First results on Oscillating Poloidal Current Drive in RFXmod

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First results on Oscillating Poloidal Current
Drive in RFX-mod

• D. Terranova, L. Zanotto, B. Zaniol, P. Scarin
• and RFX team
• Consorzio RFX, Associazione Euratom-ENEA sulla
  fusione

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OUTLINE

• Toroidal power supply circuit
• Experimental results
• Electron and ion temperature
• MHD fluctuations
• Mode locking
• Edge Turbulence
• Summary and Future plans

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Toroidal power supply circuit
12 sectors serial connection in the direct phase
(TFAT) and independent in inverse phase (Chopper
and Inverter)
From previous sector
Static switch
Capacitor bank

L
T
To next sector
TCCB
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Toroidal power supply specifications

• Power supply specifications
• Maximum voltage 3 kV/32 single turn
• Maximum current 6 kA
• Close loop ops. up to 150 Hz
• Close loop operation
• In close loop operation one can set a general
  wave form reference both in current and tension
  to the inverter.
• Open loop operation
• Limitation only from the 140 V? ms in vessel
  available for flat top control and any additional
  action.

Experimentally OPCD proved to be effective in
enhancing plasma performances temperature
increases and MHD fluctuations reduce. Optimum
oscillation frequency for RFX 125-160 Hz.
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The goal of these tests

• What we did at different F values (-0.05, -0.01,
  -0.02)
• scan in amplitude (8 mT, 15 mT, 25 mT)
• scan in frequency (100 Hz, 125 Hz, 150 Hz)
• scan in wave form (cosinusoid, sawtooth)
• A first assessment of toroidal circuit
  capabilities in plasma shots with OPCD operation,
  without aiming at performances.

No discharge optimization was done.
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OPCD and STD discharges (1)
16397 STD 16400 OPCD
A400 A, f125 Hz, C
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OPCD and STD discharges (2)
16414 STD 16408 OPCD
A400 A, f125 Hz, ST
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OPCD discharges compared
16421 16408
A400 A, f125 Hz, ST
A600 A, f100 Hz, C
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Electron temperature
16421 (OPCD) A600,f100,COS 16414 (STD)
Electron temperature increase is induced during
the co-dynamo phase. The trigger is provided by
the reduction in the amplitude of all modes.
A Limited negative effect is observed during the
counter-dynamo phase. In QSH states the dominant
and secondary modes play different roles.
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Electron temperature in QSH
In QSH discharges the electron temperature
increase is due to the reduction in the amplitude
of secondary modes, even if the amplitude of the
dominant is increasing.
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Ion temperature
16439 (OPCD) A800,f100,COS 16436 (STD)
The ion temperature increase is induced during
the co-dynamo phase in a fashion similar to the
evolution of electron temperature.
Ion temperature is measured from CV emission (0.6
? r/a ? 0.9)
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OPCD in RFX scaling with Ip
T. Bolzonella etal, Phys. Rev. Lett. 87 195001
(2001)
Maximum value of electron temperature during OPCD
oscillation.
In RFX OPCD performances improved with plasma
current. Best OPCD and optimization were done at
0.8 MA and 1 MA. The same trend is expected fro
the OPCD on RFX-mod.
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MHD fluctuations (1)
During MH discharges mode amplitudes of the
toroidal magnetic field oscillate in phase. Modes
amplitudes decrease during the co-dynamo
phase. No variation of the phase of the modes is
observed. No rotation of the phase locked
structure is observed. However low n and high n
modes seem to lock in different positions.
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MHD fluctuations (2)
In QSH discharges there is some indication that
the toroidal magnetic field mode amplitude
evolves in such a way that the dominant mode is
in anti-phase with respect to the others.
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The mode locking (1)
n?9
n?10
Double phase and wall locked structures can
appear. A delocalized structure is observed also
in configurations with MH spectra.
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The mode locking (2)
16399 16400
Shots with mode wall-locking located at the
structure-gap are generally better than those
with wall-locking at the shell-gap. Nonetheless
the OPCD seems equally effective in both cases.
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The mode locking (3)
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Edge turbulence
There is clear evidence of an effect on edge
turbulence. A reduction is observed with the
reflectometer diagnostic. The GPI diagnostic
provides indications that the observed turbulence
shares some characteristics both with QSH and MH
shots in terms of spectral analysis. A deeper
analysis is required to provide a clear
understanding of the OPCD effect.
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Toroidal circuit power input
Vq?Iq ? 0.2 MW
Vq?IqVf?If ? 20 MW
Discharges at 600 kA The toroidal circuit power
input (Vq?Iq) oscillates between about 1 MW with
an average value of about 0.2 MW. This is to be
compared with a total power input (Vq?IqVf?If)
of about 20 MW.
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Summary and Future plans

• We were able to test the effectiveness of the
  OPCD action toroidal circuit was successfully
  operated changing the amplitude, frequency and
  wave-form of the external field modulation.
• No optimized discharge was produced, but the
  operation was effective in increasing electron
  and ion temperature. Best parameters from field
  modulation
• Cosinusoid at 100 Hz (best results with amplitude
  25 mT)
• Sawtooth at 125 Hz (best results with amplitude
  25 mT)
• Clear effects on the mode locking are observed,
  together with some influence from the position of
  the wall locking.

The coming experimental campaign will be
dedicated to optimization of OPCD discharges and
creation of a data base in order to address the
effect on plasma performances both in terms of
temperature and MHD fluctuation.
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The mode locking in a STD shot
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Effect ON the mode locking (1)
In some cases the phase locking of the modes
changes due to the OPCD action. A delocalized
structure is observed also in configurations with
MH spectra.
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Effect ON the mode locking (2)