Grulke, O.

Klinger, T.

Zalach, J.

English

MPG.PuRe

ECR Heating in a Helicon DeviceJacob Zalach, Olaf Grulke and Thomas KlingerMax-Planck Institute for Plasma Physics, EURATOM Association, Greifswald, GermanyAbstract. This paper presents a microwave electron cyclotron resonance heating system for the linear magnetized VINETAhelicon device. Thereby, a relatively cold helicon wave generated plasma is heated by resonant electron cyclotron waves. Themagnetic field configuration is adjusted such that resonant energy transfer occurs locally to heat the plasma electrons. Electrontemperatures of 12 eV are obtained for an Argon plasma at a pressure of 0.02 Pa. The absorption of the microwave acrossthe resonance layer is measured with capacitive probes. Plasma parameters are characterized using compensated Langmuirprobes. Additionally, a 160 GHz microwave interferometer is used to measure the line integrated plasma density. The evolutionof these quantities for different microwave powers is compared to power balance computations, which indicate that the appliedmicrowave power is dissipated by the ionization collisions; after saturation of ionization processes the power is transferredinto electron kinetic energy.Keywords: ECR heating, linear devicePACS: 52.50.-b1. INTRODUCTIONHelicon plasma sources have attracted great attention due to the very efficient plasma production at moderate rfpower [1, 2]. With only few kW rf power plasma densities in the range of ne   1019m  3 are achieved [3, 4, 5].Therefore helicon plasmas are often used in basic research and have attracted much interest for technical applications[6]. Nevertheless, the electron temperatures in helicon plasmas are generally low (in the range of a few eV). Thecombination of high plasma density and low electron temperature leads to a highly collisional plasma, in whichCoulomb collisions play a dominant role in plasma dynamical phenomena [4]. This work presents an electron-cyclotron-resonance (ECR) heating setup for a helicon plasma produced in the linear VINETA device. The final goal isto increase the plasma temperature and thereby to decrease the plasma collisionality. The paper is structured as follows:In Sec. 2 we describe the experimental setup and the diagnostic tools used. Measurements of microwave absorptionand plasma parameters for a pure ECR and a combined ECR-helicon discharge are presented in Sec. 3. Finally theresults are discussed in Sec. 4.2. EXPERIMENTAL SETUP AND DIAGNOSTICThe ECR setup is part of the VINETA device [4, 7, 8, 9, 10] which is a cylindrical, linearly magnetized plasmaexperiment with a total length of L  4  5 m and D  0  4 m diameter. Fig. 1 shows a schematic drawing of VINETAindicating the position of the helicon source and the ECR heating setup.FIGURE 1. Schematic of the VINETA device with the helicon source and the ECR setup at opposite ends.At the one end of the device a conventional helicon source is installed for plasma production. The helical m  1antenna is placed around a cylindrical glass vacuum extension (0.5 m length, 0.1 m diameter) and is connected to an rfsource via a capacitive matching unit. The source is driven at a frequency of frf  13  56MHz with a maximum powerof Prf  max  5kW. A detailed description of the helicon setup and its operation is given in Ref. [7].At the opposite end to the VINETA device the ECR heating setup is installed. Microwaves were previously used onlinear magnetized devices as plasma sources [11, 12, 13, 14]. The goal is a direct coupling of the microwave electricfield to the electron gyro motion which allows resonant energy transfer to the plasma electrons. An overview of theprinciple is given in [15]. Our system consists of a standard industrial microwave generator (Mügge Type MS010KE-110DE) with a maximum continuous power output of Pmw10kW at a frequency of fECR  2  45GHz. The linearlypolarized microwave (H01  mode) is first transferred to a H11  mode and then passes a polarizer, which transformsit to a right-handed polarized wave (R-wave). The microwave antenna, which is a cut waveguide, is placed directly inthe plasma center and the R-wave is launched along an inhomogenius magnetic field. The magnetic field gradient andthereby the resonant absorption layer is produced by non-equidistant spaced magnetic field coils placed around thesteel vessel. For resonant absorption the magnetic field has to be adjusted such, that the electron cyclotron frequencyis equal to the microwave frequency ωecr  ωo. The required magnetic field strength is Bres  87.5 mT. The magneticfield inhomogenity is restricted to a region close to the microwave antenna (  1  4L), while the rest of VINETA ishomogeneously magnetized. The microwave antenna is vacuum