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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 TAE Damping Rates on Alcator C-Mod Compared with Nova-K J A Snipes *, N N Gorelenkov ‡, J Sears * * MIT Plasma Science and Fusion Center, Cambridge, MA USA ‡ Princeton Plasma Physics Laboratory, Princeton, NJ USA
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 First Evidence of Really Large Scale TAE Transport Presented at a Plasma Physics Meeting Far exceeds any TAE transport ever presented on JET Far exceeds any TAE transport ever presented on TFTR Appears to be relevant to accretion disks in astrophysical plasmas
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Introduction Moderate n ~ 5 – 10 Alfvén Eigenmodes (AEs) are expected to be unstable in ITER and could affect the fast population Understanding and control of AEs could improve fusion burn control and lead to improved confinement Unstable AEs are excited in C-Mod at ITER B T and n e with T e T i and up to 300 keV fast hydrogen ion tails generated by ICRH Stable AEs are excited in C-Mod with a pair of active MHD antennas that drive moderate |n| < 16 modes Damping rates of these moderate n stable AEs have been compared with MHD calculations with the Nova-K code including continuum, radiative, collisional and Landau damping
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Active MHD Antennas Excite Broad n Spectrum Two antennas above and below the outboard midplane Two amplifiers drive ~ 25 A each producing B r ~ 1 G at q=1.5 10 Hz < f < 1 MHz, broad toroidal spectrum |n| ~ 16 FWHM ~
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Stable TAE Resonances at Constant Frequency Three stable AE resonances as the toroidal field and density sweep the plasma TAE frequency through the constant active MHD frequency Resonances occur near the center of the TAE frequency gap for q=1.5
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 TAE Resonances with Sweeping Frequency Multiple stable TAE resonances with sweeping active MHD frequency, gives a damping rate / ~ -1.2% and n = -9
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Limited vs Diverted Plasma Outer Gap Scan Active TAE resonances were only observed with moderate outer gaps in inner wall limited plasmas or with small outer gaps in diverted plasmas Damping rates are in the same range as those found in JET limited plasmas but are lower than in JET diverted plasmas where | / | > 5% Compare limited and diverted plasmas to assess the role of high edge shear on /
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 TAE Damping Rate Independent of Edge Shear Elongation scan varied edge magnetic shear by a factor of 2 Resonant TAE mode numbers from 4 n < 14 For moderate n modes, no clear dependence on edge magnetic shear in contrast to low n 2 results on JET
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Damping Rate Calculations Nova-K was used without fast particles to calculate the total damping rate of Alfvén eigenmodes in the same frequency range as the experiment including plasma rotation Both inner wall limited and lower single null diverted plasmas were modeled Measured damping rates from 0.76 % < | / | < 3.0 % and measured modes from 3 n 9 were modeled Calculated q profiles from TRANSP and EFIT were compared Sensitivity of the damping rate to q was calculated by scaling the q profile with 0.9, 1.0, and 1.1 times the EFIT q profile
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Inner Wall Limited n=4 Mode Constant active MHD frequency ramping TF through the TAE resonance f TAE = 420 kHz, n=4, / ~ -3% inner wall limited case
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Agrees Better with EFIT q Profile Plasma rotation typically < 5 kHz frequency uncertainty ~ ±20 kHz Closest mode in Nova-K: 423 kHz, / = -0.35% a factor of 10 lower Shot 1021003013
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Indicates Broad Mode Structure Closest mode found by Nova-K without strong continuum interaction Broad radial eigenmode structure with large edge component Shot 1021003013 423 kHz, n=4 Radial Eigenmode Structure Perpendicular Field Perturbation
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Lower Single Null n=3 Mode Constant active MHD frequency ramping TF through the TAE resonance f TAE = 436 kHz, n=4, / ~ -0.76% lower single null case
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Agrees Better with TRANSP q Profile Plasma rotation -4 to -8 kHz f plasma =f meas +12 to 24 kHz=448 – 460 kHz Closest mode in Nova-K: 452.4 kHz, / = -0.77% in good agreement Shot 1030520026 Flat q profile y exponent = 4, q0 = 0.95 1030520026 only strong continuum interaction
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Indicates Broad Mode Structure Closest mode found by Nova-K without strong continuum interaction Broad radial eigenmode structure with large edge component Calculated / = -0.77% in good agreement with the measured / = -0.76% Shot 1030520026 Radial Eigenmode Structure 452.4 kHz, n=3
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Near Double Null Inner Wall Limited n=9 Mode Sweeping active MHD frequency through several TAE resonances f TAE = 487 kHz, n=9, / ~ -1.2% near double null inner wall limited resonance
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Agrees Better with EFIT q Profile Plasma rotation -2 to -5 kHz f plasma =f meas +18 to 45 kHz=505 – 532 kHz Closest mode in Nova-K: 513 kHz, / = -0.88% close to measured -1.2% 10% changes in q can change the calculated damping by a factor of 4 – 10 Shot 1050615011
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Nova-K Indicates Broad Mode Structure Closest mode found by Nova-K without strong continuum interaction Broad radial eigenmode structure with large edge component Shot 1050615011 513 kHz, n=9 Radial Eigenmode Structure Perpendicular Field Perturbation
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J A Snipes, 6 th ITPA MHD Topical Group Meeting, Tarragona, Spain 4 – 6 July 2005 Conclusions Moderate n stable TAEs are easily excited in C-Mod with a pair of active MHD antennas driving total perturbations ~ 1 G radial field TAE damping rates can be measured with synchronous detection of edge magnetic pick-up coil signals Damping rates of 4 < n < 14 stable TAEs in the range of 0.5 < | / | < 4.5% similar to what is found on JET for low n TAE damping rates The damping rate appears to be sensitive to the edge magnetic configuration But, there is no clear dependence of moderate n stable TAE damping rates on edge magnetic shear in C-Mod through an elongation scan Calculations of moderate n TAE damping rates with Nova-K are sensitive to changes in q(r) with order of magnitude changes in / for 10% changes in q Good agreement between measured and calculated damping rates can be obtained in some cases for reasonable q profiles calculated from EFIT
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