Simulation of Turbulence in FTU M. Romanelli, M De Benedetti, A Thyagaraja* *UKAEA, Culham Sciance Centre, UK Associazione EURATOM-ENEA sulla fusione

We have simulated with CUTIE the time evolution of field fluctuations on FTU following the injection of a deuterium pellet in an ohmic plasma CUTIE [1] is a gyrofluid ELECTROMAGNETIC – GLOBAL code which solves the electron-ion equations of continuity, momentum and energy balance for density, velocity and temperature fluctuations. The Geometry is that of a periodic cylinder, effects of toroidicity are taken into account although Shafranov shift is neglected (low  ) Important features of CUTIE are: 1. Strong nonlinear interactions between mean profiles and the turbulent fluctuations. 2. The mean profiles develope structures on a spatial scale intermediate between the ion Larmor radius and the plasma size [1] A Thyagaraja, EPS See also Theory of fusion Plasmas, Proc. Varenna-Lausanne International Workshop, 155 (1996). Proc. 26 th EPS (1999) pag and UKEA Fusion Report, FUS 419, (1999)

The FTU experiment FTU is a medium size tokamak of major radius 0.93 m, minor radius 0.3 m, maximum magnetic field 8 T and maximum current 1.6 MA R=0.93 m; r=0.3 m; B=8 T; Ip = 1.6 MA

The deep injection of deuterium pellets produce a peaked density profile and an enhanced confinement time (large increase in the neutron rate) In this discharge, the confinement time increases transiently from 60 ms in the pre-pellet phase to above 100 ms in the post pellet phase Pellet injection experiments on FTU (# T; 800 kA)

Electron and ion thermal conductivity from JETTO (m 2 /s) q-profile

In order to follow the evolution of plasma turbulence through the pellet injection, we have started by simulating pre-pellet plasma conditions until a saturated level of turbulence was reached (simulated period of 10 ms) next we have introduced a strong particle source (pellet) at r/a=0.15 lasting for 50 microsenconds. A new level of turbulence has been calculated accounting for the presence of the source. The simulation has then been continued with post-pellet plasma conditions for further 4 ms. The simulation method

We have evolved the Fourier components of the fluctuating fields on a fixed grid of 101 radial points, 32 poloidal harmonics and 16 toroidal harmonics. The time step adopted was 2 x10 -8 s and 5 x10 -8 s in the pre and post pellet phase, in the particle injection phase we have adopted a time step of 5 x10 -9 s for a period of 80  s. A 0.4 ms run lasted about 24 hours on a LINUX, Pentium III PC Simulation Parameters

80 microsec after the injection Pre pellet t=0.6 Electron density (cm -3 ) and temperature (keV) Post pellet t=0.66

780 microsec after the injection Pre pellet t=0.6 Poloidal flux contours Post pellet t=0.66

780 microsec after the injection Pre pellet t=0.6 Electric potential contours Post pellet t=0.66

80 microsec after the injection Tau-E = 35 ms Pre pellet t=0.6 Electron and ion thermal conductivity (cm 2 /s) Tau-E = 77 ms Post pellet t=0.66 Tau-E = 127 ms

The simulation predicts a dominant m=1 interacting with a large m=2 mode in the pre-pellet phase. The calculated ion thermal conductivity is about a factor 10 lower than the experimental one (from JETTO). It must be noted that the pre-pellet phase is dominated by the sowtooth activity that brings about large uncertainty on transport analysis. The electron thermal conductivity shows the presence of large spikes in proximity of the rational q surfaces, the radial size of the spikes is that of the ion poloidal gyroradius and they originate from field stochasticity near the island X-point. Simulation Results

The injection of the pellet brings about the suppression of the dominant low m modes and the growth of higher m fluctuations in the plasma periphery. The ion diffusivity decreases transiently but then recovers a value of 0.1 m 2 /s well in agreement with JETTO. The spikes in the electron conductivity are largely suppressed. CUTIE suggests that the enhanced confinement brought about by the pellet is due to the suppression of the low m modes in the fluctuation specturm which in turn drives energy into the high m mode numbers. This prediction is in qualitative agreement with the measurement of fluctuations with reflectometry Simulation Results

A poloidal correlation heterodyne reflectometer measures the turbulence properties on FTU 1. The system can work both in ordinary and lower cut-off extraordinary mode in a frequency range between 49 and 80 GHz. This allows us to probe both low and high density plasmas in a range between 3.0  and 3.0  m -3. The turbulence spectrum is usually characterised by a quasi- coherent component that is usually in the range between 50 and 300 kHz. 1) V. Vershkov et. al., Proceedings of EPS conference 2001, Madeira Time evolution of the main poloidal mode number m of the quasi-coherent component during a pellet fuelled discharge on FTU. The mode number is high (m up to 200) for a good confinement and low (m between 50 and 100) for a low confinement phase.