Measuring small fast electron populations hidden in a sea of colder “thermal” electrons in tokamak plasmas is very challenging. Why? The challenge comes from the fast electron signal being overwhelmed by thermal electron signal in most diagnostics. Physicists at the University of California-San Diego, with physicists from Oak Ridge National Lab and from General Atomics, have succeeded in measuring fast electron populations. They achieved this first-of-its-kind result by seeing the effect of the fast electrons on the ablation rate of small frozen argon pellets.