a perspective on current helicon source science...

A Perspective on Current Helicon Source Science Issues

Rod Boswell and all Helicoids

everywhere

Research School of Physical Sciences, ANU

Canberra, Australia

Monday 29th of October, 2001 Long Beach Mini-conference on Applications of Helicon Sources

Helicons of Antiquity


n A valuable prize will be awarded for the most imaginative updating

n Please hand entries to Earl after lunch


The first Helicon SourceL'héliconParoles et musique de Boby Lapointe, 1963

Mon fils tu as déjà soixante ansTa vieill'maman sucreles fraisesOn ne veut plus d'elle au trapèzeA toi de travailler il serait temps.Moi, je veux jouer de l'héliconPon pon pon pon


Overview of Basic Helicon Physics

Right hand polarised wave in magnetised plasma

Propagates between the ion and electron cyclotron frequenciesGenerally ignore ion effects (v x B) and electron effects (dJ/dt)Phase velocity <<c hence high refractive index

Can be considered a Hall wave E = j x B, ie. a motor

Isotropic propagation N2 = (ωpe/ωωce)2

Often called ‘whistlers’ in space physics

First discovered in solid state then gaseous plasmas


Variations on a theme

JJxBne

Pne

VxBEtJ

ne

me

e η∂∂

−−∇++=11

12

Full Ohm’s law in magnetised plasma

Whistler/helicon dispersion, no ion effects

)1cos(1

22

−−=

θωω

ω

ce

peN

The electron inertia introduces anisotropy and a resonance at the

resonance cone angle θ where the wave fields become electrostatic

At lower frequencies need to introduce ion terms for thelower hybrid frequency


Source Characteristics

n High density, high efficiency at low pressure

n Can achieve 100% ionisation

n Scalable from 2 cm. to 20 cm. diameter

n Cylindrical but can be linear

n Convenient rf power

n Convenient magnetic field

n Reliable


Typical Manifestations

n High magnetic field, 500 to 1500 Gauss, around the lower hybrid frequency in argon.

n Low magnetic field, 30 to 100 Gauss, electron inertia effects

n Fusion experiments in Australia and Germany


Some helicon systems


Scientific interest and challenges

n Power balance and efficiency

n Electron inertia, resonance cones and TG modes

n Lower hybrid resonance

n Cross field diffusion

n Ion heating

n Neutral pumping

n Electron-wave interactions

n Wave-wave interactions


Power balance and efficiencyn Radio frequency power into the plasma equals total

power out via ions, electrons, photons and dissociation passing through a loss surface.

n Photon and dissociation loss surfaces not the same as ion and electron loss surfaces

n An efficient helicon discharge has main loss surfaces at the two ends with little cross field charge diffusion, eg. WOMBAT with 50 Gauss, 5 x 10-4 argon, 1500 Watts at 7 MHz produces a column 200 cm long, FWHM 10 cm and 10 12 cm -3. Electron-wave interactions


Cross field diffusion

n Very low effective loss surface mainly from ends

n Electrostatic confinement

n Radial electric fields set up to trap ions

n Similar to a reflex discharge or a cathodic arc

n Also observed in helical system with very good electron confinement

n Increases ion confinement time from

n radial transit (70 µsec to axial transit (500 µsec)


Ion heating

n Perceived need for cold ions in plasma etching

n Great need for hot perpendicular ions for space thrusters

n Long ion confinement leads to high momentum transfer from electrons and isotropic heating

n Possible stochastic heating near antennas

n Radial electric fields accelerate ions

n Waves can also heat ions (lower hybrid?)


ρρ>ψψence To Mars and beyond!


Plasma Processing


1:8 Splitter manufactured at the ANU


Electron inertia, resonance cones and TG modes

n Resonance cones intersect with boundaries to produce TG modes

n TG modes ‘difficult’ to launch in high density plasmas due to poor antenna coupling to short λ

n Strong wave-electron interactions and wave-wave parametric effects on resonance cones.

n Are they related to the inductive coupling?

n Where does all the power go?


Some non-linear effects

n Bursts of emission travelling at the wave phase velocity in ‘pre blue core’ mode

n No phase change along the column in ‘blue core’ mode

n Ionisation and subsequent removal at the ion sound speed reduces pressure very quickly

n Three wave parametric decay from em and es fields

n Are the low frequency ‘idler’ waves responsible for ion heating or diffusion


Wave-wave interactions

Conserve energy: ω1=ω2 +ω3

And momentum: k1=k2+k3

Either a modulational or

parametric instability. Low frequency ‘idler’ is not ion acoustic wave as the

wavelength would be 2 mm. It may be another whistler. Much more work to be done.


Commercialisation problems

n Two patents hence ownership

not clear

n System integrators happy with what they already have

n Industry slow to accept new technology

n Helicon e-beam system

commercialised in Japan, HARE

n ULVAC?

n Ex-PMT?


The Future

Take your students surfing and demonstrate landau damping and parametric decay

See you at the GEM in Australia www.rsphysse.anu.edu.au/admin/GEM12

a perspective on current helicon source science...

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