Student Abstracts at PPPL:

Calculation of Charge-Changing Cross Sections of Ions or Atoms Colliding with Fast Ions Using Classical Trajectory Method. HARRISON MEBANE (Harvard University, Cambridge, MA, 2138) IGOR KAGANOVICH (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Evaluation of ion-atom charge-changing cross sections is needed for many accelerator applications. Ions lose energy when passing through background gasses, beam transport lines, and detectors. A classical trajectory Monte Carlo simulation has been used to calculate ionization and charge exchange cross sections. For benchmarking purposes, an extensive study has been performed for the simple case of hydrogen and helium targets in collisions with various ions. To improve computational efficiency, several integration methods, including Runge-Kutta with adaptive stepsize and Bulirsch-Stoer with Stoermer’s Rule, were compared. The algorithm was also upgraded to simulate the trajectories of two electrons for a helium target. Despite the fact that the simulation only accounts for classical mechanics, the calculations are comparable to experimental results for projectile velocities in the region corresponding to the vicinity of the maximum cross section. The accuracy of a purely classical simulation allows for simpler and faster calculations of cross sections in the vicinity of maximum cross section, avoiding slower and more complex quantum mechanical calculations. In the future, support will be added for simulations of multiple electron trajectories in more complicated targets, and the algorithms will be further refined to improve speed and accuracy.

Calculation of Divertor Thermal Response as a Function of Material Composition in the National Spherical Torus Experiment. MICHAEL CHAFFIN (Reed College, Portland, OR, 97202) RAJESH MAINGI (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Present tokamak designs use a magnetic divertor to deposit heat from the edge plasma onto Plasma Facing Components (PFCs) designed to remove the heat. Studying how this heat is distributed under various discharge conditions gives insight into how heat deposition can be optimized, and how different materials respond to plasma heating. In the National Spherical Torus eXperiment (NSTX), infrared cameras are used to measure divertor surface temperature, from which heat flux is computed using a one dimensional (1D) semi-infinite slab model with constant thermal conductivity. Here, a 1D simulation of the PFCs incorporating material-dependent thermal properties is used to compute heat flux profiles resolved across time and tile thickness. The PFC response to a given heat flux is also computed, and comparisons of resulting temperature profiles are made for a variety of materials including ATJ graphite (a low thermal expansion coefficient polycrystalline graphite presently in the NSTX divertor), pyrolytic graphite, molybdenum, and tungsten. The relatively high conductivity of pyrolytic graphite allows for greater thermal penetration of the PFCs, resulting in much lower temperatures at the PFC boundary. Using pyrolytic graphite instead of ATJ graphite in future fusion devices would mitigate the effects of higher flux deposition onto the PFCs. Further study is needed to determine the appropriateness of using high conductivity materials in particular reactor designs.

Concept to Employ Magnetohydrodynamic Conversion in a Two Gigawatt Inertial Fusion Energy Direct Drive Power Reactor. BRETT ANDERSON (St. Olaf College, Northfield, MN, 55057) CHARLES GENTILE (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

A two gigawatt Inertial Fusion Energy (IFE) direct drive power reactor, currently in conceptual design, injects deuterium-tritium targets into the reactor chamber at the rate of five hertz and uniformly illuminates each target with ultraviolet laser light, resulting in detonation. The conceptual design of this IFE reactor may provide an opportunity to directly harness the power in the post detonation ion fields. This can be accomplished by utilizing a magnetic cusp field to guide the ions into collectors located in the equatorial and polar regions of the reactor. The shaped ion fields resulting from this magnetic intervention configuration pose a distinct challenge, as their intensity may have the potential to damage certain areas within the ion collectors. One method of addressing this challenge is to employ magnetohydrodynamic (MHD) conversion to transform the internal energy of the ion fields directly into electrical energy, a process that would also attenuate the strength of the fields. In order to analyze the potential of MHD conversion in IFE, previous work on MHD conversion in other applications is examined in the context of this proposed IFE reactor configuration. Other conversion techniques are also investigated, including Compact Fusion Advanced Rankine II (CFARII) MHD conversion, radio frequency (RF) particle deceleration, and direct conversion. Analysis reveals that MHD conversion may be a promising solution depending on the intensity of the ion fields. However, a number of engineering and operational concerns need to be addressed; for example, the materials need to be able to withstand extreme conditions. In addition, some elements of the other methods for energy conversion could be incorporated into an MHD conversion design. The next logical step in the development of this aspect of the IFE reactor would be a scaled experimental test facility where material tests and methods can be advanced. This work is in support of efforts to develop an efficient, economical, and clean fusion energy source.

