Characteristics of electron cyclotron resonance plasma formed by lower hybrid current drive grill antenna |
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Authors: | P K Sharma S L Rao K Mishra R G Trivedi D Bora |
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Institution: | (1) Institute for Plasma Research, Bhat Gandhinagar, 382 428, India |
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Abstract: | A 3.7 GHz system, which is meant for LHCD experiments on ADITYA tokamak, is used for producing ECR discharge. The ECR discharge
is produced by setting the appropriate resonance magnetic field of 0.13 T, with hydrogen at a fill pressure of about 5 × 10−5 Torr. The RF power, up to 10 kW (of which ∼50% is reflected back), with a typical pulse length of 50 ms, is injected into
the vacuum chamber of the ADITYA tokamak by a LHCD grill antenna and is used for plasma formation. The average coupled RF
power density (the RF power/a typical volume of the plasma) is estimated to be ∼5 kW/m3. When the ECR appears inside the tokamak chamber for the given pumping frequency (f = 3.7 GHz) a plasma with a density (n
e) ∼ 4 × 1016 m−3 and electron temperature ∼8 eV is produced. The density and temperature during the RF pulse are measured by sets of Langmuir
probes, located toroidally, on either side of the antenna. Hα signals are also monitored to detect ionization. An estimate of density and temperature based on simple theoretical calculation
agrees well with our experimental measurements. The plasma produced by the above mechanism is further used to characterize
the ECR-assisted low voltage Ohmic start-up discharges. During this part of the experiments, Ohmic plasma is formed using
capacitor banks. The plasma loop voltage is gradually decreased, till the discharge ceases to form. The same is repeated in
the presence of ECR-formed plasma (RF pre-ionization), formed 10 ms prior to the loop voltage. We have observed that (with
LHCD-induced) ECR-assisted Ohmic start-up discharges is reliably and repeatedly obtained with reduced loop voltage requirement
and breakdown time decreases substantially. The current ramp-up rates also decrease with reduced loop voltage operation. These
studies established that ECR plasma formed with LHCD system exhibits similar characteristics as reported earlier by dedicated
ECR systems. This experiment also addresses the issue of whether ECR plasma formed with grill antenna exhibits similar behavior
as that formed by single waveguide ECR antenna. Our experimental observations suggest that the characteristics of (LHCD system-induced)
ECR-assisted Ohmic start-up discharges show similar properties, reported earlier with normal ECR-assisted Ohmic start-up discharges
and hence LHCD system may be used as ECR system at reduced toroidal magnetic field for other applications like wall conditioning.
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Keywords: | Electron cyclotron resonance lower hybrid current drive system ADITYA tokamak pre-ionization |
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