Macroscopic Stability Research on Alcator C-Mod: 5-year plan

Transcription

1 Macroscopic Stability Research on Alcator C-Mod: 5-year plan Presented by R. Granetz Alcator C-Mod PAC Feb 2008

2 Principal MHD Research Topics in the Next 5-year Plan Effects of non-axisymmetric fields (incl. proposed upgrade to coilset/power supplies) Locked modes; rotation control; NTM thresholds n=1 RMP effects on edge transport (boundary topic) Disruption mitigation Reduction of gas loads; realtime application runaway physics NTMs Threshold scalings; seed island physics Stabilization with LHCD (modify through J(r) control) Study axisymmetric stability of ITER-like equilibria to provide robust axisymmetric control for ITER

3 Principal MHD Research Topics in the Next 5-year Plan (cont.) Alfvén modes and energetic particles TAE stability; ICRF tail losses (heating degradation), incl. proposed fast ion loss detector RSAEs as a sensitive q-profile diagnostic H-mode pedestal stability (covered in Pedestal presentation) effect of higher triangularity on ELMs, regimes of small ELMs n=1 RMP effects on edge transport RF stabilization of sawteeth (covered in RF presentation) suppress NTM trigger; sustain high performance RWMs (β N = 3)?

4 Research on effects of non-axisymmetric fields Sideband and non-resonant contributions to locked mode threshold scalings Rotation braking; Test neoclassical toroidal viscosity (NTV) theory Initial experiment with non-resonant n=2 on C-Mod gave a null result similar experiment on JET supports NTV theory Plan upgraded coilset for further studies: Add multiple power supplies for increased mode spectrum flexibility and magnitude Application to n=1 resonant magnetic perturbation (RMP) studies at plasma edge

5 Disruption mitigation Experiments on C-Mod have been very successful at reducing heating of divertor and decreasing halo currents. Optimal mixture of 10-15% Ar in a helium jet has improved gas delivery time while maintaining good mitigation. Modeling with NIMROD/KPRAD has led to greater understanding of physics processes Realtime detection of VDEs and triggering of the gas jet has been successfully carried out.

6 Disruption mitigation research plans Extend realtime detection with the digital plasma control system to include other types of disruptions (locked mode, density limit, high β, etc.) Preliminary test of realtime locked mode detection shows promise Cessation of sawtoothing is a reliable indication of mode locking in C-Mod The plasma control system recognized the cessation of sawtoothing in realtime and issued a trigger to the gas jet hardware (which was purposely disabled for this initial test).

7 Disruption mitigation research plans (cont.) Find minimum level of gas mixture required for mitigation on C-Mod Reduce gas load on pumping system and diagnostics; shorten pumpout/recovery time Ultimate goal: have gas jet system regularly enabled on disruption-prone runs Continue to advance NIMROD/KPRAD modeling Provide understanding of the gas jet assimilation physics, quenching process, and timing vs gas species

8 Disruption mitigation research plans (cont.) May also prove important for understanding runaway avalanching and loss mechanisms

9 Disruption mitigation research plans (RE) Study physics of runaway electrons (RE) Quenching avalanching by density buildup alone requires gas loads of order 10 5 Pa-m 3 (Rosenbluth criterion) in ITER. This has serious implications for the cryopump systems and the tritium handling plant, particularly with mixed noble gases. Present experiments suggest that other mechanisms may suppress avalanching by enhancing RE transport losses Suggests that huge gas loads may not be required. Confinement/loss of runaways can be studied in C-Mod Use LHCD to generate a seed population prior to a disruption Spatially-resolved HXR and synchrotron imaging diagnostics can study runaway transport in conjunction with different quenching mechanisms (for example: non-axisymmetric fields)

10 Disruption mitigation research plans (RE) I () t = I exp[( γ 1/ τ ) t] RE RE, seed Rosenbluth RE Worst-case scenario No RE losses: τ RE = I RE Not seen: low magnitude since R Cmod << R ITER I RE,seed ~ dt e dt Slideaways Not measured, hyper-sensitive to dt/dt γ Rosenbluth ~ E // E c ~ η j n e Measured. Can match ITER τ 1 RE ~ B r B Stochastic losses (e.g. NIMRAD) thought important, but cannot measure due to other unknowns in rate equation

11 Disruption mitigation research plans (RE) I () t = I exp[( γ 1/ τ ) t] RE RE, seed Rosenbluth RE I RE Measured: I RE ~ I p I RE,seed I LH Set by LH fast electrons γ Rosenbluth ~ E // E c ~ η j n e Measured. Can match ITER τ 1 RE ~ B r B Stochastic (or other) loss mechanisms quantified and understood

12 Study runaway physics and quenching, cont. Numerous knobs can be varied: Applied non-axisymmetric fields (i.e. δb), including amplitude, m and n, and resonant vs non-resonant Gas jet parameters, particularly total injected atoms, and gas atomic number (total injected electrons) LHCD parameters, such as power (varies the initial amount of seed electrons), and phasing (varies the spatial profile of seed electrons) Wall deposition of runaway energy?