sealed with a quartz window. Any mismatching of themicrowave is compensated using a 3-stub-tuner system. Typical reflected power levels are a few tens of Watt.Passively compensated Langmuir probes [16, 17] are used to measure the electron temperature and density inthe plasma center by standard evaluation of probe characteristics. A capacitive probe [18] is used in the vacuumchamber center at an axial distance of 0.9 m from the microwave antenna in order to measure the electric field of themicrowaves. Its power spectrum is analyzed and the comparison between the magnitudes of the microwave peak atωECR with and without plasma gives an estimate of the microwave absorption across the resonant magnetic field layer.As an independent method a 160 GHz microwave interferometer measures the line integrated plasma density ( 	 ne dr),which is compared with the radially integrated ion saturation current profiles ( 	 ne  Te dr). Any deviation from a linearrelationship between this two independently measured quantities can be directly attributed to the electron temperatureprofile.3. MEASUREMENTSAll measurements presented here were done in Argon at a pressure of 0.02 Pa with an axial magnetic field contouras shown in Fig. 2(a). Absorption properties were investigated with a capacitive probe in a pure ECR discharge. Themicrowave signal is strongly reduced or completely disappeares as soon as the absorption layer is reached. A typicalmeasurement is shown in Fig. 2(b). The measured signal drops from values of -5 dB (without plasma) to roughly-50 dB (with plasma). Furthermore the width of the peak is strongly reduced by the wave-plasma interaction. Thisbehaviour is qualitatively the same for all microwave powers. The evolution of the radial ion saturation current profilesFIGURE 2. Left: Central axial magnetic field strength. Right: Spectrum of the capacitive probe signals for the two cases ofabsorption and free wave propagation without plasma.and of the electron temperature with increasing microwave power is shown in Fig. 3 and Fig. 4. The profiles weremeasured for a pure ECR discharge [Fig. 3(a)] and a combined ECR discharge with additional 2 kW rf power fromthe helicon source [Fig. 3(b)] (combined discharge). The comparison of the ion saturation current profiles show thatthe height of the profile increases with the microwave power for both the pure ECR and the combined discharge.Furthermore, the profiles for comparable microwave power are wider for the combined discharge. The shoulder-FIGURE 3. Ion saturation current profiles for a pure ECR discharge (left) and a combined discharge with ECR and 2 kW rf power(middle).The right diagram shows a comparison between line integrated density measured with the microwave interferometer (  ne)and radially integrated ion saturation current profiles obtained by Langmuir probes (  ne  Te). The nonlinear relationship is causedby a temperature difference. Every marker represents averaged values of several measurements, the applied microwave power isgiven in kW.like structure which occurs in all ECR plasma profiles supposedly originates from a misalignment of the microwaveantenna. The comparison of the microwave interferometer measured line integrated density with the radially integratedion saturation current profiles for the pure ECR and combined discharge is shown in Fig. 3(c). For both discharge typesthe value of the integrated ion saturation current profile increases monotonically with applied microwave power. Theinterferometer signal increases for the pure ECR discharge with microwave power of 4.1 kW but decreases for higherpowers. The linear increase in the case of the combined discharge becomes slightly stronger at microwave powersabove 2.8 kW. The central temperature dependence on the applied microwave power [Fig. 4(a)] was obtained with twoprobes separated by 1.2 m from each other in a pure ECR discharge. The temperature values of probe 2 are higherthan those of probe 1 which has a higher distance to the antenna. Both probes show a plateau like behavior up tomicrowave power of 3 - 4 kW. Higher power values results in a temperature increase up to about 12 eV at roughly 6kW. By applying additional 2 kW rf power the temperature changes as shown in Fig. 4(b) The temperature values aresignificantly lower than in the case of the pure ECR discharge.4. DISCUSSIONThe measurements with the capacitive probes show a strong damping of the microwave signal as soon as the absorptionlayer is reached which means that resonant absorption actually takes place. The ion saturation current profiles and thecentral temperature develop with increasing microwave power as expected. We note that ion saturation current isdependend on plasma density and