Commercial helicon sources inject plasma into a field-free region The MORI sourceA helicon injection expt.

We wish to optimize a source which could be part of a large array Metal or ceramic endplate Arbitrary antenna We can use a low-field density peak seen in at B  30G for n  cm -3

The low-B peak has been seen in many different experiments The field at the peak can range between 10 and 50G, and the peak is much larger in some experiments than in others. 1989, 1" tube

The low-B peak is also seen in a 7-tube array of m = 0 sources 8 mTorr, MHz

Computations using Don Arnush's HELIC code predict this peak and show how it varies HELIC calculates waves and loading with Radial density profile n(r) Collisional damping Different (thin) antenna configurations Antenna coupling Trivelpiece-Gould modes Endplates (arbitrary reflectivity) It requires Uniform B-field Uniform n(z) Cold-plasma dielectric

The plasma loading R(  ) shows a low-B peak which moves with density range Single-loop m = 0 antenna, 10 cm from endplate, 2 mTorr, Te = 4eV

At high densities, the low-B peak is gone range Single-loop m = 0 antenna, 10 cm from endplate, 2 mTorr, Te = 4eV

The low-B peak vs. density at constant B Single-loop m = 0 antenna, 10 cm from endplate, 2 mTorr, MHz

What is the cause of the low-B peak? It is not the lower hybrid resonance (1450 G) Is it due to a resonance between the helicon mode and the Trivelpiece-Gould mode, when they have comparable radial wavelengths and can interfere constructively? Is it due to constructive interference by the wave reflected from the endplate? Computations show that it is probably the latter.

"Standard" conditions for numerical tests B = G n o = cm -3 p = 10 mTorr Ar Insulating endplate KT e = 4 eV (affects collisions only) f = MHz Radial density profile

The low-B peak sharpens at lower pressure

The peak is sensitive to the density profile

The peak depends on the boundary condition

The peak depends on distance from endplate

The peak depends on the type of antenna Single loop: m = 0, bidirectional HH (half-wavelength helical): m = 1, undirectional Nagoya Type III: m = 1, bidirectional

Axial deposition profile of HH10 antenna depends on the sign of the helicity Direction of propagation:  (m = +1),  (m = -1)

This explains previous data on density enhancement by an aperture limiter Block can be solid or have a 1.2-cm diam hole. It can be conducting (carbon) or insulating (boron nitride). It can be moved to various positions behind or under the antenna.

Radial density profile shows enhancement whenever there is a limiter

At 800G, the limiter position is not critical

The end coils can also be turned off or reversed to form a cusped B-field The field lines then end on the glass tube, which forms an insulting endplate. An aperture limiter can also be added.

The cusp configuration doubles the amount of plasma created

Limiter enhancement with and without a magnetic cusp The cusp field greatly enhances the density without a limiter, but adds little when a limiter is already in place.

Density increase with end coil current is larger with a bidirectional antenna

CONCLUSION For low-field, low-density helicon injection into a large chamber, reflection of waves from an endplate can be designed to optimize plasma production. This phenomenon is probably responsible for previously unexplained density increases with aperture limiters and cusped magnetic fields. This is a significant effect.