  The role of self-generated zonal flows in the collisionless trapped-electron
mode (CTEM) turbulence is a long-standing open issue in tokamak plasmas. Here
we show, for the first time, that the zonal flow excitation in the CTEM
turbulence is formally isomorphic to that in the ion temperature gradient
turbulence. Trapped electrons contribute implicitly only via linear dynamics.
Theoretical analyses further suggest that, for short wavelength CTEMs, the
zonal flow excitation is weak and, more importantly, not an effective
saturation mechanism. Corresponding controlling parameters are also identified
theoretically. These findings not only offer a plausible explanation for
previous seemingly contradictory simulation results, but can also facilitate
controlling the CTEM instability and transport with experimentally accessible
parameters.