13 Disruption mitigation research plans (cont.) Runaway electrons have been observed on FT-U during disruptions in discharges that had LH applied prior to the disruption. J.R. Martin-Solis, et al, Phys. Rev. Lett. 97 (2006)

14 D 2 opacity for first wall survival in a reactor Reactor (Aries RS) parameters: R=5.52 m, a=1.38 m, kappa=1.89 Surface area: ~ 560 m 2 W th ~ 745 MJ If during a thermal quench, W th is radiated in ~ 1 ms, with perfect spatial uniformity, the surface loading will be 42 MJ/m 2 /s 1/2, which is right at the melt limit for tungsten and the ablation limit for carbon. Any spatial non-uniformity will result in loss of 10 s-100 s of kg of wall material. At very high densities (>10 22 m -3 ) and low temperatures (< 0.8 ev), D 2 could become a blackbody radiator. This would effectively spread out the radiation pulse in time, and assure spatial uniformity. Use D 2 gas jet injection to study radiation energy transport in highopacity conditions natural extension of our present divertor opacity studies (10 22 m -3, 1.5 ev)

15 D 2 opacity for first wall survival, cont Brightness (1018ph/s/m2/sr) Optically thin Optically thick D α brightness x A 3-1 A Ly 3-2 β Time (seconds)

16 NTM physics Higher β N and lower ν* planned in our program over the next 5 years are expected to destabilize NTMs. NTM physics could then be studied at C-Mod conditions and parameters, including: Threshold β N scalings Critical seed island physics (by ramping β N down) Rotation effects on NTM threshold β N And in particular: Effect of non-axisymmetric fields on NTM threshold NTM stabilization by external current drive (LHCD for C-Mod)

17 NTM stability studies using LHCD A potential application of LHCD on ITER is for NTM stabilization LHCD efficiency is higher than ECCD Stabilization physics is different: ECCD stabilization works mainly by driving current centered on the island (to replace missing bootstrap current) LHCD stabilization works mainly through the term, i.e. by modifying the current profile near the resonant surface

18 NTM stability studies using LHCD (cont.) We have successfully demonstrated the use of LHCD to stabilize and destabilize classical tearing modes. The same modification with LHCD should carry over directly to NTM stabilization. Destabilization n // =2.3 Stabilization n // =1.6

19 ITER needs for axisymmetric plasma control ITER needs to be confident of axisymmetric control when operating close to machine limits ITER s post-eda design is not as conservative, leading to considerable concern about the stability of high-l i plasmas. Desire smart adaptive control algorithms beyond those available today. Algorithm development and practical experience should be pursued on existing tokamaks in the next few years. ITER also has stringent limits on measurement noise and fluctuations in the feedback loop. Current machines, such as C-Mod, can address the issue of noise and how to suppress/reject it.

20 C-Mod planned contribution to axisymmetric control in ITER (cont.) Design and experimental validation of high order controllers to improve controllability of high-κ, high-l i plasmas. The Alcasim (Alcator simulator) tool will be especially useful for investigating new control algorithms. Analysis of the sources of noise and pickups. Estimation of their effects on axisymmetric control (control precision, power supply and PF coil requirements, etc.). Noise suppression through model-based filters. Design and experimental validation of "safe scenarios" and adaptive interpolation algorithms to be deployed in case of power supply saturation.

21 Energetic Particle Studies: Resources Active MHD system: Active MHD antennas can measure the damping of ITER relevant moderate toroidal mode number (n ~ 10) Alfvén eigenmodes. Two antennas excite stable Alfvén eigenmodes n ~ < 20 FWHM. Diagnostics Mirnov coils in toroidal and poloidal arrays Phase contrast imaging diagnostic (PCI) to measure core density fluctuations. CNPA to measure distribution function of confined energetic ions. Hard X-ray camera for measuring electron spatial and energy distribution. Sources of energetic particles: Flexible ICRF system that produces primarily trapped fast ions. LHRF system produces energetic electrons. Codes: NOVA-K comparisons of AE damping, drive, ICRF distribution TRANSP/TORIC models of ICRF wave solver and fast ions AORSA/CQL3D ICRF fast particle distribution + orbit effects

22 Active MHD Antenna Excites Moderate n Stable Alfvén Eigenmodes Goal: Measure AE damping rates with and without fast ion drive. Status: Measured AE damping rates as function of magnetic topology. Plans: Measure AE damping rates in presence of energetic ion drive and compare with simulations. Utilize MSE and polarimetry to constrain q profiles. Upgrade Active MHD amplifier to improve signal/noise ratio.

