Laboratory evidence of a supersonic ion beam generated by a current-free “helicon” double-layer

Abstract

An electric double-layer is generated near the open end of a high-density low pressure helicon sustained radio frequency (13.56 MHz) plasma source which expands into a diffusion chamber. Ion energy distribution functions measured with a retarding field energy analyzer placed in the diffusion chamber with its aperture facing the double-layer show the presence of a low energy peak (∼29 V) around the local plasma potential and a high energy peak (∼47 V) corresponding to a supersonic ion beam (∼2.1cs). At an axial distance 12 cm downstream of the double-layer, the beam density is 14% of the local density at that position and the ion energy gain is approximately 70% of the potential drop of the double-layer. The ion beam is observed from the center out to a radius corresponding to that of the plasma source tube (−6.8 cm⩽r⩽+6.8 cm) and is not greatly affected by the expanding magnetic field. A depression in the total ion flux just downstream of the double-layer—previously measured on the main z-axis of the reactor—is also present across the chamber diameter. Evidence of an electron beam near the closed end of the source tube, generated via “backwards” acceleration through the double-layer, has been observed on a Langmuir probe trace.

Experimental investigation of double layers in expanding plasmas

“…In order to compare the experimental beam decay to models, assumptions on the actual DL 3-D profile ar required since direct measurement of the profile is not easily achieved. We therefore consider two cases, namely a flat profile (perpendicular to the z axis) following the statements given in [7] and a hemi-spherical profile. These profiles are sketched in figure 4(a).…”

“…In these experiments, a plasma expansion was forced by a strong divergence of a high magnitude magnetic field at very low pressure. Detailed investigations of these currentfree DLs have been performed using electrical measurements [2,[6][7][8][9][10][11], laser induced fluorescence [12][13][14][15][16] and numerical simulations [17,18]. An upstream ionization instability has recently been observed in one of the experimental systems [19].…”

Sheaths: More complicated than you think

“…However, naturally occurring double layers have been identified in nonuniform magnetized plasma. [29][30][31] Representative experimental double layers in which trapped and free particles are provided at plasma boundaries are shown in Fig. 17 ͑Ref.…”

Transport of energetic electrons in a magnetically expanding helicon double layer plasma

“…8 Experiments have previously characterized the beam and bulk ions two dimensionally using retarding field energy analyzers ͑RFEAs͒ and Langmuir probes. 3,[16][17][18][19][20] Ions accelerated by DLs in low-pressure expanding plasmas are weakly divergent and magnetically detached about 10 cm downstream of the double layer. [16][17][18] The physical mechanisms leading to the dense ring of bulk plasma observed in Langmuir probe measurements downstream of the double layer has been unclear.…”

“…The formation of current-free electric double layers 1 ͑DLs͒ and subsequent acceleration of ions 2,3 in magnetically expanding, rf plasmas have been the subject of vigorous research in the electric propulsion community, due in part to the potential for constructing an innovative, long-lived plasma thruster: the helicon double layer thruster 4 ͑HDLT͒. Investigation into the formation of DLs and the related ionic and electronic behavior in one dimension have been performed experimentally, 1-10 analytically, [11][12][13] and through simulation.…”