Langmuir probe study in the nonresonant current drive regime of helicon discharge

PRAMANA c Indian Academy of Sciences Vol. 71, No. 1 journal of July 2008 physics pp. 117 131 Langmuir probe study in the nonresonant current drive regime of helicon discharge MANASH KUMAR PAUL and DHIRAJ BORA Institute for Plasma Research, Bhat, Gandhinagar 382 428, India Corresponding author. E-mail: mkpaul@ipr.res.in MS received 24 December 2007; accepted 26 March 2008 Abstract. Characterization of the current drive regime is done for helicon wavegenerated plasma in a torus, at a very high operating frequency. A radiofrequencycompensated Langmuir probe is designed and used for the measurement of plasma parameters along with the electron energy distributions in radial scans of the plasma. The electron energy distribution patterns obtained in the operational regime suggest that Landau damping cannot be responsible for the efficient helicon discharge in the present study. A typical peaked radial density profile, high plasma temperature and absence of an appreciable amount of energetic electrons for resonant wave particle interactions, suggest that the chosen operational regime is suitable for the study of nonresonant current drive by helicon wave. Successful and significant current drive achieved in our device clearly demonstrates the capability of nonresonant current drive by helicon waves in the present operational regime. Keywords. Helicon wave; radiofrequency-compensated Langmuir probe; current drive regime. PACS Nos 52.50.Qt; 52.80.Pi; 52.70.Ds 1. Introduction Waves are almost ubiquitous in plasmas, which can be driven either by electric and magnetic fields or by density and temperature gradients. Numerous possible applications of waves in laboratory plasmas (bounded plasma) have been developed. Low frequency electron whistler waves in thin bounded plasma geometry, wellknown as helicon waves, have attracted a lot of attention in the past two decades due to their ability to produce plasma with surprisingly high efficiency. Despite intensive experimental and numerical efforts [1], the discharge mechanism itself is strongly discussed [2 5]. Most of the earlier experiments focus mainly on the antenna wave coupling, mode generation and equilibrium study of such discharges. However, application of helicon waves in fusion research has been scarcely explored experimentally [6 8]. Although possibility of nonresonant current drive by helicon waves, utilizing the concept of helicity conservation due to dynamo electric field, has 117

Manash Kumar Paul and Dhiraj Bora already been explored theoretically [9], experimental study of nonresonant current drive by helicon waves has not been done experimentally in very high operating frequency regime. Present study pertains to the characterization of the current drive regime of helicon waves, which is a prerequisite for the experimental study of wave-induced helicity current drive by helicon waves in EMHD regime. Very high frequency regime is chosen for the present study so as to shun any resonant wave particle interactions in the present operational regime. Langmuir probe turns out to be a useful tool for the characterization of RFproduced plasma in the present operational regime as it relates the macroscopic (experimental) parameters to the microscopic (plasma) parameters. The primary use of the Langmuir probe diagnostics in the present experimental study is to measure the plasma parameters during different modes of discharge in the operating frequency range (31 33) MHz and then to measure the electron energy distributions (EEDF) in different density regimes so as to establish an operating regime suitable for the investigation of nonresonant current drive by helicon waves. Few essential modifications are done in the usual configuration of a Langmuir probe to avail proper plasma parameter measurements in strong magnetic and radiofrequency (RF) fields [10]. In this paper, we present the spatial and temporal measurements of plasma parameters during capacitive, inductive and helicon mode of discharges, required to characterize the very high frequency operational regime of helicon discharge in a small aspect ratio torous. The results of the spatial and temporal trends of the plasma parameters are used as guides to estimate the duration required for steady state attainment of helicon discharge and plasma uniformity during the experiment. EEDF measurements are attempted in different density regimes to verify the absence of energetic electrons required for current drive by resonant wave particle interactions. Study of current drive in plasma, during helicon discharge, is also provided at the end of this paper. 2. Experimental set-up and diagnostics 2.1 Experimental device RF power of 1.6 kw(max.) is fed to the right helical antenna placed inside the stainless steel toroidal helicon plasma device (THPD) of minor radius 10.5 cm and 30 cm, shown in figure 1. The vacuum vessel is pumped by two 2 diffusion pumps placed at 90 toroidal locations to the right and left of the antenna. A rotary pump of 200 l/min pumping speed is used to back the diffusion pumps. A base pressure of 10 5 mbar is achieved and plasma of argon gas is formed, at an operating frequency of 32 MHz, at a filling pressure of 2 10 3 mbar. The gas feed point is placed opposite to the antenna location. The magnetic field in this system is produced by four copper cables, wound on each of the quadrants of the vacuum vessel, connected in parallel to each other to minimize their net resistance. A pulsed current for 60 ms (full-width at half-maxima) is allowed to flow from a power supply to produce a toroidal magnetic field (B 0 ) of 1 kg (max.) on the axis. An ambient toroidal magnetic field (B T ) of 1 kg (max.) is used to excite 118 Pramana J. Phys., Vol. 71, No. 1, July 2008

