Micro-Particles as Probes for Laboratory Plasmas

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Microparticle Probes for Laboratory Plasmas Zhehui Wang, C˘at˘alin M. Ticos¸, Leonid A. Dorf, and Glen A. Wurden, Senior Member, IEEE

Invited Paper

Abstract—Two applications of microparticles (micron-size particles) for laboratory plasma diagnosis are discussed. The first application is about injecting hypervelocity microparticles [(HDI) for hypervelocity dust injection] for internal magnetic field measurement in high-temperature plasmas. Since the concept of HDI has already been examined in details in our previous works, the primary focus here is to compare different schemes of microparticle acceleration. A new design of HDI based on plasma-dynamic accelerator is described to inject multiple microparticles to velocities around 10 km/s simultaneously. The other application is about using microparticles to measure plasma flow [(mPTV) for microparticle tracer velocimetry]. Directions of plasma flow at multiple locations can be measured simultaneously using mPTV because ion drag dominates over other forces inside laboratory plasmas of order 1019 m 3 in density and a few electron volts in temperature. In addition to complex interactions between a microparticle with plasma, the magnitude of plasma flow may not be obtained directly from the microparticle velocity because of the time it takes for each microparticle to relax to local plasma velocity. In summary, microparticles are naturally small objects in all three dimensions and can, therefore, become useful diagnostics for laboratory plasmas with minimal perturbation. Index Terms—Hypervelocity, internal magnetic field measurement, ion flow measurement, microparticle, plasma flow measurement, plasma gun.

I. INTRODUCTION

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ROBES are routinely used for diagnosis of a variety of plasma properties, such as electron density, electron temperature, magnetic field, electric field, and plasma flows. A main advantage of a probe is its relatively simple design and construction. One of the key challenges for a probe is data interpretation which needs to take into account the characteristics of electronics, dimension and geometry of the probe, and sheath physics. Probe perturbation is unavoidable due to the probe shaft, which is both for support of the probe and data transmission. Probe perturbation can be minimized by placing the probe at the edge of a plasma. Another uncertainty comes from the difficult-to-quantify surface conditions of a probe. As an example, the conditions under which the probe arcs need more study. Arcing can compromise the probe measurement because a “secondary plasma” is created due to the local interaction of the probe with the plasma.

Manuscript received August 29, 2005; revised January 23, 2006. This work was supported in part by the Office of Fusion Energy Sciences, Department of Energy under Contract W-7405-ENG-36. The authors are with the Plasma Physics Group (P-24), Los Alamos National Laboratory, Los Alamos, NM 87545 USA. Digital Object Identifier 10.1109/TPS.2006.872161

Microparticles of micrometer size are truly miniature in all three dimensions and can become potentially useful “mini-probes” for laboratory plasmas. Microparticles, also known as dust, are ubiquitous in the universe and laboratory [1]–[3]. Dust can also be produced in both fusion and low-temperature plasmas [4], [5]. Micrometer-size dust charged in low temperature plasmas show an intriguing and interesting behavior. When the potential energy of the Coulomb interaction within a dust cloud is about two orders of magnitude higher then the energy of thermal motion, the dust grains self-align in highly ordered three-dimensional structures called “plasma crystals” [6], [7]. While being useful for studying lattice properties such as propagation of dust-lattice waves [8] or other collective processes at a convenient time scale of the order of seconds, dust crystals or just single dust grains have been successfully used as probes to infer some plasma parameters associated with particle transport [9] and with the sheath potential distribution [10]–[12]. The purposes of this paper are twofold. First, we present a new version of the hypervelocity dust injector (HDI) [13], [14], by using a plasma-dynamic accelerator with coaxial electrode geometry. One of the first plasma-dynamic accelerators for microparticle application was reported in the 1970s for hypervelocity impact experiments [15]. This newer version of HDI extends our original work to “probe” internal magnetic field and structures in high-temperature ( 500 eV) regions of fusion experiments. In addition to the electrostatic acceleration and plasma acceleration, we also list a few other possible schemes to accelerate microparticles to high speed. We will focus our discussion for the purpose of plasma diagnostics. Besides internal magnetic field mapping, we will show that microparticles can also be used to measure the directions of ion and plasma flows at multiple locations inside a plasma. A common flow diagnostic is a directional Langmuir probe, or a Mach probe [16]. Mach probes have different degrees of complexity [17]–[19]. The main concerns for Mach probes are uncertainty in data interpretation, and potential interference (probe-body in particular) with flow field. A Doppler-shift spectrometer can measure line-averaged flow, or local flow when the emitting particles are localized or when a neutral beam is used for cross-viewing [20]. Abel inversion may be required to obtain flow profile information for line-integrated measurements. Because of the limited number of chords, flow-structure is crude for line-integrated methods and details of flow structures may be lost. The structure of the paper is the following. We will first discuss the interaction of microparticles with plasma with a focus on physics relevant to HDI and plasma flow measurements in

