Session BM31: Hershkowitz Memorial

Plasma Potential: A Partial Timeline of the Life and Accomplishments of Noah Hershkowitz

A brief introduction is given to a special workshop devoted to remembering Professor Noah Hershkowitz and his many contributions to the fields of Plasma Science and Plasma Technology. Many of these contributions will be discussed in greater detail by other speakers in this session, discussion that will highlight the seminal nature of those contributions, and the extent to which areas of current research were inspired by Noah's efforts and vision. Further, an overview of Noah's life using a timeline of his activity and accomplishments, set in context of major epochs in the development of plasma science, is presented. We begin with a young high school student in New York, wanting to develop as an artist, a color-blind artist,  and end with one of the truly eminent and influential experimental plasma physicists of his generation. It is this speaker's great privilege to have worked with Noah as both a student and a colleague,  and to share, in this talk,  a little bit of the  gratitude he feels for this great plasma physicist and person--Noah Hershkowitz.   

Applications of electric probes away from the ideal limit

Throughout plasma physics, approximations are made during analysis as a means of making problems tractable.  Making sense of those approximations and their limitations in the face of laboratory realities is often vexing to the plasma newcomer.   One reason that Noah Hershkowitz's many contributions to plasma science are so impactful and enduring is his pragmatic approach and accessible presentation.  An example of these qualities can be seen in his book chapter* "How Langmuir Probes Work."  In this article, he describes numerous practical considerations in implementing and interpreting electric probes of various types in a variety of plasma environments.  Through this article, he showed that the many non-ideal cases that arise when dealing with probes can be broken down and considered in a systematic way to understand and exploit their capabilities.  Examples from the three decades since this article was published will be used to highlight the impact of his career beyond his own research results, through his pedagogical skill and the understanding and confidence he inspired in others.

*Noah Hershkowitz, "How Langmuir Probes work," in Plasma Diagnostics, D. L.  Flamm and O. Auciello, eds., Academic Press (1989).   

Noah Hershkowitz contribution to sheath experiment, problem in the sheath theory and my encounters with Noah

 

This presentation consists of three parts. 1) A century ago, Irving Langmuir put forward concept of plasma and sheath and formulated the kinetic model for a bounded, low-temperature, collisionless plasma. The solution of this model resulted in a continuous structure consisting of a quasi-neutral plasma and sheaths at the plasma boundaries. According to Langmuir, the sheath is a thin, an of order of Debye length electron depleted area where the plasma ions are accelerated to the wall. In the following decades, the science of the sheath evolved in probe diagnostics, cathode and anode electrode sheaths, the interaction of rf field with sheath in rf plasma and rf probes. These activities were primarily theoretical, with a few experimental works dealing mainly with the sheath’s integral characteristics. Experiments on the sheath structure and the sheath edge condition were especially important due to hot discussion of controversy in the sheath perception during the last decades. One among the significant contributions of Noah Hershkowitz is comprehensive experimental study of the sheath potential distributions with emissive probes developed by Noah with his collaborates. This area of Noah’s activity will be elucidated in this session by his colleagues. 2) It is shown in this presentation that an analog of the Tonks-Langmuir model for a bounded fully ionized plasma in the fluid approximation boils down to a set of equation that ultimately coincides with those derived by Bohm for collisionless sheath, from which the Bohm criterion for the sheath existence were obtained. The full solution of this system with postulated by Bohm boundary conditions resulted in the sheath and an infinitely large neutral plasma, and the “sheath” Bohm criterion is the condition of the steady-state plasma without ionization. This becomes evident after the second integration of the system that is missing in Bohm derivation and in textbooks reproducing his results. 3) My encounters with Noah will be shared during the session.   

The Technology Enabling Role of Magnetic Cusps in Plasma Science

Plasma transport in magnetic cusps remains not well understood. The problem of plasma transport in magnetic cusps was a particular problem that Noah Hershkowitz studied experimentally throughout his career. Indeed, his seminal leak width work established a basis for assessing plasma losses in multipole plasma sources. Interestingly, Noah also utilized cusp confinement to probe other plasma physics phenomena as well such as sheaths.  It was his multipole, magnetic cusp sources that provided the uniform, field-free region required to carry out his numerous plasma sheath measurements and basic probe studies.  The insight gained however from his direct study of transport through magnetic cusps has impacted a range of seemingly disparate research communities ranging from space propulsion to plasma processing to even nuclear fusion.  Here, we review some of the recent magnetic cusp work ongoing in these exciting research areas with ties to and influenced by Noah’s research.   