Langmuir probe study SS tube Glass cover RF harmonic rejection sections Probe tip 12 mm 00000 11111 00000 11111 000000000000000000000000000000000 111111111111111111111111111111111 000 111 4 mm Semi rigid co axial cable SS shield extended from coaxial cable Biasing capacitor Floating electrode/ Guard ring 20 mm Figure 1. Schematic of the RF-compensated Langmuir probe designed for our operational regime. the helicon wave. A right helical antenna made of copper strip, insulated from plasma, is used to excite helicon wave of m = +1 mode. Initially, low RF power in CW mode is applied to optimize the impedance matching unit. During the present experiments, L-type and Pi-type impedance matching networks are used for high power and broadband response respectively. The RF generator used in the present experiment can provide an output power up to 2 kw in unbalanced mode. Forward and reflected power are measured with a Bird TM power meter. The plasma is diagnosed by a set of ordinary and RF-Langmuir probes, placed opposite to each other, toroidally besides the antenna. An RF-compensated Langmuir probe is used to measure the radial variation of plasma parameters. The probes are mounted in radial ports at 50 inclination from the equatorial plane, on both sides of the helical antenna, perpendicular to the ambient magnetic field direction. The electron temperature is obtained from the logarithmic fit of the exponential part of the probe (current voltage) characteristics [11]. A floating dual Rogowski coil system [12] is placed inside the device, at 50 inclination from the equatorial plane, encircling the plasma diameter, to measure the average plasma current. 2.2 RF-compensated Langmuir probe design Due to oscillating plasma potential in RF environment, it is always difficult to get the correct values of T e and floating potential with the probe characteristics of usual Langmuir probe [1,13]. In strong RF environment, oscillating current in the antenna structure generates oscillating magnetic field in the vacuum and plasma. Owing to the plasma potential oscillation with applied RF field, the probe characteristic Pramana J. Phys., Vol. 71, No. 1, July 2008 119

Manash Kumar Paul and Dhiraj Bora Rogowski Coil B T Gas Feed and Gauges RF Compensated Langmuir Probe Dipole Probe 30 cm B dot Probe B dot Probe array To Pump Helical Antenna 10.5 cm B T To Pump RF Power Feed Figure 2. Schematic of the experimental device and probe arrangements for the study of helicon discharge. shifts back and forth along the voltage axis in accordance with the magnitude of amplitude of oscillation. This lead to a shift in the floating potential and the usual graphical methods of analysing electron currents overestimate the electron temperature. The driven RF field inside a plasma can also get spread in frequency when there are low frequency fluctuations in plasma potential. Hence, modifications must be made in the usual configuration of this probe to make measurements in strong magnetic and RF fields. For conventional Langmuir probes, electron current collected by probe (I e ) at a given bias voltage (V ) is given by I e = I 0 exp(v V p + V RF ). (1) Here, V RF is the magnitude of RF oscillation over plasma potential (V p ) and I 0 contains the rest of the physical parameters, held constant for a set of electron temperature measurements. Due to this nonlinear relation of I e with V RF, use of low pass filter is not sufficient for the elimination of errors introduced by RF fluctuations. To compensate for such errors occurring in probe measurement in RF environment, a passive approach is chosen in the present case. In this approach, the probe tip, immersed in the plasma, is driven with a slowly varying potential hence preventing an RF potential from developing across the tip sheath. Due to simplicity of construction and better performance at all harmonics of RF [14], the passive compensation technique is used in the present experimental set-up [15,16]. The design is similar in principle to one suggested by Gagne and Cantin [17], improvised by Dilecce et al [18] and Chatterton et al [19]. In order to overcome the problem of existence of RF structure on the plasma potential (V p ), the probe potential is allowed to follow the variation in V p so that the DC probe theory can be followed even during our present analysis. The variation in V p is sensed by 120 Pramana J. Phys., Vol. 71, No. 1, July 2008