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Section II. Section III is divided into four subsections, the first three for HDI acceleration with a summary on electrostatic acceleration, a survey of various microparticle acceleration methods, and the design of a plasma-dynamic accelerator. The fourth subsection is a detailed description and consideration for microparticle tracer velocimetry (mPTV). The Conclusion section follows. II. MICROPARTICLE INTERACTION WITH PLASMA Under this context, the interactions among microparticles can be neglected. In other words, the electrostatic potential between two microparticles is less than the kinetic energy of each microparticle [6], [7] (1) is the number of charges on each microparticle. is the averaged microparticle separation from each is the microparticle density. A small enough will other. guarantee the condition (1) being satisfied. Another scenario is , so that each microparticle is screened by plasma that ions or electrons. where

A. Charging and Screening of Microparticles Charge balance equation for a microparticle in a plasma is given by [2], [3] (2) , and are the electron current, the ion where , , current (including the recombination on surface), the secondary electron emission current due to the bombardment of the microparticle by the plasma ions and electrons, and the thermionic emission current which is important to microparticles at high is temperatures [21]. The microparticle “charging” time , which is small comdetermined roughly by pared with other time scales, such as microparticle transit time and duration of the plasma. Therefore, a microparticle can be charged “instantaneously” to an equilibrium state, which corresponds to a microparticle floating potential . The orbital motion limited (OML) approximation [3] may be used to calwhen the Debye length of the plasma is much greater culate than the microparticle size. For the two types of plasmas considfor electron temperatures eV ered here . for eV and and . The thermionic current is given by the Richardson–Dushman formula [21]. We found that, for hotter , the plasmas with particle densities of the order of . is, therefore, thermionic current satisfies small compared to and and may be neglected. We assume that microparticle surface temperature is 3000 . Solving (2) numerically, we found that the potential of a miin a plasma with electron temcroparticle is perature eV and . in a high temperature plasma with eV and . One consequence of a charged microparticles inside a plasma is the Debye screening of the microparticle, as expressed by

Fig. 1. Contour plot of microparticle destruction rate, characterized by the change of microparticle radius as a derivative of time, dR =dt in (3), under different plasma temperatures (T ) and densities (n). Due to the electrostatic charging of microparticles, electron heating can be reduced substantially by a reduction in the factor f .

the exponential term in (1). It is expected that screening particles (ions if the microparticle is charged negatively) can reduce the heating of the microparticles. Another consequence of microparticle screening is to modify the light-scattering properties of the microparticle. Recent theoretical work [22], [23] has indicated that screening can lead to larger light scattering cross section by a microparticle, although experimental data remain absent at the moment. B. Destruction and Erosion of Microparticles Due to Heating We neglect here possible secondary electron emission, recombination, and radiative cooling that may be favorable to the survival of a microparticle in plasmas [24]. The basic equation for microparticle erosion is [13], [14] (3) where is a screening factor of the heat flux density with being the per unit area plasma density and the plasma temperature [13], [14]. is approximated by the electron heat flux because the streaming velocity of electrons is much higher than the ions. The sum stands for average energy needed to free each atom from the microparticle. is the mass density of the microparis the mass of each atom or molecule that forms the ticle. microparticle bonding. The negative sign is for the fact that the microparticle size will always decrease due to heating. Results of microparticle erosion rate in different plasmas are shown in Fig. 1. The basic conclusion is that for high temperature plasmas, microparticles are destroyed rapidly. We can make use of the destruction process for internal magnetic field measurement [13], [14]. In relatively “benign” plasmas with densities and temperature of a few electronvolts or less than less, microparticle erosion is slow enough so that microparticles of a few micrometers and larger can survive the plasma. This result justifies possible use of microparticles as plasma flow tracer for the benign plasmas. C. Microparticle Interaction With Plasma Flow In plasmas when the destruction of microparticle can be neglected, microparticles can be used to diagnose ion flows or

