  The goal of this work is to investigate the power spectrum of the magnetic
field associated with the giant radio halo in the galaxy cluster A665. For
this, we present new deep Very Large Array total intensity and polarization
observations at 1.4 GHz. We simulated Gaussian random three-dimensional
turbulent magnetic field models to reproduce the observed radio halo emission.
By comparing observed and synthetic radio halo images we constrained the
strength and structure of the intracluster magnetic field. We assumed that the
magnetic field power spectrum is a power law with a Kolmogorov index and we
imposed a local equipartition of energy density between relativistic particles
and field. Under these assumptions, we find that the radio halo emission in
A665 is consistent with a central magnetic field strength of about 1.3 micro-G.
To explain the azimuthally averaged radio brightness profile, the magnetic
field energy density should decrease following the thermal gas density, leading
to an averaged magnetic field strength over the central 1 Mpc^3 of about 0.75
micro-G. From the observed brightness fluctuations of the radio halo, we infer
that the outer scale of the magnetic field power spectrum is ~450 kpc, and the
corresponding magnetic field auto-correlation length is ~100 kpc.
