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Optimization of Bootstrap Current Fraction in ITB Discharges in Alcator C-Mod

Optimization of Bootstrap Current Fraction in ITB Discharges in Alcator C-Mod. Paul Bonoli, Darin Ernst, John Liptac, Ron Parker, and Vincent Tang. Outline. Goals of present scoping studies:

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Optimization of Bootstrap Current Fraction in ITB Discharges in Alcator C-Mod

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  1. Optimization of Bootstrap Current Fraction in ITB Discharges in Alcator C-Mod Paul Bonoli, Darin Ernst, John Liptac, Ron Parker, and Vincent Tang

  2. Outline Goals of present scoping studies: Develop scenarios with increased bootstrap fraction based on the off-axis ICRF-heated ITB target plasma (800 kA & 4.5T) Use ACCOME code with ohmic plus bootstrap drive only Examine the effects of reduced ohmic drive initiated by either decreased total current demand or the addition of LH current drive at fixed total current. Use ACCOME code with ohmic plus bootstrap plus LH current drive Use experimentally measured temperature and density profile shapes as much as possible

  3. Profiles used in ACCOME Simulations of Ohmic plus Bootstrap Current Drive Only Reference Discharge: n(0) = 5.5  1020 m-3 , Te(0) = Ti(0) = 1.45 keV, IP = 800 kA, IOH = 672 kA, fBS = 0.16, q(0) = 0.95

  4. ACCOME Results with Ohmic plus Bootstrap Currents Only

  5. ACCOME Simulation with Ohmic plus Bootstrap Drive Only[IP = 650 kA, IOH = 153 kA, fBS = 0.76]Reduced total current drives J//(0) down resulting in higher q(0) and fBS

  6. ACCOME Results with Ohmic, Bootstrap, and LH Driven Currents

  7. ACCOME Results with Ohmic, Bootstrap, and LH Driven Currents[IP = 750 kA, ILH = 111 kA IOH = 130 kA, fBS = 0.68]Adding off-axis LH current drive reduces the central ohmic drive requirement at fixed total current

  8. Time Dependent Modeling of Bootstrap Current Fraction in ITB Discharges is UnderwayVincent Tang (with help from John Liptac) • Using TRANSP - LSC - NCLASS • TRANSP analysis of ITB discharge with off-axis and on-axis ICRF heating has already been done (shot 1001220016) • Current is being ramp down in the TRANSP analysis after the ITB is formed: • Experimental density and temperature profiles and values are used during ramp down • fBS is observed to increase

  9. Questions and Caveats • As IP decreases the confinement time will most likely decrease. • But there will also be a reduction in the local shear with q(0) > 1 as IP decreases: • Will this lead to a change in the confinement properties of the ITB mode ? [Preliminary GS2 simulations by Darin Ernst suggest this may happen].

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