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plasma flows. Momentum of plasma ions (plasma flow) couples more strongly to microparticle motion than momentum of electrons since the mass of an ion is more than one thousand times that of an electron, therefore, a microparticle preferentially samples ion flows. Besides the interactions with electrons and ions, the forces on a microparticle inside a plasma can be rather complicated and each of them can accelerate the microparticle [3]. We consider an ideal situation when plasma is homogeneous so that the predominant forces on the microparticle are from ions with a mean flow of , including the direct-impact and the long-range Coulomb force . force The equation of the motion for the microparticle inside a single-ion-species plasma is [25], [26]

basis for using microparticle as plasma velocity tracers inside laboratory plasmas. III. MICROPARTICLE PROBES FOR PLASMAS Depending on if a microparticle is destroyed or not, we can divide the microparticle probes for plasmas into two categories. In the first category, the microparticles are destroyed completely, see Fig. 1 for microparticle destruction rates which are independent of microparticle size. In this case, acceleration of a microparticle to high speed is necessary to maximize the “disof the microparticle penetration into the plasma, in tance” other words (9)

(4) is the microparticle velocity. Both the impact force in which and the Coulomb force are of the same format

is the magnitude of the microparticle velocity and the where is determined by microparticle burn time (10)

(5) with different dimensionless quantity . The normalized relative velocity between the microparticle motion and plasma flow is defined as (6)

The normalization is with respect to the ion thermal velocity . For ion impact (7) and for long-range Coulomb interactions

A model for is given by (3). The use of destructed microparticle probes may include internal magnetic field visualization [13], [14]. We will focus on acceleration methods for this type of microparticle probes. An example of the second category of microparticle probes is microparticle tracer velocimetry that will be discussed below. mPTV uses microparticles to trace plasma and ion flows. Microparticles are partly destroyed or remain intact in the second category of microparticle probes. A. Electrostatic Accelerator for Microparticles In our previous papers [13], [14], we described a hypervelocity dust injector (HDI) system for internal magnetic field mapping using a single-stage electrostatic acceleration method. The acceleration of the electrostatic method is given by [13], [14]

(8) (11) In both (7) and (8), is the magnitude of the normalized relative velocity between the microparticle and plasma flow. Relative magnitude of the direct-impact force and the longrange Coulomb force can be estimated as follows for a and eV hydrogen plasma with . A higher plasma density corresponds to even larger forces for both the direct impact and long-range Coulomb interactions. , . The forces strongly One has depend on , which measures the relative motion between the , and microparticle and the plasma. In a scenario when , , , . Therefore, the long-range Coulomb force on a microparticle is larger than the force due to direct ionic impact. In addition, we can neglect gravity in (4) for flowing hydrogen or denser and eV. For plasmas of the plasma flow scenario discussed in the previous paragraph compared with when the microparticle moves slowly , the combined force due to ions is a few plasma flow, orders of magnitude larger than the gravitation force on a carbon microparticle of 10 m in radius. The dominance of the ion drag (when is substantial) over gravity and other forces form the

where is the permittivity of vacuum. is the mass density of the microsphere. is the field emission limited field strength of the microparticle, which is determined by the material property and geometry of the microparticle. For example, a bumpy surface with sharp spots on from that of a perfectly the microsphere can reduce the spherical microsphere. is limited to to from experimental data. is the electrostatic field strength, which is limited by atmospheric breakdown voltage, for example. The theoretical limit of is 3 MV/m in the standard atmosphere. A reduction of to 0.3 MV/m are more common in practice due to sharp edges and nonideal atmospheric conditions including humidity. Because of practical limitations such as high-voltage insulation and system size, our original HDI design [13], [14] was based on a 250-kV single-stage electrostatic accelerator, which can limit the velocity of microsphere of a few micrometers and larger to less than 8 km/s, see Fig. 2. One attractive feature of a single-stage electrostatic method is that such a system does not introduce “impurities,” including gas atoms and molecules, charged particles, or sputtered materials associated with HDI.

