ELECTRON ENERGY DISTRIBUTIONS AND NON-COLLISIONAL HEATING IN MAGNETICALLY ENHANCED INDUCTIVELY COUPLED PLASMAS*

Transcription

1 ELECTRON ENERGY DISTRIUTIONS AND NON-COLLISIONAL HEATING IN MAGNETICALLY ENHANCED INDUCTIVELY COUPLED PLASMAS* Ronald L. Kinde and Mak J. Kushne Depatment of Electical and Compute Engineeing Ubana, IL 68 "Pesentations" link Octobe * Wok suppoted by NSF, SRC and AFOSR/DARPA MJKGEC_

2 AGENDA Wave heating in Magnetically Enhanced ICPs Desciption of the Model Tansition fom ICP to Helicon Heating Electon Enegy Distibutions: Continuous Acceleation Phase Matching Requiements Concluding Remaks MJKGEC_

3 MAGNETICALLY ENHANCED ICPs It is often desiable to poduce plasmas in the "volume" of lage eactos. This is difficult to accomplish using Inductively Coupled Plasmas (ICPs) due to the finite electomagnetic skin depth, typically a few cm at s MHz and cm - plasma density.. E&M WAVE -FIELD ANTENNA y applying solenoidal dc magnetic fields, the esulting tenso conductivity poduces all -components of the f electic field. SOLENOID The end esult is electomagnetic wave popagation along the magnetic field lines which deposits powe both in the volume and emotely fom the antenna. PLASMA These systems ae Magnetically Enhanced ICP, one manifestation being helicon devices. SUSTRATE MJKGEC_A

4 POWER DEPOSITION IN MEICPs The coupling of electomagnetic fields to the plasma occus though two channels: Helicon Wave and Electostatic Wave (TG). Paallel phase velocities of helicon waves may match themal speeds of - ev electons, enabling continuous, non-collisional acceleation. Chen and oswell have suggested Landau damping as such a collisionless heating mechanism. Evidence of non-collisional heating has been found expeimentally and computationally in pevious woks: A. Dageling and R. oswell, Phys. Plasmas 4, 748 (997): Ionization waves move away fom the antenna at the phase velocity of the electomagnetic waves. Y. Mouzouis and J. Schae, Phys. Plasmas 5, 45 (998): At high - field, helicon wave popagation dominates powe deposition by esonant electon acceleation. Hee we epot on computational investigations of non-collisional powe deposition in MEICPs, and consequences on electon enegy distibutions (EEDs). MJKGEC_

5 COMPUTATIONAL PLATFORM The computational platfom used in this study is the Hybid Plasma Equipment Model (HPEM). Geomety: -dimensional cylindically symmetic Electomagnetics Module: -d components of f E- and -fields based on -d magnetostatic fields Electon Monte Calo Simulation: EEDs, tanspot coefficients and souce functions Fluid Kinetics Module: Ions: Neutals: Electons: Electic Potential: Continuity, Momentum, Enegy Continuity, Momentum, Enegy Continuity, Momentum, (enegy fom EMCS) Poisson's Equation MJKGEC_4

6 ELECTROMAGNETICS MODULE MJKGEC_5 The wave equation is solved in the fequency domain: ( ) ( ) t J E t E E E = σ ε µ µ E t E i t (, ) ( )exp( ( ( ))) = ω ϕ ω τ σ = q E n e The TG-mode can be esolved by including the divegence tem based on a petubation expansion fo electon density. Conductivities ae tenso quantities poducing -d components of f E- and -fields based on -d magnetostatic fields. ( ) m e o m z z z z z z z m o m n q m q i E j q m ν σ ν ω σ ν σ σ θ θ θ θ θ θ θ, / = = = = v

7 TRIKON MORI MEICP TOOL The model geomety is based on the Tikon Moi MEICP souce. 5 ell Ja Coil ef Electomagnets 5 Coil Nozzle Flux Flux Flux Pump Pot Wafe MJKGEC_6

8 TRANSITION FROM ICP TO HELICON: AZIMUTHAL E-FIELD 5 E θ Phase G E θ Phase 6 G 5 At low static magnetic fields, simple inductive coupling dominates. 5 a) b) G G With inceasing -field, the skin depth inceases and wave popogation occus along the field lines MHz, A, mto, kw c) V / cm Phase.5.5 π d) 5 π (mjkgec_7)

