A helicon source requires a DC magnetic field.. U. Wisconsin

...and is based on launching a circularly polarized wave in the plasma UCLA

The antenna can be twisted to match the helicon's helical waveform UCLA

The R-wave propagates to the right, and the L-wave to the left (for this antenna helicity) UCLA But the L-wave is very weak, and this antenna is unidirectional

Why do helicons ionize so well? UCLA Landau damping (1985) F.F. Chen, Plasma Phys. Control. Fusion 33, 339 (1991). This was disproved (1999) F.F. Chen and D.D. Blackwell, Phys. Rev. Lett. 82, 2677 (1999). Mode-coupling to TG modes (1996) K.P. Shamrai and V.B. Taranov, Plasma Sources Sci. Technol. 5, 474 (1996). Parametric excitation of ion acoustic waves (2005). B. Lorenz, M. Krämer, V.L. Selenin, and Yu.M. Aliev, Plasma Sources Sci.Technol. 14, 623 (2005).

Two commercial helicon reactors UCLA The Boswell source The PMT (Trikon) MØRI source

Distributed source: first attempt UCLA Each tube with a solenoidal coil and helical m = +1 antenna A 7-tube circular array. This failed to produce high density.

Reason: Diverging field lines UCLA

Distributed source: Second attempt UCLA This was better

Distributed source: Third attempt UCLA The “stubby” tube This worked beautifully! But…

Plasmas merged; density is uniform UCLA …but the size is limited by the single large electromagnet.

Characteristics of permanent magnet rings UCLA Internal field External field

Experiments with 7-cm diam tube UCLA External field Internal field

Radial density profiles at Z1 and Z2 UCLA Upper probeLower probe x cm -3 Proof of principle: discharge in the external field gives much more plasma downstream.

Optimization of magnet geometry UCLA Result: Field strength  magnet volume Spacing improves uniformity slightly actual

Optimization of discharge tube: HELIC code UCLA D. Arnush, Phys. Plasmas 7, 3042 (2000). Radial profiles are arbitrary, but B and n must be uniform axially. HELIC gives not only the wave fields but also R, the loading resistance.

The low-field peak UCLA Typical HELIC result

Relation of R to plasma density UCLA Rp << Rc

Relation of R to plasma density UCLA Rp > Rc

HELIC and expt. matrices varied a, L, f, p, endplate UCLA

Examples: Tube diameter, frequency UCLA Larger diameter gives higher plasma resistance, but this is not practical MHz is much better than 2 MHz.

Final design UCLA Very similar to “stubby” tube, designed by intuition! Only improvement is the metal top. A single NdFeB magnet

The magnets are dangerous! UCLA Material: NdFeB Bmax = 12 kG Attractive force between two magnets 2 cm apart: 516 Newtons = 53 kG

Wooden frame for safe storage UCLA

Single tube, final configuration UCLA Radial density profiles at Z1 = 7.4 cm and Z2 = 17.6 cm below discharge. Radial Bz profiles at various distances below the magnet. Discharge tube

Design of array UCLA The density at Z2 is summed over nearest tubes. For a single row, a distance L = 17.5 cm between two tubes gives less than 2% ripple in density.

Computed uniformity n(x) for various y Half-way between rows1/4-way between rows Directly under a rowBeyond both rows

An 8-tube linear test array UCLA

The array source is vertically compact UCLA The magnets are to be stuck onto an iron plate, which holds them and also concentrates the flux. Once placed, the magnets cannot easily be moved, so for testing we use a wooden support.

The wooden magnet frame is used in testing UCLA

An 8-tube staggered array in operation UCLA

Possible applications UCLA Web coaters Flat panel displays Solar cells Optical coatings A web coater