Conceptual Design for a 2 GW Inertial Fusion Energy Direct-Drive Power Reactor Employing a Mechanical Vacuum Pumping System. KELSEY TRESEMER (George Fox University, Newberg, Oregon, 97132) CHARLES GENTILE (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Presented is a conceptual design for a 2 gigawatt Inertial Fusion Energy (IFE) direct-drive power reactor. The reactor operates at 5 Hz, consuming approximately 450,000 tritium-deuterium targets/day, injected at speeds greater than 100 m/s into the target chamber and uniformly illuminated by laser light, leading to detonation. Resulting post-detonation ions are directed away from the first wall of the target chamber and into equatorial and polar caches using a magnetically-induced cusp field. The reactor is designed to breed and recycle fuel through the use of breeder blankets and a fuel recovery system. To minimize target-particle interference, the chamber will be kept at less than 0.5 millitorr through the use of turbomolecular pumps (TMPs) and corresponding mechanical backing pumps. Initially, these pumps were dry-bearing TMPs, however an investigation was performed comparing bearing-based TMP’s to magnetically-levitated TMPs, revealing other vacuum pump options. All pumps were evaluated based on a wide range of specifications, the most crucial being the maximum hydrogen pumping speed, greatest mean time between failure (MTBF), and the least amount of oil (if any) present in the vacuum system. Information collected from journal articles, industry, and operational TMP experience in other fusion related venues indicate that the employment of magnetically-levitated TMP’s appears to be a superior vacuum pumping solution in the IFE environment. Thus, as a direct result of this research, magnetically levitated TMPs will be adopted into the IFE reactor design.

Experimental Study of Effects due to Perturbations on Boundary Conditions to Couette Flows. FREDERICK MANLEY (University of Illinois, Champaign, IL, 61820) HANTAO JI (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

When fluid flows between two independently rotating cylinders at low aspect ratios (the ratio of the height to the difference in radii), the flow is seen to deviate substantially from ideal Couette flow due to Ekman circulation along the end caps. In the case where the end caps are attached to the outer cylinder, fluid with less angular momentum is advected into the bulk flow, which decreases the mean velocity as predicted by the ideal case. In order to study the stability of Ekman circulation, an experiment was devised to perturb the Ekman boundary layer by modifying the inner cylinder. Water flows between an aluminum inner cylinder and acrylic outer cylinder and its velocity is measured using a Laser Doppler Velocimeter (LDV) scanned radially from underneath to obtain 2-D velocity profiles. The robustness of the Ekman layer was studied against perturbations of varying magnitudes. Though perturbing the inner cylinder boundary did produce profiles closer to the ideal Couette case, the Ekman layer proved to be more robust than predicted. Both a 7mm offset and four o-rings placed on the inner cylinder were needed to produce profiles resembling the ideal Couette case. A new apparatus will be built with a larger aspect ratio to observe the effects of similar perturbations on the less stable Ekman flow. In the future, less viscous fluids may be used to determine the effects of larger Reynolds numbers on Ekman stability.

Extensions to DivGeo, a Graphical Tool for Editing 2D Edge Plasma Computational Meshes Extensions to DivGeo, a Graphical Tool for Editing 2D Edge Plasma Quasi-Orthogonal Computational Meshes. ALAN CHIN (Princeton University, Princeton, NJ, 8540) DAREN P. STOTLER (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Transport of plasma and neutral particles across magnetic flux surfaces in tokamak fusion experiments is a highly complex dynamical system of much practical interest in designing efficient fusion reactors. Codes that have been written to simulate the behavior of such systems include B2 and Eirene, used to model plasma and neutral transport behavior, respectively, in the divertors of ITER, and DEGAS 2, used to model neutral transport during Gas Puff Imaging experiments on the National Spherical Torus eXperiment (NSTX), both of which approximate the plasma region by 2D computational meshes that are designed to be quasi-orthogonal to the poloidal magnetic flux surfaces inside the tokamak. Because the distribution of mesh cells and the topology of the mesh are specific to each experiment, a customized mesh must be created for each study undertaken. DivGeo (DG) is a graphical user interface used, in combination with mesh-generating codes such as Carre and Sonnet, to create and modify such meshes. Using the C programming language and GNU utilities in a Red Hat Linux environment, the source code of DG was modified and subjected to testing by the author and users of DG at Princeton Plasma Physics Laboratory (PPPL) and ITER. After the modifications, DG was now able to be compiled using the freely available Open Motif 2.x graphics library, which allowed it to run reliably on the Linux machines at PPPL. In addition, several new features were added to DG, including an auto-save feature, the ability to recognize concave mesh cells and the segment of the reactor determining the outer bound of the mesh, and the ability to view the mesh at arbitrary angles and aspect ratios. Together, these improvements allow precisely tailored and general meshes to be generated more quickly and easily, accelerating the progress of computational studies on tokamak plasmas.