Langmuir probe study a large remote floating electrode which is capacitively coupled to the probe tip. The probe tip is isolated from the ground by means of tuned chokes which provide high impedance (100 kω) at the tuned harmonic frequencies. The capacitor joined between the floating electrode and probe tip is chosen so that it s capacitance is greater than that of the floating electrode at the operational frequency (ω). The LC-chokes, used for the construction of tuned chokes in the RF-compensated Langmuir probe, provide an attenuation of 35 db at 30 MHz. To obtain very high impedance at tuned frequencies, the condition ω 2 CL 100 is imposed on the sheath potential. During this calculation, V p 100 V is assumed, so as to achieve a compensation within 1% of plasma potential. So after the floating electrode/guard ring, which surrounds the probe, two stages of LC chokes/filters, which consist of two pairs of ceramic capacitor and miniature inductor, are used to suppress the RF harmonics, as shown in figure 2. Such an arrangement provides a stable range of frequency compensation components to use for the modified Langmuir probe. To enhance the immunity of the probe against oscillating RF structures on plasma potential, the potential at the tip of LC-chokes should be greater than V p so as to block any RF structure riding on V p in the probe signal. The simple theory and calculations necessary for the construction of the RF-compensated Langmuir probe for our operational regime, are provided below. Schematic of the probe is shown in figure 2. For simplicity, only the first harmonic is considered, as for higher harmonics, value of β becomes negligibly small. So, the harmonics considered in the plasma potential are V p = V p sin ωt + βv p sin 2ωt, (2) where β is the fraction of first harmonic present in V p. The total impedance of the probe is Z T = 1 jωc + jωl 1 jωl 2 1 ω 2 + C T1 L 1 1 ω 2, (3) C T2 L 2 where C is the effective sheath capacitance, C T1, C T2 are the parallel tuning capacitors and L 1, L 2 are the inductors for the fundamental and the first harmonic respectively. The capacitance of the floating electrode to the plasma is C = ɛ 0A S, (4) S = λ D V Vp kt e, (5) where V is the probe bias voltage, V p is the plasma potential, S is the sheath thickness and A is its area. Now, V 0 = ω2 CL V p 1 ω 2 CL, (6) where V 0 is the potential at the junction of the probe tip and the LC-circuit for first harmonic rejection and Pramana J. Phys., Vol. 71, No. 1, July 2008 121

Manash Kumar Paul and Dhiraj Bora L = L 1 L 2 1 ω 2 + C T1 L 1 1 ω 2. (7) C T2 L 2 Hence, the voltage across the sheath is V p V 0 = V p 1 ω 2 CL. (8) In order to achieve compensation within 1% of V p, it is required that ω 2 CL 100. Capacitance for a cylindrical guard ring of 2 cm diameter and 10 mm length, with a dielectric (teflon) cylinder to isolate the probe tip from guard ring, is 300 pf for our operational regime. The second term = third term = V p, in eq. (3), is considered to make sheath drop equal to 1% of probe potential and the harmonic rejection components are calculated. The probe tip consists of a tungsten wire 0.5 mm in diameter and 4 mm in length. The cylindrical tungsten tip is capacitively coupled to the guard ring which is isolated from probe tip by a concentric ceramic cylinder that encircles the tip. Two sets of LC filters consist of ceramic capacitors and miniature inductors, constructed according to the component values calculated. Probe cleaning by resistive heating of the probe tip biased to collect electron saturation are attempted during continuous operation to improve the reproducibility of the probe characteristic curves. The probe circuit consists of a floating ±100 V DC voltage supply and a 10 kω current measuring resistor. Probe current and voltage are monitered on a 100 MHz Tektronix2014 oscilloscope and transferred to a computer by means of RS232 interface. 3. Experimental results 3.1 Floating potential Plasma is radially scanned using RF-compensated Langmuir probe to measure the floating potential during the complete duration of discharge. A typical radial profile of the floating potential of the plasma sustained by helicon waves in toroidal helicon plasma device is shown in figure 3. Floating potential peaks at the center of the plasma column and lowers near the wall of toroidal plasma device. Since during helicon discharge, electron density is concentrated towards the plasma axis, as evident from figure 4, the floating potential as well as the plasma (space) potential are expected to rise near the plasma axis. Steep elevation of floating potential near the plasma axis support helicon discharge in the present operational regime. Spatial evolution of floating potential is also essential to understand the discharge equilibrium of a pulsed RF breakdown. The closed contours obtained from the spatial profiles of floating potential, shown in figure 5, provide the requisite information about the discharge equilibrium in our device. Closed contours obtained from the spatial evolution of floating potential during helicon discharge reveal the steady state discharge obtained in our experiment. The open contours obtained at the beginning and towards the end of discharge suggest the transient nature of plasma during the initial breakdown phase and final decay phase respectively. This trend also exhibits the transient nature of sheath at the initial breakdown phase 122 Pramana J. Phys., Vol. 71, No. 1, July 2008

