Earl E Scime, Paul A Keiter, Michael W Zintl, Matthew M Balkey, John L Kline and Mark E Koepke

Transcription

1 Plasma Sources Sci. Technol. 7 (1998) Printed in the UK PII: (98) Earl E Scime, Paul A Keiter, Michael W Zintl, Matthew M Balkey, John L Kline and Mark E Koepke Department of Physics, West Virginia University, Morgantown, WV 26506, USA Received 30 September 1997, in final form 23 March 1998 Abstract. Laser induced fluorescence measurements of the parallel and perpendicular ion temperatures in a helicon source indicate the existence of a substantial ion temperature anisotropy, T -L/7i1 > 1. The magnitude of the ion temperature anisotropy depends linearly on the source magnetic field. The parallel ion temperature is independent of magnetic field strength while the perpendicular temperature increases linearly with increasing magnetic field. Bohm-like particle confinement is proposed as an explanation for the linear dependence on magnetic field of the perpendicular ion temperature. In the helicon mode, the ion temperature components are independent of RF driving frequency and power and show a trend towards isotropy at high neutral fill pressures. 1. Introduction The West Virginia University (WVU) hot helicon experiment (HELIX) was constructed as part of a larger experimental device intended to investigate space-relevant plasma phenomena. One class of space plasma phenomena of interest is electromagnetic, ion temperature anisotropy (parallel versus perpendicular with respect to the local magnetic field) driven instabilities [1]. HELIX includes a number of unique features: abroad driving frequency range ( MHz); high-power, steady-state operation (2 kw); an ion cyclotron resonant heating (ICRH) system and a laser induced fluorescence (LIp) ion temperature diagnostic system. The ICRH system is intended for controlling the ion temperature anisotropy in the plasma by enhancing the perpendicular ion temperature relative to the parallel ion temperature. However, in this paper we report measurements of the parallel and perpendicular ion temperatures in HELIX plasmas that indicate a substantial ion temperature anisotropy already exists in unheated helicon discharges. The experimental apparatus, including the source and the LIF system, is described in section 2. The scaling of the parallel and perpendicular components of the ion temperature as a function of driving frequency, magnetic field strength and operating pressure is presented in section 3. The results are discussed and interpreted in section 4. In particular, a physical model for the observed ion temperature anisotropy and the Bohm-like scaling of the perpendicular ion temperature is suggested. 2. Experimental apparatus In situ probe measurments of plasma density and electron temperature in HELIX demonstrated typical helicon source operation [2]. Those measurements were performed on a slightly different configuration of the helicon source. The experiments reported here were performed in a new vacuum chamber that was installed specifically for LIF measurements of the ion velocity distributions. Except for two pairs of opposing 2.5 cm ports arranged in a single cross configuration (figure I), the new chamber is identical to the previous chamber (1.5 m long with an inner diameter of 0.16 m). The configuration of the driving antenna is also new. Previously a Nagoya III antenna [3] was used. The new antenna is a half-turn, helical antenna designed to drive the m = 1 helicon wave. With the new antenna, the helicon resonance is achieved at even lower powers than previously reported [2], e.g., 400 W at a driving frequency of 8 MHz. When reaching the helicon resonance, the plasma density increases sharply and the discharge becomes more tightly confined along the axis of the experiment. At higher powers, a bright blue core of Ar n emission is visible along the axis of the discharge. The source magnetic field can be varied from 0 to 1300 G and typical plasma parameters are n 8 x 1012 cm-3 and re 5 ev. The absorption and emission frequencies of an atom or ion moving relative to a radiation source are Doppler shifted by its velocity.in a typical LIF measurement, the frequency of a very narrow bandwidth laser is swept across a collection of ions or atoms that have a thermally broadened velocity distribution [4]. The shift in the centre frequency and the width of the absorption spectrum feature for the entire ensemble of atoms or ions is then used to determine the temperature and flow velocity of the particle distribution. The LIF system used in these experiments consists of a 6 W Coherent Innova 300 argon-ion laser that pumps a Coherent 899 ring dye laser. The ring dye laser wavelength is measured with a Burleigh 1500 wavemeter. For argon plasmas, the output of the dye laser is tuned to 1998 lop Publishing Ltd

