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Observations of ion-scale (k_y*rho_i <= 1) density turbulence of relativeamplitude dn_e/n_e <= 0.2% are available on the Mega Amp Spherical Tokamak(MAST) using a 2D (8 radial x 4 poloidal channel) imaging Beam EmissionSpectroscopy (BES) diagnostic. Spatial and temporal characteristics of thisturbulence, i.e., amplitudes, correlation times, radial and perpendicularcorrelation lengths and apparent phase velocities of the density contours, aredetermined by means of correlation analysis. For a low-density, L-modedischarge with strong equilibrium flow shear exhibiting an internal transportbarrier (ITB) in the ion channel, the observed turbulence characteristics arecompared with synthetic density turbulence data generated from global,non-linear, gyro-kinetic simulations using the particle-in-cell (PIC) codeNEMORB. This validation exercise highlights the need to include increasinglysophisticated physics, e.g., kinetic treatment of trapped electrons,equilibrium flow shear and collisions, to reproduce most of the characteristicsof the observed turbulence. Even so, significant discrepancies remain: anunderprediction by the simulations of the turbulence amplituide and heat fluxat plasma periphery and the finding that the correlation times of thenumerically simulated turbulence are typically two orders of magnitude longerthan those measured in MAST. Comparison of these correlation times with variouslinear timescales suggests that, while the measured turbulence is strong andmay be `critically balanced', the simulated turbulence is weak.

Comparison of BES measurements of ion-scale turbulence with direct, gyrokinetic simulations of MAST L-mode plasmas