 Franz, Marrelli et al, submitted to PRL

Numerical simulation confirms ORBIT used for Poincare’ maps of the magnetic field lines (with experimental mode amplitudes as inputs) confirms the presence of two islands ORBIT used for Poincare’ maps of the magnetic field lines (with experimental mode amplitudes as inputs) confirms the presence of two islands

Outline of the talk Status of the RFX reconstruction 2. RFP mode control issues a. a. Mode rotation b. b. The monochromatic dynamo c. c. Suppression of dynamo modes d. RWMs RFX operational scenarios

RFP & RWMs (see J. Drake’s talk) EXTRAP T2R experience (thin shell device) demonstrates that RFP discharges with duration ≥2 shell times can be produced without significant effects due to RWMs EXTRAP T2R experience (thin shell device) demonstrates that RFP discharges with duration ≥2 shell times can be produced without significant effects due to RWMs If EXTRAP T2R experience is confirmed in RFX, time scales of dynamo and RWMs are separated and we should have a reasonable amount of time (≤ 100 ms) to cope with dynamo modes before dealing with RWMs. If EXTRAP T2R experience is confirmed in RFX, time scales of dynamo and RWMs are separated and we should have a reasonable amount of time (≤ 100 ms) to cope with dynamo modes before dealing with RWMs.

Outline of the talk Status of the RFX reconstruction 2. RFP mode control issues a. a. Mode rotation b. b. The monochromatic dynamo c. c. Suppression of dynamo modes d. d. RWMs 3. RFX operational scenarios

Control strategies and plans - 1 i. Benchmark and improve old RFX performance (passive or m=0 perturbations) ii. Explore new regimes with active actions on the MHD through the coils

RFX operational scenarios - 1 Benchmark and improve old RFX performance Benchmark and improve old RFX performance Actions through an applied m=0 mode (TF coils): Actions through an applied m=0 mode (TF coils): a. Synchronous driving torque for mode rotation also in closed loop mode b. PPCD c. OPCD

RFX operational scenarios - 2 Active actions through 192 saddle coils : Apply m=1 magnetic perturbations Apply m=1 magnetic perturbations Work on individual modes: one at the time or several simultaneously Work on individual modes: one at the time or several simultaneously Realize an intelligent shell Realize an intelligent shell Zeroing of radial field at the edge to maintain an effective close fitting shell. Zeroing of radial field at the edge to maintain an effective close fitting shell. Might be interesting for QSH studies, since we have evidence that a smooth magnetic boundary facilitates their onset. Might be interesting for QSH studies, since we have evidence that a smooth magnetic boundary facilitates their onset.

RFX operational scenarios - 3 Drive of m=1 magnetic perturbations Apply a monochromatic perturbation to affect one individual mode: Apply a monochromatic perturbation to affect one individual mode: i. “pumping” the mode to drive QSH states through helical fields at the plasma boundary ii. Feedback stabilization of individual modes iii. inducing rotation of a single mode Apply several simultaneous geometrical helicities (various n’s): Apply several simultaneous geometrical helicities (various n’s): a. damping of main “dynamo modes” b. feedback stabilization of RWM c. breaking phase locking among “dynamo modes” with induction of modes differential rotations

RFX operational scenarios - 4 Low current scenario Low current scenario 1. Theoretical work (Guo, Fitzpatrick et al) and experimental data (TPE-RX, EXTRAP T2R, MST) suggest that low current operation could allow for spontaneous dynamo mode rotation. 2. If dynamo modes rotate, this scenario is more suitable to concentrate efforts on RWM control High current scenario (> 1 MA) High current scenario (> 1 MA) Better for confinement improvement techniques (OPCD) and for interaction with “dynamo” modes (but higher wall-locking probability). Better for confinement improvement techniques (OPCD) and for interaction with “dynamo” modes (but higher wall-locking probability). Passive shell (and EXTRAP T2R experience) might postpone RWM issue up to ≈ ms Passive shell (and EXTRAP T2R experience) might postpone RWM issue up to ≈ ms RFX IS THE ONLY EXPERIMENT DESIGNED TO EXPLORE RFP PHYSICS IN THE MA REGIME !

Thanks to the flexibility of the coils system, different actions can coexist in the same pulse > 1 MA QSH Self-similar decay + QSH PPCD T ~50-70 ms ~50 ms Active drive of (1,n dom ) Feedback on RWM feedback on m=0

… comments, and collaborations, welcome !