Macroscopic Stability FESAC Facilities Panel Meeting June 13, 2005 E. S. Marmar for the Alcator Group
Unique C-Mod Properties Guide MHD Research Program High field, high current density, compact size, metal walls Disruption characterization Disruption mitigation High field ITER-like kinetic parameters at moderate β Avoid resistive wall modes, NTMs not yet an issue yet, but likely in AT program (β N >2) Active MHD antennas (moderate-n) + ICRF Alfvén eigenmodes and cascades Dimensionless scaling at ITER field, pressure, β Error fields/locked mode threshold studies NTM threshold and stabilization
C-Mod is a Major Contributor to Understanding, and Control of Disruptions C-Mod is key contributor to ITER halo current database Toroidal asymmetry discovered on C-Mod ITER Halo Current Disruption Database Changing inner divertor geometry had significant effects Reduced halo currents Changed toroidal asymmetry Needs to be understood Could have important potential for ITER improvement Carry out additional science and technology activities supporting ITER
Disruption Mitigation Studies Being Extended to High Plasma Pressure Goal is to reduce thermal loads and halo currents at ITER values of plasma pressure and energy density High-pressure noble gas jet technique, which gave encouraging results on DIII-D, is now implemented on C-Mod. Optimized gas jet system installed very close to plasma edge 3-D MHD modeling (NIMROD) of gas jet experiments Outlet only 3 cm from LCFS Carry out additional science and technology activities supporting ITER
Gas Jet Experiments Already Yielding Results Interesting results from very first experiments Neon more effective than helium Faster current quench, lower halo currents Will also use argon So far, have studied I P = 1 MA, <P plasma > 1 bar, P jet <70 bar Ultra-fast (up to.5 million frame/s) camera images show jet penetration Magnetics show growth of internal modes Detailed comparisons with 3-D MHD NIMROD modeling underway
Alfvén mode research on C-Mod with active MHD antennas Motivations: Study physics of excitation and damping Intermediate-n, same as expected on ITER Extract information on ICRF-generated tail ions Cascade behavior can provide sensitive measure of q min evolution Active excitation of Alfvén modes could be used in reactor to control ignition burn via α particle control Extend understanding and capability to control and manipulate plasmas with external waves; Simulate through experiment and modeling the synergistic behavior of alpha-particle-dominated burning plamas.
Stable Alfvén Modes Probed with Active MHD Antennas Example of resonances driven at the TAE frequency for q=1.5 Widths gives damping rates, proximity to unstable boundary
Alfvén Cascades Yield q-profile Evolution ICRF minority heating produces high energy (~300 kev) ion tail during current rise, which drives modes unstable, seen with PCI and magnetics (shown here) MISHKA modeling (JET collaboration) simulates mode evolution, yields q min (t)
Dimensionless and dimensional scaling: Error fields/locked mode studies Prior to C-Mod studies, the accepted size scaling of error field threshold for locking gave a very low value for ITER (2 10-5 ). Installation of error field control coils ( Acoils ) on C-Mod allowed dimensionless identity experiments at compact size to begin (joint with DIII-D, JET). Dimensional experiments at ITER field, q 95, and n/n G are also being done. Both sets of experiments indicate a more favorable threshold scaling for ITER. Carry out additional science and technology activities supporting ITER
B-field scaling of error locking threshold at constant normalized density allows extrapolation to ITER ~ / T B B Scaling of threshold is needed to extrapolate to ITER BB n B q R αn αb q αr / T It is established that α n = 1 From dimensionless scaling constraints: So, knowledge of α B determines size scaling α 5 αr = 2αn + αb 4 B 21 /n e B T (10 17 m -3 ) Implication for ITER favorable: ~ BB / T C-Mod results: α B = -1.14 α R = 0.58 ITER B-Field Toroidal Magnetic Field (Tesla) threshold ~ 1x10-4 Lower Field (JET, DIII-D) coordinated experiments consistent with these results
Summary Macrostability C-Mod MHD research program: Focused on key issues in support of C-Mod s ITER and Advanced Tokamak programs Strong collaborations through ITPA (locked modes, TAEs & energetic particles, disruption mitigation, NTMs) Leverages C-Mod s unique regions of parameter space to better determine scalings and ultimately physics understanding Excellent connections with theory and modeling