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This work reports the experimental characterization and modeling of a new generation of radiation-balanced fiber amplifier that delivers 0.6 W of output power at 1064 nm with zero net heat generation. At the radiation-balanced point, the amplifier had a single-pass gain of 20 dB, an optical efficiency of 37%, a slope efficiency of 80%, and a signal-to-noise ratio of ∼43 dB. This breakthrough was enabled by using a novel aluminophosphosilicate fiber heavily doped with Yb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> (4.2 wt.%) with low quenching and low background absorption and pumping it at 1040 nm to generate efficient anti-Stokes fluorescence (ASF) cooling. This performance was achieved after optimizing the fiber length (5.8 m) and seed power (5.8 mW). Reducing the seed power to 0.4 mW and fiber length to 4.5 m resulted in a lower output power but a record radiation-balanced gain of 32 dB. All experimental data are in excellent agreement with the predictions from a published theoretical model of ASF-cooled fiber amplifiers, demonstrating the ability to accurately predict temperature distributions and amplifier performance. It predicted, and we later confirmed experimentally, that even at an output of 5 W, the average fiber temperature remained only 1.1 K above room temperature, demonstrating the significant benefit of this new generation of rare-earth-doped fibers in conventional high-power applications. This model is a reliable tool for the future design, optimization, and analysis of radiation-balanced fiber amplifiers.