HHFW Propagation and Heating in NSTX. JEFFREY PARKER (Cornell University, Ithaca, NY, 14853) CYNTHIA PHILLIPS (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Recent experiments on the National Spherical Torus Experiment (NSTX), a fusion research device, show that the high harmonic fast wave (HHFW) core heating efficiency depends on the antenna phasing and plasma conditions. Power losses in the edge due to rf sheath formation or other parasitic absorption processes could occur if the waves propagate nearly parallel to the wall in the edge regions and intersect nearby vessel structures. To investigate this possibility, the 3D HHFW propagation in NSTX has been studied both analytically and numerically with the ray tracing code GENRAY. Initial calculations show that for certain values of the launched parallel wave number and magnetic field, the waves in NSTX are launched at a shallow angle to the vessel wall. In contrast, for ion cyclotron radio frequency (ICRF) heating in the Alcator C-Mod device at MIT or the not yet built ITER test reactor, the initial ray trajectories tend to be more radially oriented. Comparisons of the GENRAY results with 2D TORIC full wave simulations for the power deposition will also be discussed.

Improved Calculations of Particle Orbit Times in Tokamaks. ALEXANDER EGAN (University of Pennsylvania, Philadelphia, PA, 19104) JONATHAN MENARD (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Stabilizing the resistive wall mode is important to maximize the plasma pressure in tokamaks and spherical tori. Rotational stabilization of the RWM is predicted from kinetic damping theory to depend strongly on particle bounce and transit times. Previous analytic calculations of bounce and transit times have assumed high aspect ratio and circular flux surfaces. For the low aspect ratio and strongly shaped plasmas of the National Spherical Torus Experiment, recently developed calculations of the particle orbit times in general geometry find that the commonly used analytic approximation is inaccurate by as much as a factor of two. However, the analytic formula is convenient since it is based on a relatively simple elliptic integral function. General geometry extensions to the existing analytic theory are being pursued for RWM stability and other applications. It is expected that some short series of elliptic integral terms added to the current model will concisely capture the aforementioned deviation. This simple form would greatly reduce the computational overhead currently required for accurate bounce and transit time calculations. Applications of this result will include the enhancement of RWM modeling in the widely used MARS stability code.

Measurement of Copper Deposition Rate and Uniformity Utilizing Electron Cyclotron Resonance Plasma Sputtering Techniques. KELLY GREENLAND (Lock Haven University of Pennsylvania, Lock Haven, PA, 17745) DR. ANDREW ZWICKER (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

ECR (electron cyclotron resonance) plasma is used in processing such as circuit manufacturing and thin-film deposition due to its ability to produce more energetic, more dense, and more uniform plasma than other techniques. In this project, an ECR plasma, with argon as the base gas, was used to sputter copper on to silicon wafers at various pressures, powers, and geometries, and then analyzed using a scanning electron microscope to determine the thickness, uniformity and contamination of the copper layer. In addition, a spectroscopic method was developed to measure the electron temperature of the plasma by taking the intensity of certain spectral lines in the light emitted from the plasma. Typical plasma parameters were microwave power of 2500watts, a target bias of 125volts, and an argon pressure of 0.46mTorr. Measurements deduced that these conditions deposited 360 angstroms per minute of copper onto a three inch round wafer sample, and the plasma temperature was found to be approximately 7.88 eV. These results will aid in additional research, including replacing the copper target with a graphite target in order to apply ultra-hard thin films for high performance applications such as laser windows and heat resistant circuit boards.

