[en] The authors discuss current results of thermal barrier operation of TMX-U. In this device, axial particle confinement is provided by increased electrostatic potentials in the outboard mirror cells (end plugs). These potential peaks are sustained by a combination of neutral-beam injection and high-power (250 kW per plug) microwave heating (ECRH) at both the fundamental and second harmonic of the electron-cyclotron frequency. At the plug midplane, the second harmonic ECRH produces a potential minimum (the thermal barrier), which reduces the flow of cold electrons to the regions of high ion-confining potential. Recent experiments have examined thermal barrier formation at central-cell densities (n/sub cc/ -- 10/sup 12/ cm/sup -3/), using data from several new diagnostics, including an end-loss ion spectrometer and a system of X-ray spectrometers. Related experiments have sought to increase the energy confinement of hot mirror-trapped ions in the central cell. This is important for thermal-barrier operation because barrier filling by cold ions escaping from the central cell can limit high density operation. Analysis of data from a charge-exchange analyzer and Thomson scattering shows that the energy confinement is limited by a combination of charge exchange and electron drag. Current efforts at reducing the charge exchange loss are focused on control of the edge-plasma density, temperature, and fueling using both ion-cyclotron rf heating and a set of biased limiter plates to inject low energy neutrals into the core plasma. New information about neutral penetration and fueling efficiencies under these conditions has been provided by a high speed camera imaging Balmer-alpha emission from atomic neutrals