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Fig. 2. Comparison of electrostatic acceleration with plasma-dynamic acceleration under different conditions. Microparticles are made of carbon. Microparticles with radii up to 25 m are sufficient for internal magnetic field measurement in NSTX plasmas [13], [14]. TABLE I TWO CLASSES OF MICROPARTICLE ACCELERATORS: CLASS I, ELECTROMAGNETIC-FIELD BASED; CLASS II, MOMENTUM-TRANSFER BASED

When velocity requirement is not stringent (say, 1 km/s is acceptable), electrostatic HDI is still a preferred choice for microsphere injection. B. Comparison of Methods for Microparticle Acceleration Methods other than single-stage electrostatic acceleration are available that have been demonstrated or can potentially be used to accelerate micrometer-size microparticles to hypervelocities beyond 10 km/s. Several different types of microparticle acceleration are listed in Table I. Multiple-stage electrostatic acceleration is an extension of the single-stage electrostatic acceleration. The goal of using multiple-stage electrostatic is to use low acceleration voltages for each stage, while the total acceleration of voltage at each stage. Multiple-stage electrovoltage is static method was demonstrated for single microparticles [27]. Acceleration of multiple microparticles simultaneously using multiple-stage electrostatic method may pose some technical challenge. Electrostatic methods belong to the class of accelerators that utilize electromagnetic field for acceleration. Another example of this class is to use magnetic field gradient to accelerate magnetic and diamagnetic (ideally perfectly conducting) microparticles. The acceleration due to magnetic field gradient can be expressed as [28] (12) where is a geometrical coefficient. for a spherical microparticle. is the magnetic susceptibility of the microparticle. Another class of accelerators is based on momentum-transfer from the accelerator media (“propellant”) to microparticles. Light gas guns have been used for acceleration of projectiles

of a few grams to velocities approaching 10 km/s [29]. A frictionless, adiabatic “ideal gun” has an acceleration given by , where is the average pressure of the gas, is the effective density of the projectiles [30], and is the caliber of the gun, which is essentially the same as the size of the projectile. Use of a light gas gun for microparticle acceleration is yet to be seen. Thermal plasma guns are similar to light gas guns, except that plasmas, instead of hot gases, are the propellant. The acceleration for thermal plasmas can be approximated as with being a coefficient of order one that depends on the geometry and thermodynamic properties of the plasma expansion. Related to thermal plasma expansion, another type of plasma accelerator is when the directed flow energy exceeds the thermal energy of , the acceleration the plasma, in other words, due to flowing plasma drag is given by [31], [32] (13) depends on the where the dimensionless drag coefficient Reynolds number. Experimentally, plasma-dynamic accelerator for microparticles was first developed for hypervelocity impact experiments [15]. It is interesting to compare (13) with (4). Both acceleration mechanisms are due to ion drags. We expect that (13) is applicable to fluid regime while (4) is applicable to kinetic regime (including ion beams). Pure ion beam for microparticle acceleration is less effective than ion beam propagation inside plasmas because of the low density of pure ions beam resulted from repulsive Coulomb forces. Finally, we mention that when one or more microparticles collide with a large object, which has a mass much greater than the mass of the microparticles, the impact process can result in reflected hypervelocity microparticles with velocity up to two times the velocity of the large object due to momentum conservation. The acceleration during the impact is limited by material tensile strength corre, which is at a fraction of the sponding to a pressure of Young’s modulus of the microparticles [33]. Besides the velocity requirement, other key factors for a hypervelocity microparticle probe are the compact size of the accelerator, ability of the accelerator to deliver multiple microparticles simultaneously, low impurity (propellant, for example) associated with the accelerator. We have selected the plasma-dynamic accelerator for a new version of HDI to measure internal magnetic field and profiles in the national spherical torus experiment (NSTX) [13], [14]. In the following section, we discuss how the new HDI satisfies all the requirements and can deliver hypervelocity microparticles exceeding 10 km/s. C. Plasma-dynamic Accelerator for Microparticles A 1-m-long plasma-dynamic accelerator is able to accelerate multiple microparticles simultaneously to hypervelocities of 10 km/s. In Fig. 2, we compare the velocities achievable for different size microparticles for both plasma-dynamic accelerators and electrostatic accelerators. The two plasma-dynamic accelerators are for 0.25 and 1.0 m acceleration length, respectively. The two electrostatic accelerators are for a total acceleration voltage of 300 kV and 1.5 MV, respectively, assuming each microparticle can hold theoretically-permitted positive charges, corresponding to in (11). The kinetic pressure of 25 for the plasma-dynamic accelerators are