Paving an Environmentally Friendly Road to Fusion. DOREEN NUZZOLESE (The College of New Jeresy, Ewing, NJ, 8628) CARL SZATHMARY (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Princeton Plasma Physics Laboratory (PPPL) strives to sustain fusion power as a reliable and environmentally safe energy source. First, with the Tokamak Fusion Test Reactor, then the National Spherical Torus Experiment, and now the National Compact Stellerator Experiment, PPPL comes closer and closer to achieving their goal. However, these experiments use harmful substances, such as deuterium and tritium, and it is imperative that these substances not get released into the surrounding air and water. It is the responsibility of PPPL’s Princeton Environmental Analytical Radiological Laboratory (PEARL) to protect the environment and ensure safety. To do so, radio-chemists in the PEARL perform tests on air and water samples taken from areas surrounding the lab. Before being released into the environment, wastewater is tested through either Chemical Oxygen Demand (COD), testing for organic matter, or alkaline distillation purification, followed by Liquid Scintillation Analysis, testing for tritium. Air samples are tested for tritium through the Differential Atmospheric Tritium Sampler System. All test results are fed into a fiscal year report and kept on file. If any one sample shows evidence of harmful substances, it is immediately removed from site and cured before disposal. Therefore, while advancing fusion, PPPL poses no health threat, but rather advocates safe scientific practices. In conclusion, PPPL is on its way to safely revolutionizing energy through fusion, an inexpensive, inexhaustible fuel that will be sure to have an immense global impact.

Ultraviolet Induced Motion of a Fluorescent Dust Cloud in an Argon Direct Current Glow Discharge Plasma. MICHAEL HVASTA (The College of New Jeresy, Ewing, NJ, 8628) ANDREW ZWICKER (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

Dusty plasmas consist of electrons, ions, neutrals and comparatively large particles (dust). In man-made plasmas this dust may represent impurities in a tokamak or in plasma processing. In astronomical plasmas this dust forms structures such as planetary rings and comet tails. To study dusty plasma dynamics an experiment was designed in which a silica (< 2mA) when the cloud is exposed to the UV (100 watts,  = 365 nm) the mixture fluoresces, moves ~2mm towards the light source and begins rotating in a clockwise manner (as seen from the cathode). By using a Charge-Coupled Device camera, dust clouds with diameters ranging from 6-10mm have been observed with particle rotational velocities in excess of 3 mm/s near their periphery. Particle velocities decrease towards the center of the cloud. By calibrating a UV lamp and adjusting the relative intensity of the UV with a variable transformer it was found that both translational and rotational velocities are a function of UV intensity. Additionally, it was determined that bulk cloud rotation is not seen when the dust tray is electrically floated while bulk translation is. This ongoing experiment represents a novel way to control and localize contamination efficiently in man-made plasmas as well as a pathway to better understanding UV-bathed plasma systems in space.

Validating the computer simulation of the effects of secondary neutrals on the motional Stark effect diagnostic gas-filled-torus calibration. WILLIAM SCHUMAKER (Lawrence Technological University, Southfield, MI, 48075) HOWARD YUH (Princeton Plasma Physics Laboratory, Princeton, NJ, 8543)

The motional Stark effect (MSE) diagnostic, an important method of determining the magnetic field pitch angle in tokamak plasmas, measures the polarization angle of light emitted from injected neutral atoms that are affected by Stark splitting due to the Lorentz electric field. A common procedure of injecting a neutral beam into a gas-filled-torus with known magnetic fields in vacuum is one technique used to calibrate MSE diagnostics on many tokamak devices. The usefulness of this calibration has been limited on many installations due to anomalies in the measured pitch angles. Recently, this anomaly was explained as a consequence of beam neutrals that ionize after collisions, travel along the magnetic field lines, re-neutralize via a charge exchange, and rapidly produce emission spectra. Under certain conditions, these secondary neutrals emit hydrogen-alpha spectra that have the proper Doppler shift to pass through the MSE optical filters yet have a different polarization angle than those from the primary beam neutrals, thus contaminating the pitch angle measurement. In an effort to study these contaminations, computer code had previously been written to simulate the gas-filled-torus calibration of the MSE diagnostic on the National Spherical Torus Experiment (NSTX). To characterize the effects of secondary neutrals on the MSE gas-filled-torus calibration technique, new programming modules were written to extend the code to simulate other tokamak geometries and neutral beams. Several consistency checks involving numerical integration were meticulously performed on the modules, ensuring that they had been implemented correctly. Using these new modules, a sensitivity study involving various gas pressures, beam injection angles, magnetic field pitch angles, and system resolutions is in the process of validating the code against respective experimental data. If successful, this validation will help resolve a significant calibration issue with a major diagnostic used in current tokamak fusion research.