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Use of Langmuir probes in strong RF plasmas Francis F. Chen, UCLA KAIST, Daejeon, S. Korea, April 2011

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Commercial probe systems Shown here is the Hiden ESPION probe. I use the older Hiden ESP probe software, but I make my own probes. I presume you are familiar with the PlasMart probe.

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The Chen B probe To reject RF pickup, resonant chokes (inductors) and a good auxiliary electrode are needed.

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The choke must be >200 k This is a good choke. High Z is good for low density. High frequency can use lower Z. It is sometimes possible to adjust the RF frequency to match the choke.

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How RF distorts the I-V characteristic The RF shifts Vp back and forth. Since the I-V curve is nonlinear, the average current does not reproduce the curve. This shows what the uncompensated I-V curve would look as the RF pickup voltage is varied.

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Why the auxiliary electrode is needed Probe tip Cs1 causes the choke to lose part of the oscillation of the probe. The large,floating auxiliary electrode (Zx) strongly drives the choke to oscillate with the plasma’s RF fluctuations.

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The orbital-motion-limited theory Large probe, dense plasma, thin sheath Small probe, weak plasma, thick sheath Langmuir’s Orbital-Motion- Limited (OML) theory

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Langmuir’s simple OML formula OML is valid only when the sheath is so thick that there is no “absorption radius”. However, it works better than other theories even when it should not be applicable. Both Hiden and PlasmArt use this simple formula. To use this formula, probe tips should be as thin as possible to minimize R p / D.

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The simple OML formula The ion saturation current Isat is independent of T e and can be used easily to measure density n. Isat varies as the square root of V p. This is a characteristic of ion orbiting when there is no absorption radius. However, the linear I 2 – V p relation is followed even when the R p / D is not so large! A p is the probe tip area, M i the ion mass, and V p the probe potential relative to the plasma potential.

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A typical I 2 – V p curve for n < 10 12 cm -3

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. How did Langmuir get such a simple formula? This is what he started with for Maxwellian ions: s is an assumed sheath radius at which ions start with their thermal velocities

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Then he made some dubious approximations The ion temperature cancels out!

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Attempt to use the exact OML formula At high density, the curve does not fit a straight line. Using the exact OML formula gives too much curvature even if the sheath radius is adjusted to give the best fit. It is still unknown why the I 2 -V curve is so close to linear.

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The semilog electron curve 1 First, we have to fit the ions so that we can subtract them

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The semilog electron curve 2 Now we make the semilog electron curve Note that the right amount of ion current added back is essential

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False indication of electron beams

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Apparatus for helicon thruster 1 magnet, 65 gauss2 magnets, 280 gauss max (lower)

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Very thin vertical probe

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Effect of auxiliary electrode 1: no electrode The I-V curve looks more rounded. The I 2 - V curve is linear, but goes down fast. N = 15.8E11 T e = 3.65 eV T e = 10.4 eV

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Effect of auxiliary electrode 2: with electrode I-V curve more normal. n = 16.8E11 T e = 3.01 eV The temperature is more normal, but the high-T e part still exists. Need a larger auxiliary electrode.

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1000 W with a 5-mil probe The thinnest probe (125 m diam) gives I 2 -V curves that bend at high density. There is no theory that predicts the curve shape. This probe will glow in the discharge unless the sweep time is minimized. We can use a thick probe and thin-sheath theory, but the discharge will be disturbed by the probe current.

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