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TABLE II COMPARISON OF PLASMADYNAMIC ACCELERATOR DESIGNS FROM LOS ALAMOS (LANL) WITH THAT OF TECHNICAL UNIVERSITY OF MUNICH (TUM)[15]

derived from the product of the plasma density and flow speed . is the standard atmospheric pressure. Our point design to achieve this kinetic pressure is , for deuterium gas. 1) Design Parameters: The Los Alamos plasma-dynamic accelerator for HDI is based on a previous experimental design [15] while using existing Los Alamos hardware, in particular the pulsed power parts. A summary and comparison of the design parameters are given in Table. II. 2) Other Issues: One of the potential drawbacks of a flowing-plasma accelerator is the gas load associated with plasma generation. Reduction of gas load by using a surge tank in between the accelerator and the NSTX plasma is planned as a part of HDI design. Ratio of the accelerator volume to the surge tank volume determines the peak pressure inside the surge tank. Microparticle velocity will be measured using time-of-flight techniques. D. Microparticle Tracer Velocimetry Based on the discussion in Section II-C, when microparticle motion is predominantly determined by ion physics, mPTV for flow visualization is possible in plasmas. mPTV is to seed plasmas with certain number of microparticles as tracers. Plasma flow information is obtained by tracking the microparticle motion. Plasma mPTV is similar to particle tracer velocimetry (PTV) and particle imaging velocimetry (PIV) for conventional fluids and gases [34]. Velocities of the tracers are obtained through a combination of the high speed imaging of the trajectories of the tracers and time-of-flight technique. One key difference between the conventional PTV and plasma mPTV is that knowledge of tracer velocities is sufficient for PTV to obtain the fluid flow velocity (both the direction and the magnitude of the local velocity) since the flow drag is large enough to guarantee that the tracers track the fluid flow. From (4), mPTV can track the flow directions quite well. For reasons discussed below, the magnitude of microparticle velocity can not relax to the local plasma flow velocity quickly enough. Therefore, mPTV requires additional input such as plasma conditions and microparticle-plasma interaction physics to deduct plasma velocity magnitude in practice. This requirement for additional input makes the velocity magnitude measurement complex using mPTV. Further experimental and theoretical study will be necessary to make it practical to use mPTV to measure the magnitude of flow velocity. Limitation of mPTV in obtaining the magnitude of flow velocity does not prevent it from being useful. For example, mPTV will still be uniquely suitable to visualize topology of plasma flow patterns (such as vortices).

Fig. 3. Velocities of microparticles of different sizes as a function of time = 3 10 m=s. Plasma density is based on (4). Plasma flow velocity 3 10 m , temperature 10 eV. Substantial deviation between the magnitude of the microparticle velocity and the magnitude of plasma flow velocity exists.

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j Uj

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1) Motion of Microparticles in Flowing Plasmas: In steady state, the total force on a microparticle is zero, . The microparticle motion, therefore, tracks plasma flow through from (4). A key consideration for mPTV is the time that it takes for microparticles to “reach” the steady state with respect to the plasma flow. An example is shown in Fig. 3 where time-evolution of the velocities of carbon microparticles of various sizes are computed, assuming the microparticle is not moving initially. It shows that even the smallest microparticles of 0.25 m in radius cannot reach the plasma velocity of 30 km/s after 20 ms. The lower limit of 0.25 m is chosen to allow efficient scattering of the laser light by the microparticle. Laser scattering is a generic microparticle detection technique. Therefore, measurement of the microparticle velocity alone (two time-delayed images of the microparticle trajectory) does not yield the local plasma flow velocity directly. Measurement of force on microparticle (which requires at least three time-delayed images of the microparticle trajectory) is necessary to retrieve the plasma flow velocity from (4). Simultaneous contribution to ion force due to flow, charge state of microparticle, plasma density and temperature make it difficult to deduce the magnitude of plasma velocity directly from the velocity or acceleration of a microparticle. Input from computer modeling and other diagnostics will be necessary for quantitative velocity information. However, the direction of microparticle motion still coincides with the direction of local plasma flows, at least right after when the microparticle is introduced into the plasma. By putting multiple microparticles simultaneously into a plasma to generate multiple velocity vectors, we can visualize patterns such as vortices of plasma flows. Therefore, mPTV can still be quite useful to diagnose plasma flows patterns. Mass load of microparticles in mPTV to plasmas can come from two directions. 1) Contamination of plasma through disintegration. However, microparticle disintegration is not expected for low plasma temperature ( 20 eV) and millisecond duration plasmas. We can illuminate microparticles with a laser for imaging purpose. 2) Plasma momentum loss to microparticles. This is built into the concept since we rely on the momentum

