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Recent Results from Electron Cyclotron Emission (ECE) Radiometer diagnostics in the presence of Electron Cyclotron Resonance Heating (ECRH)
VARSHA SIJU 1,2, S.K.PATHAK 1,2, B.K.SHUKLA1, R.L.TANNA1, R.KUMAR1, J.GHOSH1 and ADITYA-U TEAM2,
1 Institute for Plasma Research, Bhat, Gandhinagar – 382 428
2 Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai-400094, India.
A radiometer receiver system employing a 16-channel super-heterodyne configuration is engineered to span a second harmonic frequency spectrum ranging from 64 – 83 GHz.1 This design caters to the specific demands of Electron Cyclotron Emission (ECE) measurements within the diverse toroidal magnetic field (BT) parameters of the ADITYA-Upgrade (ADITYA-U) tokamak2, which range from 1T to 1.5T. The system architecture, as shown in fig.1, is an integration of a Radio Frequency (RF) unit (64-83 GHz) – which can be varied as per the BT requirements and a 1-20 GHz, Intermediate Frequency (IF) unit that remains fixed for providing measurements at 16 radial locations within the plasma. The diagnostic is intentionally optimized for seamless implementation, enabling precise spatiotemporal analysis of plasma electron temperature evolution with a spatial resolution of 1.2cm and a temporal resolution of 10µs within the ADITYA-U tokamak environment.
Figure1: Layout of the 16-channel ECE Radiometer system.
In addition to facilitating thermal measurements for optically thick (τ>>1) plasmas, this system also facilitates qualitative exploration of non-thermal electron distributions in optically thin (τ<1) plasmas. The ADITYA-U tokamak operates a 42 GHz- 500KW Electron Cyclotron Resonance Heating (ECRH) system for plasma start-up and heating applications.3 The introduction of ECRH induces modifications in the plasma distribution function, to which the ECE radiometer is highly sensitive, allowing for the estimation of relevant parameters.4 Moreover, the system enables direction and analysis of instabilities resulting from these modified distribution functions.
Fig.2 below depicts the inferences of ECRH application on ECE signatures for 3 different discharges with different ECRH pulse widths. Various difficulties arise for the quantitative interpretation of the ECE data especially for low density discharges in the presence of ECRH. Rise in ECE intensity during low density discharges can arise owing to low optical thickness, scattering due to wall reflections etc. ECRH application also results in anisotropic distribution, accelerating the thermal electrons to a higher energy regime and thereby increasing the ECE intensity due to presence of these fast electrons. Redundancy in discharge parameters as well as modelling can help resolve these issues to a certain extent. Relaxation oscillations obtained during instabilities owing to the application of ECRH are also observed in the ECE signatures that can help understand the dynamics of plasma.
Figure2: Temporal evolution of ECRH pulse widths and ECE signatures for 3 different discharges at ADITYA-U tokamak
This manuscript provides a preliminary overview of the observations derived from the ECE radiometer signatures, focussing on the dynamics of plasma influenced by both heating processes and instabilities induced by ECRH.
References
[1] S. Varsha, et al., Journal of Instrumentation. Vol.16 (2021).
[2] R.L. Tanna et al 2019 Nucl. Fusion 59 (2019)
[3] Braj Kishore Shukla et al. EPJ Web of Conferences 277, 02005 (2023)
[4] A.E.Costley, Nucl.Fusion 30 2185 (1990)