Recent Directions in Sheath Theory Inspired by Prof. Hershkowitz

This talk will recount the author’s introduction to plasma physics through experiments on sheaths in Prof. Hershkowitz’s laboratory and how it inspired theoretical research. Three topics will be discussed: the role of global confinement conditions on determining sheath structure, the role of electrostatic instabilities in determining how a plasma interacts with a material, and the unconventional electron and fireball sheaths found near positively biased electrodes. All of these examples remain current topics of research interest and were significantly influenced by Prof. Hershkowitz’s pioneering contributions. The importance of feedback between experiment and theory in order to advance scientific understanding was something that he stressed. It remains essential.    

Langmuir probe: how to handle expanding ion collecting area when the probe is negatively biased

Prof. Hershkowitz, being one of the greatest experimental plasma physicists, had taught me many plasma diagnostics such as Langmuir probes, double probes, emissive probes, laser induced fluorescence (LIF) and an ion-acoustic wave launcher/detector. We are all aware of importance of these techniques, and I appreciate very much for his invaluable lessons on them. However, if I am asked to say the one most important lesson from him, then I choose his skepticism before making conclusions: when I showed him data, he would often tell me that "something is wrong!" Following his skepticism, we have re-visited a well-known problem on expanding ion collecting area of a planar Langmuir probe. Ion currents collected when the probe is negatively biased tend to increase with the bias associated with sheath expansion. Typically, these effects are eliminated by fitting a linear fit or a power law fit to obtain ion saturation current which is, then, used to obtain electron information from the I-V characteristic. These fitting results are quite sensitive to the fitting range which inevitably introduces subjectivity, and quite often we observe inconsistent ion and electron densities. In this Noah Hershkowitz Memorial Session, we discuss how to properly obtain ion currents consistent with the experimental data. Note that our proposed method may not necessarily reflect the views and opinions of Prof. Hershkowitz and cannot be confirmed as he is not with us anymore.   

Coffee Break

Coffee Break   

Sheaths, Microelectronics Fabrication and the Founding of PSST

The many contributions of Prof. Noah Hershkowitz to low temperature plasmas span from fundamental science to service to the profession.  His investigations of sheaths and ion acceleration into surfaces provided great insights into the fundamental processes that we need to understand to optimize reactant fluxes incident onto wafers for microelectronics fabrication.  The competing influence of multiple ion species in formation of the pre-sheath is the launching pad for accelerating the many different types of ions produced in plasma etching mixtures through the sheath proper.  In this talk, we will discuss the evolution of fundamental sheath studies by Prof. Hershkowitz to the modeling of multi-component, multi-frequency and non-sinusoidal capacitively and inductively coupled plasmas for microelectronics fabrications.  These early discussions on sheaths and ion energy distributions occurred at a time when Prof. Hershkowitz was in the process of founding the journal Plasma Sources Science and Technology, one of his greatest services to the profession.  PSST was founded in acknowledgement of the need for translational research from the fundamental to the applied.  Several "behind the curtain" debates on the founding of PSST will be shared and Prof. Hershkowitz's influence on editors-in-chief following him will be discussed.   

Go Forth and Build a Versatile Plasma Source – Words of Wisdom from Noah Hershkowitz

In this talk I will describe the origins of the West Virginia University helicon source program. The focus will be on the key results from the program over the past two decades and the collaborative role played by Professor Noah Hershkowitz throughout that time. The West Virginia University Hot hELIcon eXperiment (HELIX) provided variable density and ion temperature plasmas, with controllable levels of thermal anisotropy, for space relevant laboratory experiments as well as fundamental studies of helicon source physics. Through auxiliary ion heating, the ion temperature anisotropy (T_⊥/T_||) was variable from 1 to 20 for parallel plasma beta (β=8πnkT_i||/B²) values that spanned the range 0.0001 to 0.01. Through measurements of sub-mm wave fluctuations localized to the plasma edge; enhancements in plasma density, electron temperature, and ion temperature at specific antenna frequencies and magnetic field strengths; measurements of energetic electrons; ion thermal anisotropy measurements; and electric field measurements; the key role of slow waves in helicon sources was identified. Neutral pumping, another topic championed by Professor Hershkowitz, was also studied extensively using single photon and two-photon measurements of neutral species in helicon sources.   