2 (a) " \ large Ion temperature anisotropy in helicon plasma ring dye laser chamber.c c,,\-t\ir---i \{::=::}-j pump laser I \\\)e.t 0\' chopping wheel '" 11 yerpendicular injection optics / -, collection optics source chamber / beam dump I -- ""' B / (b) " Figure 1. (a) The experimental geometry for ion temperature measurements perpendicular to the magnetic field. (b) The experimental geometry for parallel ion temperature measurements. The polarizer and quarter wave plate combination selects a single circular polarization of the injected light. The magnetic field shown in (a) is radially uniform to within 0.5% across the plasma column and axially uniform to within 1% over the central half of the plasma column. The helical antenna is shown in (b) nm to match the 3d to 4p transition of singly ionized argon ions. For helium plasmas, the dye laser was tuned to the 2p to 3d transition of neutral helium at nm. As the dye laser sweeps through 10 GHz, roughly 0.01 nm, the fluorescent emission from the upper metastable level (see figure 2) is measured with a filtered photomultiplier tube detector. The filter in front of the photomultiplier has a 1 nm passband centred around the emission line (459 nm for singly ionized argon and 389 nm for neutral helium). The output of the dye laser is chopped at 1 khz. The chopping signal is used as the reference for a Stanford Research SR830 lock-in amplifier that monitors the photomultiplier tube signal and isolates the fluorescence signal from the intense background emission at the same wavelength. The argon absorption transition includes a number of Zeeman-split components. For perpendicular measurements (figure l(a», only the linearly polarized Jr transitions are excited. The splitting of the Jr lines is negligible for the magnetic fields of HELIX. For parallel temperature measurements (figure l(b», the two circularly polarized a transitions are excited and the Zeeman splitting is of the order of the thermal broadening of the absorption line. Since the dye laser light is conveyed to HELIX through a multimode fibre optic cable that does not preserve the laser's polarization, a linear polarizer followed by a quarter waveplate is used at the output of the fibre to select a single circular polarization for the parallel measurements (figure l(b». The polarizer and quarter waveplate significantly reduce the available light intensity, thus the parallei measurements suffer from a lower signal-to-noise ratio than the perpendicular measurements. Typical perpendicular and parallel LIP spectra are shown in figure 3(a) for singly ionized argon and in figure 3(b) for neutral helium. Drifts in the source plasma are determined by subtracting the expected Zeeman shift from the measured shift of the parallel ion velocity distribution. Except at very low operating pressures, approximately 1 mtorr, no significant drifts were observed. 3. Ion temperature as a function of experimental parameters As shown in figure 4, the increase in the perpendicular ion temperature with increasing magnetic field is significantly larger than the increase in the parallel ion temperature for an argon helicon plasma. For each temperature measurement, ten sweeps across the absorption line are averaged together and then fitted to a single, shifted Maxwellian distribution. For argon, the FWHM of the intensity distribution, I(v), is related to the temperature by I(v) = I(vo)e-o.0719(v-f/T (I) where T is the temperature in e V, v is the laser frequency in GHz, Vo is the frequency of the absorption line in the rest frame of the ion and the coefficient in the exponential accounts for the mass of the ion and the conversion of units. For helium the relationship is I(v) = I(vo)e-O.07718(\J-f/T (2) Previous in situ measurements [2] demonstrated that the argon plasma density in HELIX increases only slightly with magnetic field above 500 a, i.e., once the helicon resonance is achieved. Therefore, the more than fourfold increase in perpendicular ion temperature with increasing magnetic field (figure 4) is not accompanied by a comparable increase in ion density.perpendicular ion temperatures of approximately 0.4 ev and parallel ion temperatures 187