Langmuir probe study Floating Potential (Volts) 15 10 5 0-10 -8-6 -4-2 0 2 4 6 8 10 Radial distance (cm) Figure 3. Radial profile of floating potential of plasma measured in the current drive regime. Increasing values on horizontal axis indicate the probe positions from inboard to outboard. 20 15 Plasma current (100 ua) 10 5 Uncompensated > Compensated > 0 5 150 100 50 0 50 100 Bias Voltage (Volts) Figure 4. Comparison of the probe characteristics obtained using RFCLP and ordinary Langmuir probe shows a significant difference in the electron temperatures obtained. Each point is averaged over five pulsed plasma shots to minimize error. and final decay phase. These contours also show the movement of the plasma column during the entire period of the discharge. Presence of closed contours during (5 45) ms period and the absence of any transient during this time indicate the proper alignment of antenna and stable plasma. 3.2 Plasma density and temperature Plasma temperature is measured using both the Langmuir probes and is compared at various power levels. The probe characteristics measured using ordinary Pramana J. Phys., Vol. 71, No. 1, July 2008 123

Manash Kumar Paul and Dhiraj Bora Plasma Temperature (ev) 16 14 12 10 8 6 4 2 0-10 -8-6 -4-2 0 2 4 Plasma Radius (cm) 6 8 10 Plasma density (m -3 ) 4x10 18 3x10 18 2x10 18 1x10 18 0-10 -5 0 5 10 Radial distance (cm) Figure 5. Radial profile of electron temperature (T e ) measured during helicon breakdown, shows a variation of T e in the range (9 12) ev during current drive experiments. Increasing values on horizontal axis indicate the probe positions from inboard to outboard. Figure 6. Radial profile of plasma density reveals plasma density rises near the plasma axis and low on both sides of antenna, as expected in the case of a typical helicon breakdown. Increasing values on horizontal axis indicate the probe positions from inboard to outboard. Langmuir probes and RFCLP reveal a marked difference between the plasma temperatures (T e ), as shown in figure 6 [20]. The probe characteristics are obtained over different pulsed plasma shots, biasing the probe at desired fixed voltage during each shot. Proper repeatability of discharge during every shot is ensured by almost the same magnitude and structure of temporal evolution of ion-saturation signal. The T e values obtained by ordinary and RF-compensated Langmuir probe are 18 ev and 12 ev respectively. Radial profile of the plasma temperature is shown in figure 7. However, the radial T e profile remains approximately flat within the probe measurement limits indicating that the plasma is resistive due to collisional effects. Difficulty in T e measurements near the edge regions lead to bigger error bars near plasma boundary on both sides of plasma. Plasma temperature varies between (10 12) ev during current drive experiments. An array of Langmuir probe is used to measure the radial variation of plasma parameters during a single pulsed plasma shot. Plasma density is obtained from the ion saturation current using the Bohm formula. A typical radial plasma density profile, shown in figure 4, measured at a gas pressure of (2 5) 10 3 mbar, input RF power of 1.4 kw and a magnetic field of 800 G, shows a sharp rise in density towards the plasma axis and low on both sides of antenna, as expected in the case of helicon breakdown. Variation of plasma density n e is observed with the ambient magnetic field B T. During experiment, the fill pressure of argon gas is 2 10 3 mbar. The input RF power is kept constant at 1 kw. A linear relationship between the plasma density and magnetic field during the helicon mode of discharge is shown in figure 8. Signature of discharge mode transition is clearly observed in (0.2 0.4) kg. The linear trend observed during the radial measurement of plasma density variation with ambient magnetic field, carried out on the plasma axis, comply with the dispersion relation for the helicon mode [1,8]. The helicon mode is obtained in the range (0.5 1) kg, but at progressively lower RF powers for higher ambient magnetic field strengths. 124 Pramana J. Phys., Vol. 71, No. 1, July 2008

