arXiv subjects

Saeid Houshmandyar

Publications and source records attributed to Saeid Houshmandyar.

3 recordsLinked to original sources

arXiv2026

ITER ECE front-end design, alignment and in-situ calibration

The electron cyclotron emission (ECE) diagnostics suite at ITER utilizes a front-end quasi-optical (QO) system whose design is fundamentally constrained by a field-stop concept. The field-stop defines the Gaussian beam variation throughout the optical system and within the plasma, thereby setting the ECE sampling volume and spatial resolution. An in-situ hot calibration source, optimized using Gaussian beam transmission criteria, provides independent and absolute electron temperature measurements. The QO system extends beyond the front-end to include the polarization splitter unit (PSU), transmission lines, and switchyard, forming an integrated optical path to the ECE instruments. Misalignment between the front-end and PSU reduces the effective field-stop size, degrading spatial resolution and measurement fidelity. The oblique ECE view, a key feature of the ITER design, enhances sensitivity to non-thermal electron populations and complements the diagnosis of neoclassical tearing modes. Integrated QO design and plasma physics understanding are essential for reliable ITER ECE measurements.

physics.plasm-ph↗

arXiv2026

Assessing the role of ITER ECE oblique view in resolving non-thermal emissions

Systematic discrepancies between electron temperature (Te) measurements from radially viewing electron cyclotron emission (ECE) and Thomson scattering (TS) diagnostics have been observed in multiple tokamaks and are widely attributed to non-Maxwellian features in the electron velocity distribution function (EVDF). As the International Thermonuclear Experimental Reactor (ITER) is expected to operate at much higher core temperatures than present devices, accurate Te measurements from ECE become increasingly critical, particularly in the presence of non-Maxwellian EVDFs. This work presents ECE spectra simulations performed to assess the diagnostic capability of the ITER oblique view under ITER conditions where non-Maxwellian distributions are present. The results show that above a critical oblique angle, Doppler broadening becomes the dominant effect, masking fine-scale spectral signatures of non-thermal distortions across a wide range of conditions. Furthermore, at higher EC harmonics in either polarization, the spectra remain largely unaffected by non-thermal emissions, enabling reliable reconstruction of Te-profiles. These findings demonstrate that the ITER oblique ECE view retains sufficient sensitivity to non-thermal electrons while providing robust and accurate Te-profile measurements under reactor-relevant conditions. The results presented here also have direct implications for ITER ECE system operation and channel selection, particularly in the presence of non-Maxwellian electron populations.

physics.plasm-ph↗

arXiv2026

Microtearing Thresholds and Second-Stable Ballooning in the DIII-D Pedestal: Reduced Modeling and Core-Edge Implications

Global and local linear gyrokinetic simulations of 42 pedestal equilibria from three DIII-D discharges are used to investigate pedestal stability and its impact on pedestal structure and confinement. Microtearing modes (MTMs) and kinetic ballooning modes (KBMs) represent the main ion scale instabilities. For all three discharges, MTMs lie near a stability boundary in the mid-pedestal and exhibit threshold behavior, with growth rates increasing at and beyond pre-ELM pressure gradients. Pedestal MTMs retain conventional signatures but also show enhanced particle transport and partial density-gradient drive, indicating they can constrain pedestal {\it pressure} rather than electron temperature alone. KBMs are typically second-stable in this region due to low magnetic shear and large pressure gradients, though they can become active near the pedestal foot where magnetic shear is higher. These findings suggest MTMs play the role of inter-ELM pressure limit in the mid-pedestal when KBM is second stable. A preliminary quasilinear mixing-length transport model, with properly tuned free parameters, reproduces experimental temperature and density profiles when coupled to ASTRA. When applied to a case with doubled separatrix density, the model predicts reduced pedestal pressure consistent with ITPA H-mode confinement trends, attributable to increased MTM and ETG transport. These results clarify pedestal-limiting mechanisms and establish a physics-based link between separatrix conditions, pedestal structure, and global confinement. This work lays the foundation for new predictive modeling capabilities for core-edge integration in burning plasma regimes.

physics.plasm-ph↗