3 E E Scime et al 459. n fluorescence / emission / / 4p2Fm nm pump laser L 3p3p 3d3D c;ut;t;;nal transfer..\, \ -' 4s2D 3d2G9/2 2s3S (a) (b) Figure 2. LlF schemes for (a) singly ionized argon and (b) neutral helium. Figure 4. Perpendicular (filled circles) and parallel ion temperatures (open squares) in argon plasmas as a function of source magnetic field. The error bars for the data are smaller than the circles and squares. Measurements taken at an RF power of 2 kw and pressure of 4 mtorr. Figure 3. (a) Singly ionized argon LlF spectra for parallel and perpendicular laser injection. The parallel signal is noticeably smaller and shows evidence of the expected Zeeman shift (assuming any axial drifts are ignorable). (b) Neutral helium LlF spectrum for perpendicular laser injection. The horizontal axes are the same for both plots. The different labels illustrate the relationship between frequency shift and particle velocity for the nm argon line. of 0.2 e V in an argon helicon plasma are consistent with the measurements of Nakano et al [5] when they operated their helicon source with a uniform magnetic field geometry. Boswell et al [6] reported spatially unresolved, spectroscopically determined, perpendicular ion temperatures of approximately 0.8 ev for similar argon plasma parameters in a pulsed helicon source. From microwave transmission measurements at 9.25 and 10.5 GHz, the density of these HELIX plasmas is known to exceed 1.4 x 1012 cm-3. Thus the ion-ion collision frequency for a 0.5 e V ion is of the order of a few MHz, i.e., a mean free path less than 1 cm. Intuition would suggest that under such conditions anisotropic ion temperatures cannot be sustained, yet the measurements clearly indicate a wide range of anisotropy, 1 < T 1./ TII < 4. It is important to note that the ion temperature anisotropy can be reproducibly set by adjusting the magnetic field. An explanation for both the linear increase of the perpendicular ion temperature with increasing magnetic field and the existence of such a substantial ion temperature anisotropy is suggested in section 4. The ion temperature components in argon plasmas are independent of driving frequency (figure 5(a» and weakly dependent on RF power (figure 5(b». Previous measurements showed a strong dependence of plasma density on driving frequency in the helicon mode [2], yet figure 5(a) indicates no measurable change in the ion temperature for the driving frequency range 7.5 to 12 MHz. There is a slight upward trend in the perpendicular ion temperature with RF power and the parallel ion temperature doubles as the RF power increases from 1 to 2 kw. The lack of strong correlation between the ion temperature and RF power confirms that the ion temperature increase is not an artifact of ion sloshing in the RF electric field. Preliminary 188

4 Ion temperature anisotropy in helicon plasma Figure 6. Perpendicular (filled circles) and parallel (open squares) ion temperatures in argon plasmas versus fill pressure. Measurements taken with a magnetic field of 1000 G and a RF power of 2 kw. Lines through points above 2 mtorr are to guide the eye and suggest thermal equilibration at higher fill pressures. Figure 5. (a) Perpendicular (filled circles) and parallel (open squares) ion temperatures in argon plasmas versus driving frequency. Measurements taken with a magnetic field of 800 G and a pressure of 2.6 mtorr. (b) Perpendicular (filled circles) and parallel (open squares) ion temperatures in argon plasmas versus RF power. Measurements taken at a magnetic field of 1000 G and a pressure of 2.6 mtorr. ion heating experiments using an additional antenna tuned to the ion cyclotron frequency indicate that after the antenna is turned off, the ion temperature increase due to the ion heating circuit decays over a period of milliseconds. The power in the heating circuit decays in microseconds, thus the ion heating experiments provide additional evidence that the LIP measurements accurately reflect the thermal distribution of the ions. As expected, the perpendicular and parallel ion temperatures appear to equilibrate as the neutral gas pressure is increased and the ion-neutral collision frequency increases (figure 6). It is worth noting that