7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

ECR assisted ICRF plasma production in ADITYA-U tokamak

Kishore Mishra1,2, J. Ghosh1,2,K. M. Patel, K. A. Jadeja, S. Kumar, H M Jadav, R. Kumar, R. Tanna,K. Singh, H. Raj1,2, B. Kadia, K. M. Parmar, M. Singh, N. Ramaiya, M. B. Chowdhuri, S. Aich, R.
Joshi, D. Rathi, A. Varia, R. P. Yadav, G. Ashok, B. Hegde, A. Kumawat
1Institute for Plasma Research, 2Homi Bhabha National Institute (HBNI)
e-mail (speaker): kmishra@ipr.res.in

Plasma start-up with assistance of preionized plasma
has long been recognized as a fundamental requirement
for a successful and controlled plasma current ramp up.
Requirement of preionization is more prominent insuperconducting continuous vessel machines, whereavailable electric field (loop voltage) to initiate
breakdown is severely limited. The Spherical Tokamak(ST) based concept also have limited central solenoid
space, and hence available loop voltage is also small.
While Gyrotron based Electron Cyclotron Resonance(ECR) breakdown is the most successful and utilized
preionization method in fusion devices for low loop
voltage start-up, alternative methods of preionization are
also being developed for a long time. Ion Cyclotron
Resonance Frequency (ICRF) plasma production is one
such case, where successful plasma production in thevessel and its usefulness in wall conditioning and low
loop voltage start-up has been demonstrated in several
machines [1-3]. Though, ICRF system is present inalmost every machine, achieving successful plasma
production through ICRF alone is not guaranteed.
Unlike ECR, plasma production through ICRF antenna is
not a resonant phenomenon. ICRF plasma is produced if
the matched oscillation of stray electrons under theinfluence of high frequency electric field around theantenna and supported by ponderomotive potential
created by it along with surrounding metallic
components are met with favorable condition of neutral
density, local error field, and RF electric field. For
example, the ICRF antenna that was used to produce
successful plasma in Aditya torus, could not form plasma
when the vacuum vessel is upgraded to Aditya-Umachine even under an expanded operational regime. ARF electric field modelling indicated that the revised
antenna vessel wall distance in old and new machine,
modified the RF electric field and thus prevented a
successful breakdown. Considering the unalterable
nature of the RF electric field structure without changing
the antenna, one is prompted to ponder: 'Could theintroduction of a few seed electrons potentially facilitate
the avalanche?'
To investigate this aspect, a 2.45 GHz magnetron, 2kW
is used to form plasma in the vessel. The toroidal field(Bt) ~ 876 G corresponding to the EC frequency iscreated by a programmable DC power supply before
applying EC pulse. Hydrogen fill pressure is kept
between 4x10-5 torr to 2x10-4 torr. No vertical field orerror compensating field is applied in this set ofexperiments. After ECR plasma is formed, ICRF power
40-80kW, 36MHz is applied to the antenna and promptly
plasma is formed near the antenna and filled the torus
both toroidally and radially. Once it is formed, ICRF
plasma continues to remain as long as ICRF power isapplied independent of ECR plasma. The minimum
required overlapping time period between EC and ICpulses is found to be ~30ms and minimum power
required is 200W. The location of ECR is varied toinvestigate the effect on ICRF plasma formation atsimilar conditions. It is observed that, ECR should not bevery near to the antenna as it detunes the RF source andleads to high reflection. The ICRF plasma could be
produced by keeping ECR even at high field side wall.

Figure-1: EC pulse is followed by IC pulse with 30ms
overlapping. Density is measured near IC antenna, Visible lightand H emission during EC and IC phase.

Since the operating Bt for 2.45GHz is one order lower
than tokamak operation including the present machine,
the next obvious objective is to produce and sustain
ICRF plasma during the Bt ramp-up phase. This scheme
could also be useful for ICRF wall conditioning utilizing
both ramp-up and ramp down phases of Bt during eachplasma discharge. This scheme may be developed andutilized, where EC cannot be used due to restriction ofoperating Bt.
References[1] Lyssoivan et. al. Plasma Physics (13), 2007, p. 30-34[2] K Mishra, et. al, EX-P4-28, IAEA, 2018[3] S. Kamio et al Nucl. Fusion, 2021, 114004