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Plasma Current and Pressure Profile Effects on Tearing Mode Onset in Steady-state Hybrid Scenarios on DIII-D Tokamak
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- Authors
- Advisor
- 나용수
- Major
- 공과대학 에너지시스템공학부
- Issue Date
- 2017-02
- Publisher
- 서울대학교 대학원
- Keywords
- Tearing mode onset ; Tearing stability index ; DIII-D steady-state hybrid scenarios ; Stability diagram
- Description
- 학위논문 (박사)-- 서울대학교 대학원 : 에너지시스템공학부, 2017. 2. 나용수.
- Abstract
- Tearing mode (TM) instability is a kind of resistive magnetohydrodynamic
instability that limits the performance of the tokamak plasma. A correct
understanding of this phenomenon is essential for high performance steady-state
operation, especially the onset of TMs. Based on the steady-state hybrid
experiments performed on the DIII-D device, the occurrence of TM is identified
and analyzed, and the effect of the plasma current and the pressure profiles on the
tearing stability is investigated in terms of the tearing stability index ∆".
The characteristics of the mode is mainly investigated using the system of magnetic pick-up Mirnov probes. The FFT analysis and the phase-fitting method
are applied to identify the mode onset, the mode amplitude, and the mode number
in the experiments. The mode onset was defined for this thesis that the phasefolding
disappears in the phase-fitting results of poloidal probe array.
Since the tearing stability is sensitive to the equilibrium current and pressure
profiles, the more accurate and tightly constrained equilibrium is reconstructed
using the well-measured plasma profiles using various diagnostics for the
discharges in the database. The characteristics of plasma current and pressure
profiles at tearing mode onset in the database of DIII-D steady-state hybrid
discharges seems to be more sensitive to the global feature of plasma profile
through li and fp, and the effect of plasma resistivity at the mode surface through
'( than the local feature of plasma profile through Lq and Lp. From the result of
the best fit equation in the dimensionless form using rs and 'TA the global feature
of profiles and the current profile can affect more than the local feature of profiles
and the pressure profile, respectively.
The tearing stability index ∆" is calculated with the experimental equilibria in
two ways, by MHD codes (PEST-III and resistive DCON) and by MRE. They are
verified and validated in reasonable agreement. It is noteworthy that the determined
∆′ can be positive regardless of the mode onset, so the conventional wisdom of
∆" > 0 for tearing destabilization may not be the sufficient condition, rather a
positive value greater than a certain threshold could replace this under the toroidal
geometry. The analytical formula of the tearing stability threshold ∆1
" is reviewed for the semi-collisional regime and the collisionless banana regime. By comparing the
analytical ∆1 " with the ∆" calculated from PEST-III code near the TM onset, it is
found that the condition of ∆" > ∆1 " is required for the mode onset in the experiment. A preliminary study on the ∆1
" estimation from the normalized mode growth rate is performed by NIMROD code. The ∆2
" can be fitted for negative or marginal growth rate using the relation between the normalized mode growth rate
3'- from NIMROD and the ∆" from PEST-III.
Finally, a stability diagram of n = 1 tearing mode onset is suggested with
local and global features of plasma profiles. Onset condition of n = 1 TM is
analyzed by the difference between ∆" and ∆1
" , and its stability diagram is derived
in terms of the local (▽/II and ▽p) and global (BN) variations for steady-state
hybrid scenarios. To calculate ∆" − ∆1
" for the stability diagram, a novel modeling
package has been developed by integrating IPS/FASTRAN for equilibrium
reconstruction, PEST-III/DCON for linear stability ∆" calculation, and the ∆1
"
solver for analytical ∆1 " calculation. The stability boundary at the mode onset,
∆" = ∆1 " , is mapped on Lp-Lq diagram. Characteristics of this stability boundary
show that the TM unstable area expands, then the stability boundary moves as BN
increases. This stability diagram can be used to design and control experiments to
avoid n = 1 TM.
- Language
- English
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