once the helicon mode is achieved, the plasma density increases only slightly with increasing neutral pressure [2, 7]. Surprisingly, at very low neutral pressures, such as 1.2 mtorr, both ion temperature components increase significantly and are nearly equal (figure 6). As the pressure was decreased to 1.2 mtorr, the plasma source switched from the helicon to the inductive mode of operation [2]. Therefore, the 1.2 mtorr measurements are representative of an inductively coupled, non-resonant, plasma. Previous floating potential measurements indicated dramatic differences in the particle confinement properties of helicon and inductive plasmas [2]. Perhaps a difference in the axial confinement of helicon and inductive plasmas is responsible for the sharp change in ion temperature as the source transitions between the inductive and the helicon modes. Temperature measurements in helium plasmas are problematic. Since the helium LIP scheme relies on absorption by a neutral helium atom, the measured temperatures are representative of the helium ion temperatures only if the neutral atoms and helium ions are in thermal equilibrium. The perpendicular neutral atom temperature in helium plasmas as a function of magnetic field is shown in figure 7. Although the measured temperature is very low, there is evidence of a linear dependence of perpendicular temperature on magnetic field. It is highly unlikely that the helium ion temperature is only 0.07 ev (540 C). The 0.07 ev temperature is the temperature of the neutral helium gas. The helium ion-neutral collision frequency is approximately 200 Hz for 0.07 ev ions hitting cold neutrals and the electron-neutral collision frequency is approximately 2000 Hz for 5 e V electrons hitting cold neutrals. Even after charge exchange collisions are included, the mean free path for, 0.07 ev neutral helium is of the order of 10 cm for I mtorr pressures. Therefore, each neutral helium atom will make only a few collisions each time it passes through the plasma while bouncing from wall to wall inside the source. Only during the rare, longer passage down the axis of the source will a neutral atom undergo enough collisions to gain an appreciable amount of energy. If we assume that the helium ion temperature increases with increasing magnetic field in the same fashion as in argon plasmas, this weak collisional coupling is probably responsible for the linear increase in neutral temperature with magnetic field seen in figure 7. These helium observations are consistent with neutral argon temperatures an order of magnitude smaller than singly ionized argon ion temperatures in helicon plasmas as reported by Boswell et al [6]. To measure the helium ion temperature accurately will require direct measurement of a helium ion line. Singly 189

5 E E Scime et al > t f-o c,g, Figure 7. Neutral temperature in helium plasmas versus magnetic field. Note the expanded veltical scale. Measurements taken at an RF power of 2 kw using the perpendicular optical configuration (figure 1 ). ionized helium has two visible lines at and nm. In neither case is it possible to observe the emission from a different fluorescence line. LIF is possible with a single line, but the intense background light in our helicon source will substantially reduce our signal-to-noise ratio. We anticipate attempting singly ionized helium LIF measurements during the next year. 4. Discussion Experimentally we observe perpendicular ion temperatures substantially greater than the parallel ion temperature. If we assume that there is no direct ion heating by the RF circuit, the ions can only be heated through electronion collisions or wave-particle interactions. Since the perpendicular particle confinement time can increase with increasing magnetic field, it seems plausible that the longer ions are confined the higher their temperature, i.e., the ions experience more heating through collisions with the hotter electrons. The linear dependence of the perpendicular ion temperature (figure 4 and figure 7) on magnetic field strength is suggestive of Bohm-Iike particle confinement. For Bohm radial diffusion in a cylindrical, magnetized plasma, the exponential time constant for particle loss is given by [8] R2 v- (3) 2DB where R is the radius of the plasma column and the Bohm diffusion coefficient, DB, is given by KTe DB = (4) 16eB Combining equations (3) and (4) yields a Bohm confinement time