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...

I. ABSTRACTComplex structures on a material surface can significantly reduce the total secondary electron emission yield from that surface. A foam or fuzz is a solid surface above which is placed a layer of isotropically aligned whiskers. Primary electrons that penetrate into this layer produce secondary electrons that become trapped and not escape into the bulk plasma. In this manner the secondary electron yield (SEY) may be reduced. We developed an analytic model and conducted numerical simulations of secondary electron emission from a foam to determine the extent of SEY reduction. We find that the relevant condition for SEY minimization isū ≡ AD/2 >> 1, where D is the volume fill fraction and A is the aspect ratio of the whisker layer, the ratio of the thickness of the layer to the radius of the fibers. We find that foam can not reduce the SEY from a surface to less than 0.3 of its flat value.

Complex structures on a material surface can significantly reduce the total secondary electron emission from that surface. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at one point of the structure and intersecting another. We performed Monte Carlo calculations to demonstrate that fractal surfaces can reduce net secondary electron emission produced by the surface as compared to the flat surface. Specifically, we describe one surface, a "feathered" surface, which reduces the secondary electron emission yield more effectively than other previously considered configurations. Specifically, feathers grown onto a surface suppress secondary electron emission from shallow angles of incidence more effectively than velvet. We find that, for the surface simulated, secondary electron emission yield remains below 20% of its unsuppressed value, even for shallow incident angles, where the velvet-only surface gives reduction factor of only 50%.