Applications of permanent- magnet sources and arrays Francis F. Chen INER, February 24, 2009

Helicon sources are ICPs with a DC B 0 This is a commercial helicon source made by PMT, Inc. and successfully used to etch semiconductor wafers. It required two large and heavy electromagnets and their power supplies. Computer chips are now etched with simpler sources without a DC B-field. New applications require larger area coverage.

Possible uses of large-area plasma processing Roll-to-roll plastic sheets Smart windows OLED displays Solar cells, mass productionSolar cells, advanced

Distributed helicon source: proof of principle Achieved n > 1.7 x cm -3, uniform to  3%, but large magnet is required. F.F. Chen, J.D. Evans, and G.R. Tynan, Plasma Sources Sci. Technol. 10, 236 (2001)

The problem with small magnets A small solenoidField lines diverge too rapidly Annular permanent magnets have same problem

However, the external field can be used Note that the stagnation point is very close to the magnet Place plasma in the external field, and eject downwards

External field Internal field The bottom curve is when the tube is INSIDE the magnet PM helicons: proof of principle

Evolution of a multi-tube PM helicon source 1.Antenna design 2.Discharge tube geometry 3.Permanent magnets 4.RF circuitry Next: construction and testing of Medusa 2 Medusa Medusa 1

Helicon m = 1 antennas Only the RH polarized wave is strongly excited Nagoya Type III antenna: symmetric, so RH wave is driven in both directions. RH helical antenna: RH wave is driven only in the direction matching the antenna’s helicity. This antenna has the highest coupling efficiency

Why we use an m = 0 antenna A long antenna requires a long tube, and plasma goes to wall before it gets out. An m = 0 loop antenna can generate plasma near the exit aperture. Note the “skirt” that minimizes eddy currents in the flange. Now we have to design the diameter and length of the tube.

The low-field peak: an essential feature The peak occurs when the backward wave is reflected to interfere constructively with the forward wave. R is the plasma resistance, which determines the RF power absorbed by the plasma,

Designing the tube geometry Adjust a, H, and  RF so that n and B are in desired range.

This is done with the HELIC code D. Arnush, Phys. Plasmas 7, 3042 (2000). L c is made very large to simulate injection into a processing chamber. The code computes the wave fields and the plasma loading resistance R p vs. n and B

Choose a peak at low B, mid cm -3 density Low-field peak

Typical R(n,B) curves at the low-field peak Vary the B-fieldVary the tube length Vary the tube diameterVary the RF frequency

Final tube design for MHz Material: Pyrex or quartz With aluminum top

Reason for maximizing R p : circuit loss R c R c = 1.0  R c = 0.1 

Magnet design for G Vary the outside diameter Vary the vertical spacing

Final magnet design NdFeB material, 3”x 5”x1” thick B max = 12 kG

RF circuitry For equal power distribution, the sources are connected in parallel with equal cable lengths. The problem is that the cable lengths, therefore, cannot be short. The length Z2 and the antenna inductance L are the most critical.

C1, C2 for N=8, L = 0.8  H, Z1 = 110 cm, Z2 = 90 cm (unless varied) Allowable values of C1, C2 in match circuit There is an upper limit to each antenna’s inductance L. The range of Z2 can be restrictive for large arrays

Layout of 8-tube test module, Medusa 2 Compact configurationStaggered configuration The spacing is determined from the single-tube density profiles to give 2% uniformity

Side view Probe ports Aluminum sheet Adjustable height The source requires only 6” of vertical space above the process chamber Z1 Z2

Medusa 2 in operation at 3 kW CW

Radial profile between tubes at Z2

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

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

An linear array of 15 probes UCLA H. Torreblanca, Multitube helicon source with permanent magnets, Thesis, UCLA (2008).

Density profiles along the chamber Staggered configuration, 2kW Bottom probe array

UCLA Density profiles along the chamber Compact configuration, 3kW Bottom probe array Data by Humberto Torreblanca, Ph.D. thesis, UCLA, 2008.

Application to light gases, like hydrogen

Hydrogen RnB scans for MHz No stable solution for hydrogen. Here, H is distance from antenna to endplate.

