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Electron cyclotron resonance heating (ECRH) is important in tandem mirror devices which incorporate thermal barriers to enhance plasma confinement. ECRH in the end cells generates hot mirror-confined electrons to form the thermal barrier and creates the plugging potential to reduce ion end loss from the central cell. Important considerations for heating include a) the proper choice of wave polarization and harmonic number for wave penetration and efficient absorption, b) the possibility for control of hot electron energy and anisotropy by spatially limited heating, and c) the particle feed for hot electrons due to RF trapping processes. Fokker-Planck and Monte Carlo computer codes are useful for understanding these effects. The Tandem Mirror Experiment Upgrade (TMX-U) was constructed to investigate plasma confinement for the thermal barrier mode of operation. We carried out experiments to generate the hot electrons that are required for the thermal barrier. Heating efficiencies as large as 30 percent were observed. By combining the hot electron population with neutral-beam-injected hot ions and ECRH at the location of the desired potential peak, we strongly reduced the end loss of ions and measured plugging potentials up to 700 V. These results are consistent with the thermal barrier model.