Langmuir probe study 6x10 17 Plasma Density (m -3 ) 4x10 17 2x10 17 0 0.0 0.2 0.4 0.6 0.8 1.0 Magnetic Field (kgauss) Figure 7. Variation of density with ambient magnetic field. Mode transitions are clearly observed during density variation. The line represents the estimated dispersion relation in the present operational regime. 8 Plasma Radius (cm) 6 4 2 0 2 4 6 20 15 10 5 8 0 10 20 30 40 50 Time (msec) Figure 8. Spatial and temporal variations of plasma density, measured in radial direction, shows a steady state attainment within 10 ms. As reported earlier [21], electron energy distribution (EED) measurement using RFcompensated Langmuir probe (RFCLP) may lead to strong suppression of energetic electrons in the probe characteristics, an extended calculation is provided below to prove that our RFCLP is capable of measuring the full EED, without losing any vital information. The sheath radius can be calculated from the simple formula S r = 500V 3/4 b n 1/2 e T 3/4 e, (9) where V b is the bias voltage on the probe, n e is the plasma density and T e is the electron temperature. Sheath radius (S r ) in our case turned out to be 0.1 mm and the probe radius (P r ) is 0.5 mm. So, Pramana J. Phys., Vol. 71, No. 1, July 2008 125

Manash Kumar Paul and Dhiraj Bora S r P r = 0.2 (10) which is small enough to neglect the sheath effect in our case. The effect on the Langmuir probes due to the applied magnetic field is negligible when the probe dimensions are much smaller than the Larmour radius (r L ) of the collected plasma species. In the present operational conditions, r L turned out to be 0.1 mm which is small in comparison to the probe dimensions. Electrons with Larmour radius smaller than the dimensions of the probe can either be collected through cross-field diffusion or from flux tubes intersecting the probe. It has been shown that the electron saturation current is reduced by a factor [11] [ ] 1/2 D 1 + (T i /T e ) s = 16λ ei, (11) D 2πa where λ ei is the mean free path for electron ion collisions and D is the diffusion coefficient across the magnetic field lines given by ( ) D = D 1 + ω2 ce νei 2, (12) where ω ce is the electron angular cyclotron frequency, ν ei is the electron ion collision rate and D is the diffusion coefficient along the magnetic field lines given by D = T e ν ei. (13) The minor radius a of our experimental system is 10.5 cm. For a typical argon plasma, the reduction factor s in our case is 0.1 and can be neglected. In the present parameter regime, ν ei 10 6 s 1 and ω ce = 3 10 9 s 1. For electrons (ω ce /ν ei ) 10 3 and the electron radial diffusion is strongly inhibited. But for ions, (ω ce /ν ei ) (m e /M i ), the ion diffusion is not severely limited. So, the usual standard expressions for the electron and ion probe currents will be used with appropriate corrections for the effective probe area. Moreover, in the presence of a toroidal magnetic field, the tip of Langmuir probe should be aligned perpendicular to the magnetic field so as to eliminate any discrimination of low energy particles by the probe. For a cylindrical Langmuir probe, aligned along the magnetic field, the electric field (E) between plasma and the probe acts perpendicular to the magnetic field (B) lines which results in changing the direction of E B drift of the guiding center of the plasma particles in azimuthal direction with respect to the axis of the probe. Thus, the low energy plasma constituents, owing to a smaller Larmour radius than the probe sheath thickness, are restricted to reach the probe due to the superposition of cyclotron motion on the guiding center motion. The resulting discrimination of low energy particles by the probe leads to a higher measured value than the actual value of temperature. This effect is eliminated by orienting the probe tip perpendicular to the magnetic field so that the orbit effects are taken off the scenario by keeping E B zero. 126 Pramana J. Phys., Vol. 71, No. 1, July 2008