Hydrogen helicons in Medusa 2 tube The lower hybrid frequency  LH) is 6.5 times higher for H than for Ar and is not >  (LH). Need to decrease B to have lower  (LH), but low B means bad coupling, like ICPs. Since k  is same if we keep 2” diam tube, we have to increase  (RF) and change n and k z.

Meaning of the lower hybrid frequency The exact lower hybrid frequency  LH is given by where  p is the ion plasma frequency. The last term is negligible except at very low density, so  LH is proportional to B/  M. In simple helicons,  is >>  LH and  c, so the ions cannot move with the RF. When  LH approaches  RF, the ions will move and contribute to the helicon current. Scime et al. have seen increased ion temperatures when  ~  LH, but HELIC does not show any great effect there. At  LH, the ion and electron orbits  to B look like this: The blue line is the ion cyclotron orbit, which has been distorted by the LH wave. The red line is the orbit of the electron guiding-center E x B drift. The cyclotron orbits of the electrons is too small to see.

There are stable solutions, but n has to be high, requiring LOTS of power. Hydrogen RnB scans for MHz

Compare hydrogen at MHz with argon at MHz to get an idea of how the discharges behave in the standard 2” diam tube H is essentially the tube length

How does the power deposition look in normal Ar discharges? Here P(z) and P(r) are the power deposition profiles in z and r, and P(k) is the power spectrum. The cases are at two low-field peaks, and the spectrum is almost a pure mode. The dashed line is the location of the antenna. * *

This compares the profiles for argon and hydrogen in the same 2 x 2” tube and at the same conditions: B = 50G and n = 3 x cm -3. However, f = MHz for argon and MHz for hydrogen. Compare similar H and Ar discharges

Power deposition profiles for two very different cases P(r) is dominated by the TG mode and does not vary much. P(z) peaks near the antenna (dashed line in each case). High P near endplate is not good, since plasma created there is lost fast. The k-spectrum is pure for H = 1.5” but has other modes for H = 3.5”, as seen by the wiggles in the RnB curve on the last page.

Comparison of waves in 1.5 in. and 3 in. long tubes The short tube has higher P(z), but it is high near the endplate. The electric field |Ez|, however, fits properly, whereas it is too short for the 3” tube. The maximum of Ez at the endplate causes strong reflection, which gives a higher low-field peak. Thus, the short tube is better even though a lot of useless ionization occurs near the endplate. This shows that computing Ez may be the best way to fit the tube length to the half- wavelength of the helicon wave and optimize the loading.

Comparison of 3 optimized systems of different diameters For hydrogen at MHz Tube: 2” diam, 1.5” high Magnet: 3 x 5”, 2” high Tube: 3” diam, 2” high Magnet: 4 x 6”, 2” high Tube: 6” diam, 3” high Magnet: 7 x 10”, 4” high Note: antenna inductance has to be adjusted

Application to spacecraft thrusters

A Hall-effect thruster It requires an electron neutralizer

Generation of a “double layer” The Bohm velocity is reached when  = ½, and sheath forms

Potential jump observed by Charles et al.

B-field in Boswell’s helicon machine

Medusa source adapted to VASIMR The optimized 9-cm diam source is shown with dimensions in cm, together with a NdFeB magnet designed for 400G at the antenna. D is the distance from the midplane of the magnet to the midplane of the antenna. The magnet is made in two pieces supported by a non-ferrous metal plate. The B-field can be adjusted by changing D either by hand or remotely with a motor.

A stronger B-field for higher density Layout of magnet and tube for 600G operation, showing a gas feed line and a DC bias supply.

A small diam source with for testing high-field operation A 5-cm diam helicon tube and a 600-G magnet designed for a small overall system diameter.

Conclusion on spacecraft thrusters “Ambipolar” sources can eject ions with automatic space-charge neutralization. Helicon sources can generate ions efficiently. Permanent magnets can reduce the complexity of helicon sources. However, for the fields and densities considered for the VASIMR project, the magnet may be too large to be practical.