University:
Boston University
Course:
CAS PY 106 | Physics 2
Academic year:

2019
Views:

335

Pages:

13
Author:

Clint W.

Low Temperature Plasmas !D = ("0Te/e2ne)1/2 !D~L vThi~vsystem ne~nneutral sheaths wakes collisions Low Temperature Plasmas “Typically” Studied Plasmas Lab experiments: fusion applications Reconnection modelling: no solid surfaces MHD and PIC codes: rarely consider !D dominated rgions Low Temperature Plasmas From NRC Plasma 2010 study: Future designs for microelectronics devices require fabrication of intricate structures such as this trigate transistor fabricated in silicon having dimensions of only tens of nanometers. Source: M. Mayberry, Intel Corp ! Non-invasive diagnostics provide insights to complex phenomena occurring in plasmas. Here electric fields above the electrodes of a semiconductor processing plasma are measured using laser-induced fluorescence. Low Temperature Plasmas Planetary formation and rings Medical physics Industrial plasmas Dusty plasmas Ionospheric plasmas Threaded by common theme of interactions with neutrals and surfaces Main Electrons (eV) Main Ions (eV) VLF12 (Hz) Main he, up-going energy [eV] normalized density VLF12 (kHz) 10 10 2 10 1 10 0 3 10 10 2 10 1 energy flux [ergs/cm^2-sec] 15 0.10 0.01 5 1.0 0.8 0 1000 0.6 0.4 8000.2 600 0.0 1000 400 100 200 10 0 300 400 300 400 time [s] 500 600 500 600 700 700 Sheaths and charging •! Cold, dark ionosphere: payload potential from current balance of thermal fluxes: ! ! !sc = “few” kTe “few” = ln (sqrt(mi/me)) •! Since je(!sc) + ji(!sc) = 0 ne vthe exp(e!sc/kTe) – ni vthi = 0 (or vbohm or vram) •! And, auroral precipitation can enhance !sc by contributing to electron current collection •! Typically 2-4 V negative where Te, Ti < 1eV; a significant perturbation to the observation Sheaths and charging •! How then to measure the thermal plasma density? •! Not with the electrons –! Even a positive probe can be surrounded by a negative payload sheath (MacDonald, JGR 2006; Frederick-Frost, PoP 2007) –! This effect may be mitigated by photoelectron emission but not in lower dark ionosphere –! Te easier (Frederick-Frost, JGR 2007) •! Moen MNP: density in the sheath (Jacobsen IOP 2010) •! Sheath problems: –! Perturbations (Montgomery); Wakes (Hutchinson) •! Thermal ions? Core, but sheath effects •! RPA nicely low-resource: no high voltage! •! Where put steps on I-V curve if !sc dominant and varying? Charging models: complications Photoemission: charge positive, density dependent Secondary emission: more positive or more negative Area ratios: how big is your biased probe? how flat is your surface? Upshot: hidden or poorly measured populations Ionospheric parameters Dark ionospheric plasma: spacecraft charge a few kTe negative: Jtot = -e ne vthe exp(-ephi/kTe) + e nivthi = 0 Solve for e phi = -kTeln[vthe/vthi] ~ few kTe In sunlight or aurora, varies (-5V to +2V) ne a few 103 or 104/cc; !D a few to 10 cm And another thing…!sc in arcs •! •! •! •! Thermal ion measurements sensitive to !sc !sc sensitive to Te (“5 kTe”) Te strongly raised in soft precipitation Thermal current balance: –! Jth(e-) + Jth(ions) = 0 •! Thermal current balance plus auroral e-: –! Jth(e-) + Jth(ions) - Jaur(e-) + Jsecondary = 0 •! Jsecondary dominates Jauroral, so less negative •! !sc more negative in arc, but less than it would be from thermal current balance !sc in arcs on C2 …repeatable in the lab! Ionospheric thermal plasma facility

Auroral Ionospheric Plasma Processes Ch 4

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