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Design of PM helicon arrays

Design of PM helicon arrays. Optimization of the discharge tube Design of the permanent magnets Design of a multi-tube array Design and construction of a test chamber Antennas and the RF distribution system Experimental results Design of a compact module

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Design of PM helicon arrays

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  1. Design of PM helicon arrays • Optimization of the discharge tube • Design of the permanent magnets • Design of a multi-tube array • Design and construction of a test chamber • Antennas and the RF distribution system • Experimental results • Design of a compact module • Ideas for further improvements to be tested UCLA

  2. A commercial helicon etcher (PMT MØRI) It required two heavy electromagnets with opposite currents.

  3. Previous experiment with 7 tubes The “stubby” tube It required a large electromagnet UCLA

  4. Plasmas merged; density is uniform High density and uniformity were achieved UCLA

  5. Optimization of discharge tube: HELIC code 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. D. Arnush, Phys. Plasmas 7, 3042 (2000). UCLA

  6. The HELIC user interface UCLA

  7. The Low-field Peak UCLA

  8. Mechanism of the Low Field Peak Basic helicon relations UCLA

  9. The peak is sensitive to the density profile

  10. The peak depends on the boundary condition

  11. The peak depends on distance from endplate

  12. 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

  13. Typical scan of Rp vs n, B Each point requires solving a 4th order differential equation >100 times. A typical scan takes ~ 3 hours on a PC. UCLA

  14. Matrices for optimizing discharge tube Vary the tube length and diameter Vary the RF frequency Vary the endplate conductivity UCLA Vary the pressure and frequency

  15. Vary H (endplate distance) for 3” diam UCLA

  16. Vary diam for H = 2” at 100G UCLA

  17. Vary the frequency UCLA

  18. Not much variation with pressure UCLA

  19. Vary the endplate material Initially, it seems that the conducting endplate is better. However, it is because the phase reversal at the endplate has changed, and the tube length has to be ~1/4 wavelength longer to get constructive interference. By changing H, almost the same R can be achieved. UCLA

  20. Relation of R to plasma density Rp << Rc UCLA

  21. Relation of R to plasma density Rp > Rc UCLA

  22. Final design UCLA

  23. The “New Stubby” tube UCLA

  24. Design of PM helicon arrays • Optimization of the discharge tube • Design of the permanent magnets • Design of a multi-tube array • Design and construction of a test chamber • Antennas and the RF distribution system • Experimental results • Design of a compact module • Ideas for further improvements to be tested UCLA

  25. Internal field External field Characteristics of permanent magnet rings UCLA

  26. The B-field of annular PMs The field reverses at a stagnation point very close to the magnet. Plasma created inside the rings follows the field lines and cannot be ejected. UCLA

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

  28. The field of 4 stacked magnet rings The internal and external fields at various radii. The individual rings can be seen at large radii. Calibration of the calculated field with a gaussmeter. UCLA

  29. For the designed tube, B ~ 60G is good UCLA

  30. External field Internal field Proof of principle on 3” diam tube UCLA

  31. Radial density profiles at Z1 and Z2 Upper probe x 1010cm-3 Lower probe Proof of principle: discharge in the external field gives much more plasma downstream. UCLA

  32. The final design for 2” tubes Material: NdFeB Bmax = 12 kG Attractive force between two magnets 2 cm apart: 516 Newtons = 53 kg The magnets are dangerous! UCLA

  33. Wooden frame for safe storage UCLA

  34. Single tube, final configuration Radial Bz profiles at various distances below the magnet. Discharge tube UCLA

  35. Design of PM helicon arrays • Optimization of the discharge tube • Design of the permanent magnets • Design of a multi-tube array • Design and construction of a test chamber • Antennas and the RF distribution system • Experimental results • Design of a compact module • Ideas for further improvements to be tested UCLA

  36. Design of array Radial density profiles at Z1 = 7.4 cm and Z2 = 17.6 cm below discharge. The density at Z2 is summed over nearest tubes. UCLA

  37. Computed uniformity n(x) for various y Half-way between rows 1/4-way between rows Directly under a row Beyond both rows

  38. A tube spacing of 7” is chosen For a single row, a distance L = 17.5 cm between two tubes gives less than 2% ripple in density. UCLA

  39. Design of PM helicon arrays • Optimization of the discharge tube • Design of the permanent magnets • Design of a multi-tube array • Design and construction of a test chamber • Antennas and the RF distribution system • Experimental results • Design of a compact module • Ideas for further improvements to be tested UCLA

  40. An 8-tube linear test array Top view UCLA

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

  42. 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

  43. The wooden magnet frame is used in testing UCLA

  44. Water and RF connections These will be shown in detail later UCLA

  45. An 8-tube staggered array in operation UCLA

  46. Design of PM helicon arrays • Optimization of the discharge tube • Design of the permanent magnets • Design of a multi-tube array • Design and construction of a test chamber • Antennas and the RF distribution system • Experimental results • Design of a compact module • Ideas for further improvements to be tested UCLA

  47. Antennas The antennas are m = 0 loops made of three turns of 1/8” diam copper tubing. The reason for m = 0 is that m = 1 antennas are too long, and much of the plasma is lost by radial diffusion before exiting the tube. The antenna must be close to the exit aperture and be tightly wound onto the tube. The helicon wave pattern for m = 0 is a peculiar one but theory is straightforward. The wave changes from pure electromagnetic to pure electrostatic in each half cycle. UCLA

  48. The RF system The critical elements are the junction box and the transmission lines. UCLA

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