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shows the signal-to-noise ratio (SNR) versus the dust radius, , eV. calculated for plasmas with The two curves correspond to and , respectively. , (distance to the detector), (background plasma volume associated with each microparticle detection), and considering the transmission factors 0.8 in both interference filter and lenses. The largest size of microparticles suitable for mPTV is determined by the magnitude of the ion drag force compared with other forces, including gravity. Additional design of an mPTV system using a laser includes proper selection of laser power and detector exposure time (longer exposure time can reduce the effect of dark current from the detector). Polarized laser light may offer an additional choice over unpolarized laser beams to improve SNR [35]. IV. CONCLUSION Fig. 4.SNR of detected scattered light as a function of microparticle (dust) radius. Plasma parameters are n = 3 10 m , T = 10 eV. Smallest microparticle radius is 0:25 m to maintain a SNR of about 20.



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transfer from plasma to microparticles to measure flow. We can minimize the perturbation by a) controlling the total mass of the microparticles so that microparticle mass is no more than 10% - m partiof the total plasma mass and b) avoiding large cles as much as possible. 2) Microparticle Selection: It seems natural and convenient to use spherical microparticles for mPTV since microspheres of different materials are readily available from a number of commercial resources. Cylindrically shaped microparticles and other nonspherical microparticles can possess additional degrees of freedom of motion, such as rotation, for the benefit of plasma diagnostics. The prerequisite is that rotation can be resolved experimentally. Flexibility in selection of materials of microparticle can also allow additional information to be retrieved about the plasma. Chemically stable (for easy handling) microspheres of low atomic number materials, such as graphite or diamond, can be used for initial mPTV studies. The microparticle sizes are determined to be in the range of 0.25 and 10 m. The lower limit is determined by the light scattering, the upper limit is to ensure that the ion force is much greater than other forces, in particular, gravity. 3) Microparticle Tracking Using Lasers: Lasers are widely used to image and track dust and microparticles due to the light-scattering ability of these particles. In a standard setup, an incident laser beam is spread into a sheet beam using a cylindrical lens to track microparticle motion in a two-dimensional plane by fast imaging cameras. Since microparticle size under this context is comparable to the laser wavelength, the Mie scattering model applies. Scattered light decreases with the microparticle size. For example, the Mie scattering cross-sections are , , and for , 1 and 5 m, respectively. Besides the Mie scattering cross section, the combined effects of plasma background radiation, optical setup (optical transmission efficiency) and detector characteristics (photoelectronic conversion efficiency, noise/dark current) limit the smallest microparticles appropriate to mPTV to m in radii. Fig. 4

Microparticles have great potential for diagnosis of laboratory plasma ranging from a few electronvolt (low temperature) to 1-kV temperatures. HDI and mPTV are two examples discussed in this work. Both HDI and mPTV can be less invasive to plasmas than conventional macroscopic probes. Destruction of hypervelocity microparticles can be used to measure internal magnetic field in fusion-relevant high-temperature plasmas. Many acceleration schemes are available. Plasma-dynamic accelerator offers one of the most promising schemes for multiple microparticle injection. Potential problems associated with gas load in a plasma-dynamic accelerator are mitigated by using surge tanks. mPTV can be used to visualize the directions of plasma flows. mPTV is uniquely suitable to reveal complex flow patterns such as vortices in plasmas. The fact that substantial “slipping” between microparticles and plasma flow exists makes the magnitude of flow measurement difficult using mPTV. Laser scattering appears to be adequate for microparticle tracking. Capabilities derived from microparticle diagnostics can enhance our scientific understanding of plasma flows, internal magnetic field structure and dynamics, and turbulent structures inside plasmas. In addition, experimental data about microparticles in plasmas may be used to compare with the modeling of microparticles, which are an important ingredient of the cosmic environment. ACKNOWLEDGMENT The authors would like to thank Dr. G. M. Lapenta and Dr. G.-L. Delzanno on microparticle for the stimulating discussions on interaction with plasmas. REFERENCES [1] R. L. Merlino and J. A. Goree, “Dusty plasmas in the laboratory, industry, and space,” Phys. Today, vol. 57, p. 32, 2004. [2] V. E. Fortov, A. G. Khrapak, V. I. Molotkov, and O. F. Petrov, “Dusty plasmas,” Phys. Uspekhi, vol. 47, p. 447, 2004. [3] P. K. Shukla and A. A. Mamun, Introduction to Dusty Plasmas. Philadelphia, PA: Inst. Phys., 2002, ch. 3. [4] G. S. Selwyn et al., “In situ laser diagnostic studies of plasma-generated particulate contamination,” Appl. Phys. Lett., vol. 57, p. 1876, 1980. [5] J. Winter, “Dust in fusion devices—A multi-faceted problem connecting high- and low-temperature plasma physics,” Plasma Phys. Control. Fusion, vol. 46, p. B583, 2004.