9 TRANSITION FROM ICP TO HELICON: AXIAL E-FIELD a) EZ Phase EZ Phase G 6 G b) G G With inceasing magnetic field, adial and axial components of the electic field ae poduced. Helicon opeation occus when powe deposition is dominated by axial electic field. The lage axial electic fields povide fo efficient acceleation of electons along the -field lines. c) V / cm Phase.. π d) π.56 MHz, A, mto, kw (mjkgec_8)

10 TRANSITION FROM ICP TO HELICON: POWER AND [e] 5 Powe [e] G Powe [e] 6 G 5 The tansition fom ICP to helicon esults in powe deposition shifting downsteam. 5 a) b) G G The peak plasma density follows the shift in powe deposition downsteam MHz, A, mto, kw c) Powe (W/cm ) [e] (cm - ).. 4 x d). 4 x 4 x (mjkgec_9)

11 ELECTRON ENERGY DISTRIUTION (ev -/ ) F F F Enegy (ev) 5 ELECTRON ENERGY DISTRIUTIONS ( mto) Electons on flux line F expeience little "tail lifting" nea the axis....on F e's ae esonantly acceleated with downsteam tail lifting....on F e's have less tail lifting due to lage column density of tavesed gas and lowe fields in peiphey of eacto. 5 5 ell Ja Coil Coil Nozzle ef Pump Pot Wafe mjkgec_ Flux Flux Flux Electomagnets.56 MHz A mto kw

12 ELECTRON ENERGY DISTRIUTION (ev -/ ) Enegy (ev) mto 5 mto mto 5 ELECTRON ENERGY DISTRIUTIONS vs PRESSURE With deceasing pessue, electon mean fee paths incease as does noncollisional acceleation. The tails of the EEDs ae pogessively lifted with distance downsteam. 5 5 ell Ja Coil Coil Nozzle ef Pump Pot Wafe mjkgec_ Flux Flux Flux Electomagnets.56 MHz A Fluxline kw

13 PHASE MATCHING Fo electons to be continuously acceleated downsteam, the phase velocity of the axial f field must be well matched to the speed of the electons to avoid slipping into phases of deceleation. The paallel phase velocity of the E-fields is popotional to the f fequency. 5 f = 7. MHz RADIALLY AVERAGED EED (ev -/ ) f =.6 MHz f =.6 MHz HEIGHT (cm) -8 ENERGY (ev) 4 ENERGY (ev) 4 ENERGY (ev) 4 As the f fequency deceases non-collisional heating is moe significant due to bette phase matching with themal electons. A, mto, G, kw MJKGEC_

14 FRACTION CAPALE OF PHASE MATCHING A measue of the total amount of collisionless heating is the faction of electons with themal speeds capable of phase matching to the wave. This faction not only inceases with deceasing fequency, but also extends acoss a lage popotion of the eacto MHz PERCENT OF ELECTRONS IN PHASE.6 MHz.6 MHz % 5 %.% A, mto, G, kw MJKGEC_

15 CONSEQUENCES OF TG MODE The Tivelpiece-Gould mode can be suppessed at high magnetic field and high powe deposition. At high magnetic fields (> s G) EEDs with and without electostatic tems in Maxwell's equation, as well as most bulk plasma popeties, ae simila. WITHOUT TG MODE WITH TG MODE EED (ev-/ ) - -4 Inceasing Distance Fom Coil Inceasing Distance Fom Coil Enegy (ev) A, mto, G, kw 4 5 Enegy (ev) MJKGEC_4

16 CONCLUDING REMARKS A model has been developed to investigate EEDs in wave heated MEICP eactos. The tansition fom ICP to helicon heating occus when a signficant faction of the powe deposition is poduced by axial and adial components of the wave. Non-collisional heating esults in significant aising of the tail of the EED downsteam of the antenna above s G and below mto. "Tail-aising" equies matching of the phase velocity of the wave and themal speed of the electons. This can be facilitated by adjusting the fequency of excitation fo a given powe deposition. MJKGEC_5