  For the measurement of the dynamics of fusion-born alpha particles $E_\alpha
\leq 3.5$ MeV in ITER using collective Thomson scattering (CTS), safe
transmission of a gyrotron beam at mm-wavelength (1 MW, 60 GHz) passing the
electron cyclotron resonance (ECR) in the in-vessel tokamak `port plug' vacuum
is a prerequisite. Depending on neutral gas pressure and composition,
ECR-assisted gas breakdown may occur at the location of the resonance, which
must be mitigated for diagnostic performance and safety reasons. The concept of
a split electrically biased waveguide (SBWG) has been previously demonstrated
in [C.P. Moeller, U.S. Patent 4,687,616 (1987)]. The waveguide is
longitudinally split and a kV bias voltage applied between the two halves.
Electrons are rapidly removed from the central region of high radio frequency
electric field strength, mitigating breakdown. As a full scale experimental
investigation of gas and electromagnetic field conditions inside the ITER
equatorial port plugs is currently unattainable, a corresponding Monte Carlo
simulation study is presented. Validity of the Monte Carlo electron model is
demonstrated with a prediction of ECR breakdown and the mitigation pressure
limits for the above quoted reference case with $^1$H$_2$ (and pollutant high
$Z$ elements). For the proposed ITER CTS design with a 88.9 mm inner diameter
SBWG, ECR breakdown is predicted to occur down to a pure $^1$H$_2$ pressure of
0.3 Pa, while mitigation is shown to be effective at least up to 10 Pa using a
bias voltage of 1 kV. The analysis is complemented by results for relevant
electric/magnetic field arrangements and limitations of the SBWG mitigation
concept are addressed.
