Operation of a Segmented Hall Thruster with Low-sputtering Carbon-velvet Electrodes

Raitses, Y.; Staack, D.; Dunaevsky, A.; Fisch, N.J.

doi:10.2172/934607

Cited by 10 publications

(17 citation statements)

References 11 publications

(28 reference statements)

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“…Furthermore, our analysis assumes that velvet fibers are perfectly normally oriented. In actuality, velvet fibers are observed to lie at angles to the normal, and even to curve and change their angles, to "flop" over near their tops 9 . This effect is not considered by our model.…”

Section: Introductionmentioning

confidence: 99%

Modeling of reduced effective secondary electron emission yield from a velvet surface

Swanson

Kaganovich

2016

Journal of Applied Physics

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I. ABSTRACTComplex structures on a material surface can significantly reduce total secondary electron emission from that surface. A velvet is a surface that consists of an array of vertically standing whiskers. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at the bottom of the structure and on the sides of the velvet whiskers. We performed numerical simulations and developed an approximate analytical model that calculates the net secondary electron emission yield from a velvet surface as a function of the velvet whisker length and packing density, and the angle of incidence of primary electrons. We found that to suppress secondary electrons, the following condition on dimensionless parameters must be met: (π/2)DA tan θ ≫ 1, where θ is the angle of incidence of the primary electron from the normal, D is the fraction of surface area taken up by the velvet whisker bases, and A is the aspect ratio, A ≡ h/r, the ratio of height to radius of the velvet whiskers. We find that velvets available today can reduce the secondary electron yield by 90% from the value of a flat surface. The values of optimal velvet whisker packing density that maximally suppresses secondary electron emission yield are determined as a function of velvet aspect ratio and electron angle of incidence.PACS: 52.59.Bi, 52.59.Fn, 52.77.-j II. INTRODUCTIONSecondary electron emission (SEE) from dielectric and metal surfaces under bombardment of incident electron flux is important for many applications where incident electron energy can reach tens or hundreds of electron volts. Under these conditions secondary electron emission yield can exceed unity and therefore strongly modify wall charging or cause multiplication of secondary electron populations. The multipactor effect causes accumulation of SEE population in RF amplifiers and limits the maximum electric field in these devices 1 . Clouds of secondary electrons have been also found to affect particle beam transport in accelerators. As a result, researchers at SLAC and CERN have studied effective ways to suppress Secondary Elecron Yield (SEY, γ) for example by cutting grooves into the accelerator walls 2-5 . SEE processes are also known to affect Hall thruster operation due to contribution to so-called near-wall conductivity or due to reducing wall potential and increasing plasma energy losses 6 . Wall conditions can also affect instabilities in plasmas and electron energy distribution functions 7,8 . Therefore, researchers investigate the possibility of using complex surface structures to minimize SEY for electric propulsion devices 9,10 . The surface geometry of a material can affect its SEY just as much as its chemical composition. Note that there is a significant difference in micro-pores configuration as opposed to velvet; in micro-pores configuration electrons cannot penetrate arbitrary far along perpendicular distances into the pore array, unlike in velvet. As an important consequence, we show that velvets can give much higher reduction in SEY as compar...

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Section: Introductionmentioning

confidence: 99%

Modeling of reduced effective secondary electron emission yield from a velvet surface

Swanson

Kaganovich

2016

Journal of Applied Physics

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“…Hall thrusters are the prime candidate for many future missions because of their higher thrust and lower cost compared to traditional ion thrusters [3,4]. Every Hall thruster fabricated today in the US and Europe has dielectric walls made of either boron nitride (BN) or BNSiO 2 [5], or segmented walls that contain both carbon and ceramic material typically in a layered structure [6][7][8]. This is because these ceramic materials provide: 1) Insulating surfaces so as to not short out the electric field in the thruster acceleration region, 2) Low sputtering yield under ion bombardment to minimize the erosion and extend thruster life, 3) Low secondary electron yield to minimize the electron power loss to the wall.…”

Section: Introductionmentioning

confidence: 99%

Conducting Wall Hall Thrusters

Goebel

Hofer

Mikellides

et al. 2013

49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference

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A unique configuration of the magnetic field near the wall of Hall thrusters, called "magnetic shielding", has recently demonstrated the ability to significantly reduce the erosion of the boron nitride (BN) walls and extend the life of Hall thrusters by orders of magnitude. The ability of magnetic shielding to minimize interactions between the plasma and the discharge chamber walls has for the first time enabled the replacement of insulating walls with conducting materials without loss in Hall thruster performance. The boron nitride rings in the 6 kW H6 Hall thruster were replaced with graphite that self-biased to near the anode potential during operation. The thruster efficiency remained over 60% (within two percent of the baseline BN configuration) with a small decrease in thrust and increase in Isp typical of magnetically shielded Hall thrusters. The graphite wall temperatures decreased significantly compared to both shielded and unshielded BN configurations, leading to the potential for higher power operation. Eliminating ceramic walls makes it simpler and less expensive to fabricate a thruster to survive launch loads, and the graphite discharge chamber radiates more efficiently which increases the power capability of the thruster compared to conventional Hall thruster designs. Nomenclature g = acceleration of gravity Isp = specific impulse ! ˙ m p = total propellant mass flow rate T = thrust P T = Total input power P d = Discharge power P mag = Power into the magnets

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“…These were first observed in our cylindrical Hall thruster [5], but here we study the 2kW segmented electrode annular Hall thruster as described in Ref. [6]. Segmented electrodes are one of the ways to tailor the electric field profile in Hall thrusters [7], though here the segments remained floating.…”

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confidence: 94%