Emissive Probes in Plasma Propulsion: Noah Hershkowitz's Legacy

For plasma propulsion devices such as Hall thrusters, knowledge of the ion accelerating electric field is essential for understanding of how thrust is generated. Emissive probes have become a widely used tool for diagnosing these devices with flowing plasmas. This is in great part thanks to Noah Hershkowitz, who developed and refined emissive probe techniques for measuring plasma space potentials. By emitting electrons below the plasma potential, but not above it, emissive probes can be used to make measurements in a wide variety of conditions, making them well suited for electric propulsion plasmas. Noah taught the plasma community how to account for space-charge effects, which make interpreting emissive probe data more complex than it first appears [1]. He also pioneered a careful technique that uses emissive probes to provide insights into electric propulsion devices with the accuracy that cannot be obtained in any other way [2]. In this talk, we will highlight results from the Hall thruster research which we conducted together with Noah [2,3]. Noah’s remarkable technique is standing the test of time. Despite the development of recent theories of inverse sheath for the floating emissive walls [4] which clears the way for the use of a simple floating probe measurements in some quiescent plasmas [5], it remains that Noah’s method of emissive probe measurements using the inflection point in the limit of zero electron emission [1] is critically needed to minimize space charge effects particularly in flowing plasmas. Among many other areas of flowing plasmas, these conditions pertain to our own area of research, plasma thrusters [2,3,5]. 

[1] J. R. Smith, N. Hershkowitz, P. Coakley, Rev. Sci. Instrum. 50, 210 (1979)

[2] J. P. Sheehan et al., Phys. Plasmas 18, 073501 (2011)

[3] J. P. Sheehan et al., J. Prop. Power 33, B35697 (2017)

[4] M. D. Campanell and M. V. Umansky, Phys. Rev. Lett. 116, 085003 (2016)

[5] B. F. Kraus and Y. Raitses, Phys. Plasmas 25, 030701 (2018)   

Analytical Solutions of Space Charge Problems for Different Geometries Including Collisions

Space charge effects are very important in vacuum electronics and plasma boundary interactions and desing of plasma systems. Here, a new solution technique is presented for finding analytic solutions to the nonlinear Poisson equation with a single charge species. Using this technique, currents, current density, sheath lengths, Debye lengths are obtained for collisional and collisionless charge transport in one dimensional Cartesian , cylindrical and spherical system. The method is also applicable to other nonlinear differential equations having power-like solutions.   

From Multi-ion Species Bohm Criterion to Detachment Sheath Physics: Our Work with Prof. Noah Hershkowitz and Beyond.

In this presentation, the speaker's works with the late Prof. Noah Hershkowitz in the field of low temperature basic plasma physics, particularly on the formation of plasma sheath/presheath will be reviewed. The work reviewed here includes experimental investigations of how the Bohm criterion is satisfied in multi-ion species plasmas, formation of virtual cathodes near small electrodes, and their relations with the recently developed theory of instability enhanced collisional friction, as well as pulsing fireballs near grids, and discusses the work of some of Prof. Hershkowitz's previous students. This presentation will also introduce recent developments in the speaker's group in the Chinese Academy of Sciences, Institute of Plasma Physics (ASIPP) inspired by Prof. Hershkowitz's previous work and recent collaboration with our group, which include their recent work in strongly electron emitting cathodes and emissive probes, collisional Bohm criterion and sheath formation of detached plasmas in linear devices.   

Reminiscences of Prof. Noah Hershkowitz: Open microphone

This session will consist of a series of short reminiscences of Prof. Hershkowitz’s life and career. Contributions are invited from any audience members. We ask that they remain short (<5 minutes) so that there will be time for anyone who would like to contribute. For planning purposes, we ask that potential speakers please contact the workshop organizers ahead of time, but it is not a requirement to do so. It should be possible to present a slide or two. Priority will be given to in-person contributions and those who contacted the organizers prior to the meeting. However, if the conference AV equipment and time allow, contributions from remote participants may also be possible.