Langmuir probe study 3.3 Electron energy distribution function A valid application of the Druyvesteyn formula [22] requires collisionless electron motion near the probe. This needs the Debye length (λ D ) to be much less than the electron mean free path (λ e ). Along with this condition, it is also necessary that probe radius must be smaller than λ e to be nonintrusive effectively. In the present case, maximum λ D is 0.1 mm, minimum λ e is 10 mm and the probe radius is 0.5 mm. So, the necessary condition to allow this probe for electron energy distribution determination is sufficed. Electron energy distribution function (EEDF) is determined using the RFCLP from the second derivative of the IV characteristic that requires collisionless electron motion near the probe [22]. I e = e3/2 A 8m V p V b (U + V b V p )F (U)dU, (14) di e = e3/2 A dv b 8m V p V b F (V b V p )dv b, (15) d 2 I e dv 2 b = e3/2 A 8m F (V p V b ), (16) where A is the probe area, U is the kinetic energy and F (U) is the normalized isotropic component of EEDF. By setting U = (V p V b ), F can be written in terms of the kinetic energy, as F (U) = 8m e 3/2 A d 2 I e. (17) dv 2 b Although determination of EEDF by Langmuir probes in RF plasmas is subject to misinterpretation, cautious design and meticulous operating procedure result in a very reliable and versatile experimental tool. The plasma potential value obtained by emissive probe is (80 ± 5) V and the floating potential value obtained by RFCLP is (18 ± 2) V. From figures 3 and 6, it can be easily deduced that the plasma potential is approximately constant in radial direction during helicon discharge. This observation indicates the absence of appreciable amount of energetic electrons during the helicon discharges at the present operational regime. EEDF patterns measured at different plasma density regimes (10 16 10 18 m 3 ), shown in figure 9, also reveal the absence of appreciable super-thermal species necessary for current drive by resonant process. Probe measurements done during high RF power helicon discharges indicate the prominence of thermal species in the current drive regime of the bounded whistlers. For effective Landau damping, the resonant energy should lie near or above 50 ev, which is the optimum energy for electron impact ionization of argon [23,24]. For small aspect ratio (length of antenna to the radius) source, the resonance energy is given by Pramana J. Phys., Vol. 71, No. 1, July 2008 127

Manash Kumar Paul and Dhiraj Bora EEDF (m -3 ev -3/2 ) 5x10 18 4x10 18 3x10 18 2x10 18 0.8kW 1kW 1.2kW 1.5kW 1x10 18 0 40 60 80 100 120 140 160 180 Electron Energy (ev) Figure 9. EEDF obtained at different power levels show a small rise in bulk electron temperature due to collisional effects. ( 32.1BT E(eV) = an e ) 2 7.2af, (18) where antenna radius (a) is in cm, magnetic field (B T ) in kg, density (n e ) is in cm 3 and operating frequency f is in GHz. However, since the presence of significant amount of energetic electrons is negated in our present parameter regime of helicon discharge, Landau damping is easily eliminated as a possible mechanism for electron energization in our parameter regime. Absence of appreciable amount of super-thermal electrons during high RF power and high B T discharges can also be attributed to the extension of path length of primary electrons with increase in B T. This effect leads to a phase mismatch for initiation of any secondary effects [25]. 3.4 Current drive by helicon wave Absence of appreciable energetic electrons in the present operational regime makes it suitable for the study of nonresonant current drive by helicon wave. In our device, a single source is used for plasma production, discharge sustainment as well as current drive. Successful excitation of m = +1 helicon mode in our device, at the present operational regime, is confirmed by the measurement of wave magnetic field components using B-dot probes. The radial variation measurements of the wave magnetic field components are in good agreement with the numerically computed counterparts [26]. To measure the plasma current generated during helicon discharge, dual Rogowski coil system is introduced radially into the toroidal vacuum chamber. The coil housing is mounted in a radial port at 50 inclination from the equatorial plane, with the major opening of coil housing encircling the plasma cross-section. The Rogowski coil diagnostic port is located opposite to the helical 128 Pramana J. Phys., Vol. 71, No. 1, July 2008

