Due to the need to reduce the power requirements and thus the weight of large space-based neutral particle beam (NPB) space platforms, current systems studies propose that these NPB accelerators be maintained at cryogenic temperature levels. As such, the ground test articles that will be used to develop many of the technological advancements necessary prior to engineering development of an operational NPB space platform, will also address the issues of operating cryogenically. In support of a number of these programs, a cryogenic cooling test of a typical accelerator component was performed. This paper discusses the various modes of cryogenic cooling proposed (two-phase convective boiling and single-phase supercritical convection), the advantages and disadvantages of each, gives details of the test, and compares the test results to analytical predictions.IN RECENT STUDIES OF THE OPERATIONAL NPB SPACE PLATFORM, it has been determined that large amounts of radio frequency (RF) power and prime power will be required to generate a beam with adequate intensity to meet mission objectives. These large amounts of power place a huge weight penalty on the space platform.One viable way to reduce the required power would be to reduce the power losses in the accelerator. These losses are a result of the surface currents flowing in the RF cavity and the inherent electrical resistance of the RF surface. Since this electrical resistance varies as a function of temperature, the lower the accelerator temperature can be kept, the lower the losses will be. By operating cryogenically (specifically near liquid hydrogen temperatures - 20 °K), the accelerator losses can be reduced by a factor of from three to five. If one were to operate at liquid helium temperatures (4 °K), the accelerator would become superconducting and losses could be practically eliminated. In the superconducting case, the total power would only be the power required by the beam.This paper addresses cryogenic linear accelerator (linac) operation. No linac with beam parameters such as those required for the space-based NPB has ever been designed or built to operate cryogenically. Work is currently ongoing in this area to demonstrate feasibility. The concept of a superconducting accelerator is only feasible in the far term and is not discussed further in this text.