Radial compression of a non-neutral plasma in a cusp trap for antihydrogen synthesis

We present the radial compression of an electron plasma by using a rotating wall in a spindle cusp region of an axisymmetric magnetic quadrupole. The compression rate depended on the rotating frequencies and had a broad peak extending on both sides of a longitudinal (1,0) mode frequency, which was the only observed characteristic frequency. The formation of the high-density plasma is one of the important milestones for the antihydrogen formation and extraction of ultraslow spin-polarized antihydrogen beams in the spindle cusp.

Figure 1

(Color online) (a) Schematic view of the trap electrodes, field lines, and confined plasma. Electrodes G4 and F4 produce rotating wall rf fields. (b) Magnetic field strength profile at the coil current of $60\mathrm{A}$, (c) electrostatic potential profile in the confinement phase on the device axis $r=0$.

Figure 2

(a) Response of plasma on the external longitudinal rf electric field at the resonance frequency. Ch1: External rf voltage of $f=8.9\mathrm{MHz}$ and $V=0.4\mathrm{V}$ added on electrode F5, and ch2: excited fluctuation received on electrode F3. Resonant frequencies $f_{\mathrm{z}}$ as functions of (b) gradient of the external electrostatic potential well, represented by ${\varphi }_0∕L^2$, and (c) cusp magnetic field strength.

Figure 3

(Color online) Temporal evolution of the line-integrated density profiles of a plasma (a) with and (b) without the rotating wall, and (c) $n_l$: line-integrated density at $r=0$. The solid and dotted lines are to guide the eyes only.

Figure 4

Total electron number $N_t$ and line-integrated density on the device axis $n_{l(r=0)}$ at $t=20\mathrm{s}$ as a function of applied rotating wall frequency. Directions of the applied rotating wall were equal to (solid circles and solid squares) and opposite to (open circles and open squares) the $\mathbf{E}\times \mathbf{B}$ plasma rotation.

Figure 5

Temporal evolutions of $T_{e\parallel }$ when the rotating wall was applied from $t=0$ to $200\mathrm{s}$ (triangles) and not applied (circles).