Langmuir probe study Figure 10. Toroidal magnetic field pulse for 50 ms (FWHM), input RF power pulse for 60 ms, time evolution of the central plasma density, temporal evolution of the plasma current observed in normal coil position and when the coil is rotated by 180. antenna, at a toroidal distance of 90 cm. When the direction of toroidal magnetic field (B T ) is reversed, magnitude of plasma current declines sharply due to poor coupling of RF power to the left-handed counterpart (m = 1) of helicon wave. However, reversal of B T with in situ reversal of antenna results in the reversal of plasma current signal with almost exact magnitude in that direction, as shown in figure 10. However, early decay of plasma current (before plasma density), suggests that a threshold magnetic field is required to sustain the plasma current during helicon discharge. To substantiate the absence of pick-up due to any closed loop of current formed inside THPD, measurements of plasma current are done at different toroidal locations. This experiment confirms that significant current drive in the helicon discharge regime leads to a finite change in net plasma energy. This set of experiments, performed in the present operational regime, validates the plasma current drive due to helicon waves in the present parameter regime in THPD. Investigation of parametric dependence of plasma current is in good agreement with the analytical results. Numerical estimation of contribution from various current drive factors to the net plasma current, obtained in the present parameter regime, is done using the following momentum balance equation obtained in EMHD regime [6], from the equation of motion, taking electron fluid into consideration. Pramana J. Phys., Vol. 71, No. 1, July 2008 129

Manash Kumar Paul and Dhiraj Bora n 0 J 0 qη = q[n e E] [J e B] + [P e ] + m n 0 q 2 [(J e )J e ], (19) where [ ] implies time average over wave oscillation period, n 0, q, J e, E, B, P and η are plasma density, electronic charge, plasma current density, generated electric field, applied magnetic field, pressure and plasma resistivity respectively. This analysis leads us to the contribution of each current drive component in terms of the wave magnetic field components and the variations of the same in toroidal coordinates. This analysis reveals significant contribution (80 90)% of wave-induced helicity (J e B) [26] during the nonresonant current drive by helicon waves in the present parameter regime. 4. Conclusion In this paper, important preliminary experimental investigations, performed in a toroidal discharge chamber of small aspect ratio, at a very high operational frequency, are presented which establish proper conditions for nonresonant current drive by helicon wave-induced helicity. In order to delineate the novel wave-induced helicity current drive phenomena from usual resonant wave particle interactions, EEDF measurements, performed using the RF-compensated Langmuir probe, are extremely helpful. The EEDF measurements performed, clearly demonstrate that Landau damping is not responsible for the efficient ionization in the present helicon discharge. Proper design and measurement procedure of an RF-compensated Langmuir probe, designed for the present operational regime, lead to proper and accurate measurements of plasma parameters and electron energy distribution during helicon breakdown, which are helpful in the novel, exclusive, experimental study of nonresonant current drive by helicon wave in EMHD regime. References [1] R W Boswell and F F Chen, IEEE Trans. Plasma Sci. 25(6), 1229 (1997) [2] F F Chen, Plasma Phys. Controlled Fusion 33(4), 339 (1991) [3] S Cho, Phys. Plasmas 7(1), 417 (2000) [4] K P Shamrai and V B Taranov, Plasma Sources Sci. Technol. 5, 474 (1996) [5] J G Kwak, H D Choi, H I Bak, S Cho, J G Bak and S K Kim, Phys. Plasmas 4(5), 1463 (1997) [6] S K P Tripathi and D Bora, Nucl. Fusion 42, L15 (2002) [7] Shuichi Takamura, Takuo Kojima and Takayoshi Okuda, Plasma Phys. 25(12), 1469 (1983) [8] B C Zhang, B D Blackwell, G G Borg and V Petri zílka, Phys. Plasmas 4(11), 3986 (1997) [9] V Petri zílka and J A Tataronis, Plasma Phys. Control. Fusion 36, 1027 (1994) [10] I D Sudit and F F Chen, Plasma Sources Sci. Tech. 3, 162 (1994) [11] Noah Hershkowitz, Plasma diagnostics: Discharge parameters and chemistry edited by D Auciello and D L Flamm (Academic Press Inc., 1989) vol. 1, chap. 3, p. 113 [12] Manash Kumar Paul, P K Chattopadhyay and D Bora, Meas. Sci. and Tech. 18, 2673 (2007) 130 Pramana J. Phys., Vol. 71, No. 1, July 2008

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