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PRODID:UW-Madison-Physics-Events
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UID:UW-Physics-Event-9757
DTSTART:20260730T190000Z
DTEND:20260730T210000Z
DTSTAMP:20260725T172130Z
LAST-MODIFIED:20260722T162904Z
LOCATION:ERB 106 or https://uwmadison.zoom.us/j/95902445303?pwd=pvySl9
 EOsfp6SaQ9hD24J1eVHE2djV.1
SUMMARY:Verification of Magnetohydrodynamic Computations in Stellarato
 rs and Analytical Modeling of Pressure-Driven Instabilities in Low Mag
 netic Shear\, Thesis Defense\, Sanket Patil\, Physics PhD Graduate Stu
 dent
DESCRIPTION:While the risk of current-driven instabilities and disrupt
 ions is diminished in stellarators\, macroscopic instabilities can lea
 d to a range of consequences from a non-disruptive increase in transpo
 rt to crashes in the pressure profile. Evaluating stability with verif
 ied numerical models and theoretical analysis can elucidate plasma beh
 avior and aid the development of future stellarators.<br>\n<br>\nIn th
 is vein\, computational routines are developed to support stellarator 
 simulations with the initial-value magnetohydrodynamics (MHD) code NIM
 STELL. Developments include mapping ideal MHD equilibria from the DESC
  code and efficient preconditioning for typical stellarator configurat
 ions. Subsequently\, NIMSTELL is verified through benchmarks against i
 nitial-value reduced-MHD simulations in JOREK and full-MHD eigenvalue 
 calculations in CASTOR3D. Linear growth rates are compared for tearing
  and ballooning modes in W7-A\, a tearing mode in QA and a near-resona
 nt interchange mode in QH. NIMSTELL and CASTOR3D growth rates agree to
  within $3%$ for most cases\, verifying the accuracy of both codes. La
 rger deviations (∼ 10%) in JOREK growth rates are attributed to the 
 use of a reduced-MHD model. For the QA tearing mode\, the three codes 
 demonstrate the theoretical $\\eta^{3/5}$ scaling for tearing modes. B
 ecause ballooning growth rates are higher at shorter wavelengths\, the
  challenge of spatially resolving the W7-A ballooning eigenmode is con
 sidered in detail.<br>\n<br>\nInformed by the prevalence of low magnet
 ic shear in optimized configurations\, pressure-driven instabilities a
 re investigated in the low-shear regime. Linear and nonlinear simulati
 ons with NIMSTELL indicate that such modes can severely degrade confin
 ement\, consistent with results from prior research. Following the sta
 ndard analysis of ballooning modes\, which is valid for magnetic shear
  of order unity\, a modified theoretical model valid for low shear is 
 developed using the Wentzel–Kramers–Brillouin (WKB) method. The an
 alysis yields a system of two coupled ordinary differential equations 
 (ODE) that constitutes an eigenvalue problem analogous to standard bal
 looning analysis. In contrast to the latter\, the eigenvalue of the lo
 w-shear ODE system does not reduce to a three-dimensional (3D) functio
 n in the 4D phase space. This avoids the “lock-in” phenomenon and 
 corresponding singular solutions found in standard analysis. As a resu
 lt\, eigenvalues of the ideal MHD normal mode equation can be determin
 ed using conventional quantization methods such as the Einstein-Brillo
 uin-Keller (EBK) conditions. The low-shear model is verified for an ax
 isymmetric configuration by comparing the linear growth rates given by
  the EBK conditions with NIMSTELL results.
URL:https://www.physics.wisc.edu/events/?id=9757
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