temperatureFIGURE 4. Electron temperature taken in the plasma center: left) at two different axial positions for a pure ECR discharge (probedistance 1.2 m) and right) comparison of a pure ECR and a combined discharge with ECR and 2 kW rf power.Isat  r   keAprobeni  r  eTe  r Mi(1)where Mi is the ion mass, Aprobe the projection of the effective area of the collecting probe in the direction of themagnetic field and k  0  49 is a constant which includes the influence of the magnetic field [19]. The comparison ofthe radially integrated ion saturaton current profile with the line integrated density as measured with the interferometer[Fig. 3(c)] indicates, that at microwave power levels above 4kW the temperature increases. This finding is supportedby probe measurements (Fig. 4), where an electron temperature increase is observed only for microvave power levelsPECR  4kW. This behavior can be understood with the help of power and particle balance computations [20]. Inthe computation the main sources and sinks of particles and energy - ionization, recombination, excitation of atoms,radiation and elastic electron-ion collisions - are considered as well as the open ended linear geometry of the VINETA.They result in a relationship between ne and Te where the density decreases with increasing electron temperature. Itturns out that for the ECR discharge the applied microwave power is first absorbed by ionization processes whichincrease the density, as observed in Fig. 3(c). At a particular power level the ionization mechanism saturates and theremaining power heats the plasma electrons and the density decreases. For the pure ECR discharge this point is reachedat 4.1 kW. This interpretation is supported by the temperature measurements shown in Fig. 4(a). The values for thecombined discharge show a different behavior. The interferometer data show that the linear increase above 2.8 kWbecomes slightly stronger. This behavior corresponds with the temperature measurements shown in Fig. 4(b) whichalso show a slight increase.5. SUMMARYThe present setup is suitable to obtain a ECR heated plasma with significant higher electron temperatures than in thecase of a pure helicon discharge at plasma densities in the range of 1017  1018m  3. Resonant absorption takes placeand the ion saturation current profiles as well as the microwave interferometer data show a relationship which canbe understood with the help of power balance computations where first the absorbed power is mainly used by theionization mechanism which eventually saturate. Beyond that point power increase leads to an increase of electrontemperature. This explanation is supported by temperature measurements which develop with the applied microwavepower as expected.REFERENCES1. F. F. Chen, and R. W. Boswell, IEEE Trans. Plasma Sci. 25, 1245–1257 (1997).2. R. W. Boswell, and F. F. Chen, IEEE Trans. Plasma Sci. 25, 1229–1244 (1997).3. F. F. Chen, J. Vac. Sci. Technol. A 10, 1389 (1992).4. C. Schröder, O. Grulke, T. Klinger, and V. Naulin, Phys. Plasmas 11, 4249–4253 (2004).5. F. F. Chen, I. D. Sudit, and M. Light, Plasma Sources Sci. Technol. 5, 173–180 (1995).6. I. Tempermeister, J. Vac. Sci. Technol. B 12, 2310 (1994).7. C. M. Franck, O. Grulke, and T. Klinger, Phys. Plasmas 10, 323–325 (2002).8. C. M. Franck, O. Grulke, and T. Klinger, Phys. Plasmas 9, 3254–3258 (2002).9. C. M. Franck, O. Grulke, T. Klinger, A. Stark, E. E. Scime, and G. Bonhomme, Plasma Sources Sci. Technol. 14, 226–235(2005).10. O. Grulke, C. M. Franck, T. Klinger, C. Schröder, A. Stark, T. Windisch, and J. Zalach, Contrib. Plasma Phys. 45, 385–395(2005).11. V. P. Bhatnagar, G. VanOost, A. M. Meissiaen, and P. E. Vandenplas, Plasma Physics 18, 535–540 (1975).12. D. Hemmers, M. David, H. Kempkens, and J. Uhlenbusch, J. Phys. D: Appl. Phys. 31, 2155–2164 (1998).13. V. Yadav, and D. Bora, Plasma Sources Sci. Technol. 13, 231–236 (2004).14. P. Reinke, S. Schelz, W. Jacob, and W. Möller, J. Vac. Sci. Technol. A 10, 434–438 (1992).15. M. A. Lieberman, and A. J. Lichtenberg, Principles of plasma discharges and materials processing, Wiley-Interscience, 1994.16. I. D. Sudit, and F. F. Chen, Plasma Sources Sci. Technol. 3, 162–168 (1994).17. U. Flender, B. H. Nguyen-Thi, K.Wiesemann, N.A.Khromov, and N.B.Kolokolov, Plasma Sources Sci. Technol. 5, 61–69(1995).18. S. E. Savas, and K. G. Donohoe, Rev. Sci. Instrum. 60, 3391–3395 (1989).19. D. Batani, S. Alba, P. Lombardi, and A. Galassi, Rev. Sci. Instrum. 68, 4043–4050 (1997).20. C. Lechte, J. Stöber, and U. Stroth, Phys. Plasmas 9, 2839–2846 (2002).

ECR Heating in a Helicon Device

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ECR Heating in a Helicon Device

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