that increases linearly with increasing magnetic field 1:= 8eBR2 KTe (5) For a 1 kg field and a 8 ev electron temperature, equation (5) predicts a particle confinement time of 0.6 ms. The classical confinement time for a fully ionized gas with the same plasma parameters, [8]: t"=r 21JJ.n(kTe R2B2 + K1i) where F7J. is the perpendicular resistivity, is approximately 2 ms. Along the magnetic field, the particle confinement time can be estimated by calculating the time needed for a plasma ion to travel from the centre of the source to the end while moving at the ion acoustic speed Cs = '.;ykte/m;, II= 'II 1..1.'7' ms. \ vykte/m; Since the perpendicular ion temperature clearly does depend on the magnetic field strength, either the processes governing the ion energy confinement are fundamentally different than the processes governing the particle confinement, or the particle confinement is not described by equation (7). In reality, a typical ion moves along the magnetic field at the ion thermal speed, not at the ion acoustic speed. At the parallel ion thermal speed, it requires approximately I ms for an ion to travel from the middle of HELIX to the end. The increase from 0.3 ms to 0.6 ms of the Bohm confinement time as the magnetic field is increased from 0.5 kg to 1 kg, i.e., equation (5), could have a marked effect on the perpendicular ion temperature. For ions travelling at the ion thermal speed along the magnetic field, the parallel diffusion is comparable to the perpendicular diffusion. Therefore the linear dependence of the perpendicular ion temperature on the magnetic field strength can be explained by Bohmlike perpendicular diffusion. Preliminary measurements of electromagnetic fluctuations in HELIX indicate wave activity at a variety of frequencies well below the driving frequency. Thus, the necessary level of turbulence required to drive Bohm-like diffusion may exist in HELIX. One difficulty with this hypothesis is that the ion heating due to ion-electron collisions is isotropic. The (6) '7) 190

6 Ion temperature anisotropy in helicon plasma parallel ion temperature should increase in the same manner as the perpendicular temperature. However, as an individual ion's parallel velocity increases, the time the ion spends inside HELIX decreases-resulting in less time to undergo additional heating. This type of selflimiting process should result in a limit on temperature of the parallel ion distribution without placing any restriction on the perpendicular ion temperature, consistent with our measurements. At higher neutral pressures, the mean free path of 'parallel' ions is reduced by ion-neutral collisions and ions in the centre of the discharge no longer make an uninterrupted trip to the open ends, resulting in higher parallel ion temperatures as the neutral pressure increases (figure 6). To test this hypothesis, experiments using magnetic mirror configurations to increase the parallel ion confinement time are planned. Why the ion temperature becomes isotropic in inductive mode plasmas also remains an open question. The measurements reported here indicate that it is possible to independently adjust operating parameters such as density, ion temperature and ion temperature anisotropy in the WVU helicon source. This feature makes the helicon an ideal platform for a wide range of space-relevant, laboratory plasma experiments. Acknowledgments The work was performed with support through National Science Foundation grant A1M-96l6467 and US Department of Energy grant DE-FGO5-97ER We would also like to acknowledge the support of the NSF EPSCoR program which provided the initial funding for the dye laser system. We thank Carl Weber and Doug Mathess for fabricating much of the experimental apparatus and our many colleagues for their advice and support during the construction of the helicon source. References [1] See, for example, Gary S P 1993 Space Plasma Instabilities (New York: Cambridge University Press) [2] Keiter PA, Scime E E and Balkey M M 1997 Phys. Plasmas [3] Watari T et al 1978 Phys. Fluids [4] Hill D H, Fornaca Sand Wickham Ma 1983 Rev. Sci. Instrum [5] Nakano T, Sadeghi N, Trevor D, Gottscho Rand Boswell R 1992 J. Appl. Phys [6] Boswell R, Porteous R, Prytz A, Bouchoule A and Ranson P 1982 Phys. Lett. 91A 163 [7] Kim J-H and Chang H-Y 1996 Phys. Plasmas [8] Chen F F 1985 Introduction to Plasma Physics and Controlled Fusion (New York: Plenum) 101