Commercial helicon sources need heavy magnets and cannot cover large substrates The PMT (Trikon) MØRI source

Solution 1: Use distributed sources Experiment at PMT, ca 1995 However, still need a large electromagnet

Solution 2: Use permanent magnets Material: NdFeB Bmax = 12 kG The magnets are dangerous! The innovation involves two parts: 1. Novel use of PMs 2. Use of the low-field peak effect. UCLA

The field of annular permanent magnets Internal field External field Plasma created inside the rings follows the field lines and cannot be ejected. The field reverses at a stagnation point very close to the magnet.

Proof of principle on 3” diam tube External field Internal field The bottom curve is when the tube is INSIDE the magnet

Mechanism of the Low Field Peak Constructive interference of reflected wave Basic helicon relations UCLA

Design of discharge tube: maximize the loading resistance Low-field peak Calculations are done using the HELIC code of D. Arnush

Final design of “stubby” discharge tube

Medusa 2: An 8-tube linear test array Top view UCLA

The array source is vertically compact Side view Probe ports The magnets can be made in two pieces so that they hold each other on an aluminum sheet. Once placed, the magnets cannot easily be moved, so for testing we use a wooden support. UCLA

Wooden frame for safe storage UCLA

The wooden magnet frame is used in testing UCLA

Antenna connections UCLA For CW operation, all connections were solidly soldered, and RG/393 teflon-insulated cable was used. Cable connectors cannot take the startup voltage. UCLA

Matching circuit for N tubes in parallel The problem with array sources is that the cable lengths cannot be short. The match circuit cannot be close to all the tubes. UCLA

There is a “sweet spot” for tube design when RF is considered. Matching sets limits on antenna inductance and cable lengths. There is a “sweet spot” for tube design when RF is considered. C1, C2 for N=8, L = 0.8mH, Z1 = 110 cm, Z2 = 90 cm

A water-cooled, 50-W, low resistance, rectangular transmission line

Medusa 2 in operation UCLA

Density profiles across the chamber 3.5 Compact configuration, 3kW Side Langmuir probe << 4” below tubes << 7” below tubes UCLA

Density profiles across the chamber 7 -7 14 Staggered configuration, 3kW Bottom probe array UCLA

Density profiles along the chamber Staggered configuration, 3kW Bottom probe array UCLA

Density profiles along the chamber Compact configuration, 3kW Bottom probe array UCLA

A compact, stackable module The match circuit fits on top of the array