Plasma resistivity determination in runaway discharges from positive voltage spikes on TCABR tokamak

Kuznetsov, Yu.K.; Nascimento, I. M.; Galvão, R. M. O.; Tsypin, V. S.

doi:10.1590/s0103-97332002000100020

Cited by 3 publications

(1 citation statement)

References 1 publication

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In this figure, we can see that V loop crosses zero at the same time instant as the plasma current. However, V loop is a rather spiky signal, which could be a manifestation of the presence of runaways, as caused by the intermittent loss of REs [16,17]. In phase II, the plasma current is close to zero (I p ∼ 0 A) corresponding to a situation where the plasma is cold (few eV) and the density has dropped about sixfold from the flat-top phase; in principle, in this phase atomic physics processes dominate.…”

Section: Characteristics Of the Ac Transitionmentioning

confidence: 99%

Experiments during AC current transition in ISTTOK and the hypothesis of ballistic runaway electrons

Malaquias¹,

Hole²,

Henriques³

et al. 2022

Plasma Phys. Control. Fusion

View full text Add to dashboard Cite

The operation of the Instituto Superior Técnico Tokamak (ISTTOK) in the alternating current (AC) regime is characterized by the presence of a residual plasma density during the current transition, i.e. when the plasma current crosses the zero value. Contrary to what has been reported in other AC experiments, the pressure-like profiles obtained by the heavy ion beam diagnostic (the product of plasma density with a known function of plasma temperature) do not show evidence of the co-existence of two anti-parallel currents. This is also confirmed by the fact that in these experiments the plasma current and the V loop both cross the zero value at the same instant. The Murakami–Hugill plots reveal that during the transition the plasma current decays faster than the Hugill limit and that the subsequent ramp-up phase is sustained by runaway electron (RE) currents. It is hypothesized that during plasma current decay a population of REs can enter the following semi-cycle of the discharge. A trajectory simulation model including the forces at play shows that the retardation effect of the friction and molecular forces can cause the dampening of REs (within 20 μs) with energy below 1 keV inside the chamber (without external vertical correction magnetic fields), thus ionizing the background gas. Faster REs (above 1 keV) can also contribute to the ionization of the background gas (e.g. within 50 μs for 3 keV energy electrons) but require a vertical magnetic field to balance the gradB and curvature drifts. This could explain why tuning the vertical field and dosing the background pressure to obtain successful AC discharges is usually challenging.

show abstract

Section: Characteristics Of the Ac Transitionmentioning

confidence: 99%

Experiments during AC current transition in ISTTOK and the hypothesis of ballistic runaway electrons

Malaquias¹,

Hole²,

Henriques³

et al. 2022

Plasma Phys. Control. Fusion

View full text Add to dashboard Cite

show abstract

Recombinative plasma in electron runaway discharge

et al. 2005

View full text Add to dashboard Cite

Cold recombinative plasma is the basic feature of the new regime of runaway discharges recently discovered in the Tokamak Chauffage Alfvén Brésilien tokamak [R. M. O. Galvão et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)]. With low plasma temperature, the resistive plasma current and primary Dreicer process of runaway generation are strongly suppressed at the stationary phase of the discharge. In this case, the runaway avalanche, which has been recently recognized as a novel important mechanism for runaway electron generation in large tokamaks, such as International Thermonuclear Experimental Reactor, during disruptions, and for electric breakdown in matter, is the only mechanism responsible for toroidal current generation and can be easily observed. The measurement of plasma temperature by the usual methods is a difficult task in fully runaway discharges. In the present work, various indirect evidences for low-temperature recombinative plasma are presented. The direct observation of recombinative plasma is obtained as plasma detachment from the limiter. The model of cold recombinative plasma is also supported by measurements of plasma density and Hα emission radial profiles, analysis of time variations of these parameters due to the relaxation instability, estimations of plasma resistivity from voltage spikes, and energy and particle balance calculations.

show abstract

Runaway discharges in TCABR

Kuznetsov¹,

Galvão²,

Bellintani³

et al. 2004

Nucl. Fusion

View full text Add to dashboard Cite

It is found in experiments carried out in Tokamak Chauffage Alfvén Brésilien (TCABR) that two regimes of runaway discharges (RADs) with very different characteristics are possible. The RAD-I regime, which is similar to that observed in other tokamaks, can be obtained by a gradual transfer from a normal resistive to a RAD by decreasing the plasma density. This regime can be well understood using the Dreicer theory of runaway generation. The total toroidal current contains a substantial resistive component and the discharge retains some features of standard tokamak discharges. The second runaway regime, RAD-II, was recently discovered in the TCABR tokamak (Galvão R.M.O. et al 2001 Plasma Phys. Control. Fusion 43 1181). The RAD-II regime starts just from the beginning of the discharge, provided that certain initial conditions are fulfilled and, in this case, the runaway tail carries almost the full toroidal current. The background plasma is cold and detached from the limiter due to the recombination process. The primary Dreicer process is suppressed in the RAD-II and the secondary avalanche process dominates, even at the start-up phase, in the generation of the toroidal current. It is possible to trigger a transition from the RAD-I to the RAD-II regime using plasma cooling by gas puffing. The experimental results are shown to be in reasonable agreement with theoretical predictions based on the runaway avalanche process.

show abstract

Plasma resistivity determination in runaway discharges from positive voltage spikes on TCABR tokamak

Cited by 3 publications

References 1 publication

Experiments during AC current transition in ISTTOK and the hypothesis of ballistic runaway electrons

Experiments during AC current transition in ISTTOK and the hypothesis of ballistic runaway electrons

Recombinative plasma in electron runaway discharge

Runaway discharges in TCABR

Contact Info

Product

Resources

About