  The paper reports on a new plasma startup scenario in the Gas Dynamic Trap
(GDT) magnetic mirror device. The primary 5 MW neutral beam injection (NBI)
plasma heating system fires into a sufficiently dense plasma target ("seed
plasma"), which is commonly supplied by an arc plasma generator. In the
reported experiments, a different approach to seed plasma generation is
explored. One of the channels of the electron cyclotron resonance (ECR) heating
system is used to ionize the neutral gas and build up the density of plasma to
a level suitable for NBI capture. After a short transition (about 1 ms) the
discharge becomes essentially similar to a standard one initiated by the plasma
gun. The paper presents the discharge scenario and experimental data on the
seed plasma evolution during ECR heating, along with the dependencies on
incident microwave power, magnetic configuration and pressure of a neutral gas.
The characteristics of consequent high-power NBI discharge are studied and
differences to the conventional scenario are discussed. A theoretical model
describing the ECR breakdown and the seed plasma accumulation in a large scale
mirror trap is developed on the basis of the GDT experiment.