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[35] G. Gebauer and J. Winter, “In situ nanoparticle diagnostics by multiwavelenght Rayleigh-Mie scattering ellipsometry,” New J. Phys., vol. 5, p. 38.1, 2003.

Zhehui Wang was born in Jiangxi, China. He received the B.S. degree in space physics from the University of Science and Technology of China, Hefei, China, in 1992, and the M.S. and Ph.D. degrees in astrophysical sciences from Princeton University, Princeton, NJ, in 1994 and 1998, respectively. He is a technical staff member with the Plasma Physics Group at Los Alamos National Laboratory, Los Alamos, NM. His research interests and contributions include hollow cathode magnetron, coaxial plasmas, microparticle diagnostics of laboratory plasmas, and basic plasma experiments to study magneto-rotational instability, magnetic field generation, ion and plasma flows.

C˘at˘alin M. Ticos¸ was born in Bucharest, Romania, on July 19, 1972. He received the B.S. and M.S. degrees in physics from the University of Bucharest, in 1995 and 1996, respectively, and the Ph.D. degree from the University of Miami, Coral Gables, FL, in 2002. After completing the Ph.D. degree, he worked as a Postdoctoral Associate at Oxford University, Oxford, U.K., on a project which was part of a large European collaboration financed by the EU (FP5) called “Complex Plasmas: The Science of Laboratory Colloidal Plasmas and Mesospheric Charged Aerosols.” He is now a Research Associate in the Plasma Division, Los Alamos National Laboratory, NM, working on developing a diagnostic device for magnetic field mapping in a spherical tokamak based on novel dust injection technology. Dr. Ticos¸ was the recipient of several research and teaching awards while he was a graduate student at the University of Miami.

Leonid A. Dorf was born in Nizhny Novgorod, U.S.S.R. (now Russia), on November 19, 1976. He received the B.S. degree in physics from Nizhny Novgorod State University (NNSU), Nizhny Novgorod, Russia, in 1997, the M.S. degree in astrophysical sciences/plasma physics from NNSU and Princeton University, Princeton, NJ, in 2000, and the Ph.D. degree in astrophysical sciences/plasma physics from Princeton University, in 2004. He is a Postdoctoral Fellow in Group P-24 at Los Alamos National Laboratory, Los Alamos, NM. His research interests include innovative approaches to magnetic fusion, plasma propulsion and plasma sources, plasma–wall and plasma–dust interaction, sheath phenomena, and plasma diagnostics. Dr. Dorf is a member of the American Physical Society (APS) and the American Institute of Aeronautics and Astronautics (AIAA).

Glen A. Wurden (M’94–SM’00) was born in Anchorage, AK, on September 9, 1955. He received three simultaneous B.S. degrees, in physics, mathematics, and chemistry (summa cum laude) from the University of Washington, Seattle, in 1977, and the M.S. and Ph.D. degrees in astrophysical sciences (plasma physics) from Princeton University, Princeton, NJ, in 1979 and 1982, respectively. He is Program Manager for Fusion Energy Sciences at Los Alamos National Laboratory, Los Alamos, NM. His research interests include magnetized target fusion, plasma diagnostics, alternate confinement concepts, including burning plasma issues. Dr. Wurden is a member of the American Physical Society (APS), the American Association for the Advancement of Science (AAAS), and Phi Beta Kappa.