Influence of background gas on flute instability produced at interface between laser plasma and external magnetic field

Abstract

Diamagnetic cavity and flute instability generated by plasma expansion in an external magnetic field are important phenomena in space and fusion physics.We use a nanosecond laser to irradiate a carbon planar target to generate plasma, and at the same time apply a 7T transverse pulsed strong magnetic field to the plasma. The flute instability generated on the surface of the diamagnetic cavity when the plasma expands in an external magnetic field is studied experimentally. Data analysis shows that under our experimental parameters, the radius of gyration of electrons(<i>ρ</i><sub>e</sub>) is much smaller than the density gradient scale length of the diamagnetic cavity(<i>L</i><sub>n</sub>), while the ion's gyration radius(<i>ρ</i><sub>i</sub>) is much larger than <i>L</i><sub>n</sub>, indicating that the electrons are magnetized while the ions are non magnetized. The relative drift between electrons and ions provides free energy for the development of instability.The drift velocity is composed of the gravity drift velocity and the diamagnetic gradient drift velocity. The calculation shows that the gravity drift velocity is much larger than the diamagnetic gradient drift velocity in our experiment, so the instability belongs to the Large Larmor Radius Instability. By filling the target chamber with helium, we found that the background gas can significantly inhibit the development of flute instability. When the background gas pressure exceeds 50Pa (about 1% of the interface plasma density), the flute instability is almost is completely suppressed. Kinetic dispersion equations show that ion-ion collisions and electron-ion collision effects are the main factors that inhibit the development of instability. Calculations on the dispersion equation show that ion-ion collisions are the main factor that inhibits the development of instabilities.