Experiments with Dense Plasma in the Central Solenoid of Ambal-M

At present the axisymmetric ambipolar mirror trap AMBAL-M consists of a central solenoid which is attached to a plugging and MHD stabilizing end system and is filled from the other end by a plasma stream generated by a gas-discharge source. In the first experiments we obtained the plasma in the solenoid with ~0.4 m diameter, density ~6·10¹²cm⁻³, electron temperature ~50 eV, and ion energy ~250 eV. In order to enhance the plasma flow from the source into the solenoid, the distance between the entrance throat of the solenoid and the plasma source was gradually decreased, and the plasma density was increased to ~2·10¹³ cm⁻³. Installation of a second source from the opposite end of the machine allowed us to increase the plasma density up to ~2.5·10¹³ cm⁻³ in the solenoid and up to ~1.5·10¹³ cm⁻³ in the mirror trap of the end system. For better propagation of the plasma stream from the second source into the trap the coil of the MHD-stabilizer semicusp was switched in the same direction as all other coils, thus the magnetic configuration consisted of a series of simple mirrors. However, the plasma remained MHD stable owing to its line-tying to conducting ends. When this line-tying broke during the fast cut-off of the source current, the density profile in the solenoid abruptly rearranged pointing to possible MHD activity, and independently of the initial shape it became almost flat up to the limiter.

Further enhancement of the plasma density was achieved using hydrogen puffing into the solenoid plasma while only the first source was functioning. Two methods of gas puffing were used – through a ceramic tube to the solenoid axis and into a gas-box surrounding the plasma. Optimization of the hydrogen puffing rate led to the density increase up to ~ 6·10¹³ cm⁻³ without noticeable degradation of the ion temperature which remained at a high level of ~200 eV (the ratio of the plasma pressure to the magnetic field pressure β~0.1), which is provided by stochastic ion heating from electrostatic oscillations when the source is working. The obtained solenoid plasma density is the highest one achieved in ambipolar mirror traps.