23 Modeling Alfvén Cascades Provides q Evolution J A Snipes, et al, 2005 Phys Plasmas, The frequency evolution of Alfvén cascades can be modeled to determine the evolution of the minimum q value to compare with MSE Alfvén cascades indicate very low or reversed shear so that they can be used to sensitively diagnose changes in the current density profile in the current rise due to LHCD and ICRH

24 Measured Radial Structure of Alfvén Cascades Phase Contrast Imaging (PCI) measures 32 vertical chord integrals of the density perturbations of Alfvén cascades Through synthetic PCI calculations and NOVA modeling, the PCI measurements constrain the radial structure of the modes A 32 channel ECE system will also be enhanced to provide local measurements of the radial structure of Alfvén eigenmodes with high time resolution

25 MHD Induced Fast Ion Loss Diagnostic fast ions Fast ion 3D-Collimator v tot = v + v // A magnetic spectrometer using the TF images lost fast ions onto a scintillator plate as a function of their energy (gyroradius) and pitch angle The high time resolution (1 MHz) will allow measurements of the ICRF heated fast ions lost due to Alfvén eigenmodes to better quantify fast particle transport and compare with modeling plasma Scintillator housing M Garcia-Munoz, IPP-Garching

26 Summary of Goals Disruption mitigation, including runaway electrons Study effects of nonaxisymmetric fields Characterize and improve axisymmetric stability of ITER-like equilibria Neoclassical tearing modes (NTMs) TAEs and fast particle instabilities Finalize optimization of gas mixture, pressure, and injected quantity Continue development of real-time disruption prediction and mitigation activation for additional types of disruptions Continue collaboration with modelers on NIMROD/KPRAD simulations of gas jet disruption mitigation Achieve D 2 opacity conditions and study radiation rates and spatial uniformity Study physics of runaway electron physics during disruptions and mitigation using unique C-Mod tools (LHCD, hard x-ray and synchrotron diagnostics, error-field coils) Magnetic braking of rotation; tests of neoclassical toroidal velocity (NTV) theory Explore resonant magnetic perturbations for affecting/controlling edge pedestals and ELMs Explore effects on NTM seed islands Upgrade A-coil power supplies and coilset Develop advanced controllers for stability at higher elongation Characterize effects of noise on feedback and develop noise rejection/suppression algorithms Develop safe scenarios and adaptive algorithms in case of power supply saturation/failure LHCD stabilization of NTMs by modification of ICRF stabilization of NTMs by eliminating sawtooth seed islands Characterize dependence of intermediate-n TAE damping rate on plasma parameters Benchmarking codes (NOVA-K, AORSA/CQL3D) against experiment Study TAE-induced fast ion loss (ICRF-generated ions) Combine measurements and modeling of reversed shear AEs (Alfven cascades) to provide information on q- profile modification by LHCD Diagnostic upgrades (two more antennas; CNPA array, fast ion loss diagnostic, fast ion D α diagnostic, 2 nd PCI)

27 ITPA MHD contributions Description Disruption mitigation by massive gas jets Disruption runaway physics and mitigation Vertical stability of ITER-like equilibria Error field effects on NTMs JOINT Experiments MDC-1 (DSOL- 11) Notes on C-Mod Contributions Optimise gas mixture; injection quantity, real-time detection and triggering Under discussion LHCD-driven seed; hard x- ray and synchrotron diagnostics MDC-13 MDC-3 Advanced controllers; noise characterization and rejection Sawtooth control for NTM suppression Current drive prevention/stabilisation of NTMs TAE intermediate-n damping rates Fast ion losses and redistribution from TAEs Rotation effects on NTMs MDC-5 ICRF; LHCD MDC-8 LHCD modification of MDC-10 Compare with JET MDC-11 New fast ion loss detector MDC-14 Non-resonant magnetic braking MDC-12 Compare with JET Resonant magnetic perturbation effects on ELMs and pedestal PEP-19 C-Mod will do n = 1