Browsing by Subject "Plasma (Ionized gases)"

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ECE radiation analysis of the Hall thruster
(2007-12) Kim, Minkyu, 1970-; Hallock, G. A.
In this study, we developed three computational techniques for the ECE radiation analysis of the Hall thruster. The first one is the single particle approximation analysis. This is the simplest one among the approaches. We modeled the plasma region of the Hall thruster with three parameters, the magnetic field, electron temperature, and electron density distributions. These parameters are constant in a cell. We calculated the radiation with the parameter distributions according to the observation angle. The frequency of a cell is determined by the magnetic field of the cell. This analysis is easy to approach and does not require a high computing performance. However, the results of this analysis don’t have detail results. The radiated electric field is derived from the power, so there is no polarization information on the electric field. We moved on more sophisticated analysis. The next one is the Particle-In-Cell (PIC) analysis. PIC is for analysis of microscopic phenomena. Particle motions in the thruster channel region is simulated with the PIC method. We selected electrons from the Maxwell-Boltzmann distribution for the speed of electrons. The Monte Carlo method was adopted in this selection. We solved the Lorentz force equation to get the motion data of the electrons and analyzed the radiated electric field with the particle motions. Then, we took the Fourier transform of the electric field to consider the radiation in the frequency domain. This approach is from definition, the radiation is from charge acceleration. It is more realistic approach to the plasma. It uses same parameter distributions, but the parameters in a cell is not constant any more because of adopting the Monte Carlo method. It also shows the polarization information of the radiation. However, we assume in this analysis that the radiation is in free space. The channel plasma is considered as current sources for radiation. The material constants of the plasma is concerned as free space. The last approach adopted is to consider the non free space and inhomogeneous media. The hybrid FEM/MoM (hybrid element method) was suggested to exploit advantages of finite element method (FEM) and method of moment (MoM), the representative methods for the radiation analysis, and to compensate their disadvantages. The hybrid element method was introduced to analyze the ECE radiation by using EMAP5. In this analysis, the plasma was considered as dielectrics, and the source currents were from the plasma parameters.
Experimental studies of a helicon plasma
(2008-08) Lee, Charles Anton; Bengtson, Roger D.; Hallock, G. A.; Pitchford, L. C. (Leanne C.)
The goal of this dissertation is to provide experimental insight into the mechanism behind the efficient power absorption of helicon plasmas. This work presents evidence which is consistent with the Radially Localized Helicon (RLH) theory put forth by Breizman and Arefiev. Helicon discharges produce peaked density profiles with radial density gradients creating a potential well that sets up the RLH waves, which we believe is the major power absorption mechanism in the plasma. The experimental data presented in this dissertation must be taken in totality along with parallel efforts in theory and computer simulation. We show photographic evidence along with Langmuir probe measurements of the axial density that shows an asymmetric, right-hand circularly polarized wave being launched in a direction consistent with RLH theory. Additionally, we are able to show, through Langmuir probe measurements, that significant radial density gradients exist in the plasma which is required by the RLH dispersion but contrary to the uniform density assumption of current theory. Furthermore, using the two-dimensional density profile obtained from experiment, we are able to use that data as input into a model which confirms key features of the power absorption in terms of location and magnitude. The time-varying magnetic field is measured and analyzed against the RLH dispersion relation. Using a Fourier decomposition technique, the analysis indicates the proper scaling of the wavenumbers with the RLH dispersion. Finally, using the experimental density as input to a computer model, simulations show very good agreement with the amplitude and phase of the experimentally measured RF magnetic fields.
Impurity transport studies on Alcator C-Mod tokamak using charge exchange recombination spectroscopy
(2008-08) Bespamyatnov, Igor Olegovich, 1978-; Rowan, William L.; Gentle, Kenneth W.
A Charge-Exchange Recombination Spectroscopy (CXRS) diagnostic has been installed on Alcator C-Mod to study the transport of light impurities in plasma. The system provides spatially (1 cm) and temporally (12.5 msec) resolved measurements of the impurity density, temperature and flow velocities of the particular impurity. Two optical arrays: poloidal (19 channels) and toroidal (10 channels), collect the light emitted from excited impurity ion populated by charge exchange process from the Diagnostic Neutral Beam (DNB) particle. The attention of this dissertation is focused on the B⁴⁺ (n = 7 [-->] 6) spectral line emitted by B⁴⁺ ion formed in the following charge exchange reaction (H⁰ + B⁵⁺ [-->] H+ + B⁴⁺*). A complex spectral model was developed to simulate emission. The high magnetic fields of C-Mod result in broad Zeeman patterns which must be taken into account for the interpretation of the line shift and broadening in terms of impurity ion velocity and temperature. After the spectral line fitting and careful identification of the charge exchange component, the calculated Doppler broadening and shifts of the spectral line profile yield information on the ion temperature and rotation. Together with the calculation of the beam density, the absolute calibration of the CXRS optical system provides us with B⁵⁺ density measurement capabilities. One of the main objectives of this work was to use the acquired impurity density, temperature and flow velocity profiles to investigate plasma transport behavior and infer the radial electric field E[subscript R] from plasma force balance equation. The focus here was placed on the region of the Internal Transport Barrier (ITB) formation 0.35 < p < 0.8. Radial electric field E[subscript R] is readily calculated in the region of the ITB foot using measured B⁵⁺ profiles. ExB velocity shearing turbulence stabilization are believed to play an important role in the physics of the ITB formation. The computed E[subscript R] profiles demonstrated the large difference between the H-mode and ITB discharges. Linear gyrokinetic stability analysis (GS2) demonstrated that shearing rate w[subscript ExB] prevails over the linear Ion Temperature Gradient (ITG) growth rates [gamma subscript max] in the region where ITB forms.
Rotating mirror plasmas in the quest of magnetofluid states
(2006) Quevedo, Hernan Javier; Bengtson, Roger D.; Mahajan, Swadesh M.
The goal of this dissertation is to describe and discuss the first steps taken by the Magneto Bernoulli eXperiment (MBX) to create magnetofluid states in the laboratory using a rotating plasma in an external mirror magnetic field. The terminology magnetofluid has been introduced to characterize a plasma model, based on 2-fluid theory, that treats the flow and the magnetic field in a symmetrical way. Many interesting astrophysical and laboratory problems involve large flows and fall in this category. Based on the set of parameters where MBX should run, we set up the experiment, and added different probes to diagnose the rotating plasma. We have also installed a data acquisition system, and set up an archive system (to store the data) that can be accessed worldwide. Experimental results demonstrate that supersonic flows can be generated with biasing electrodes at the throat of the mirror magnetic field. Alfvenic flows needed for a transition to magnetofluid states could not be reached because the initial plasma density was too low. At low bias (slow rotational speed) the plasma has E × B/B2 drift rotation and the magnetic fields lines are equipotentials. With a higher bias, we observed large potential drops along the field lines. We also observed an asymmetry in the polarity of the bias which leads to constraints in the control of the sheared plasma flow. We present a model that captures many of these features. In conjunction with experimental efforts we develop a theory for a rotating plasma embedded in an external mirror magnetic field. An analytic solution that involves rigid rotation of the plasma shows important differences between a 2-fluid system and ideal MHD. We find high non equipotential magnetic lines and asymmetry to compare with the experimental results.
Self-consistent dynamics of nonlinear phase space structures
(2004) Eremin, Denis; Berk, H. L.
This thesis investigates the self-consistent dynamics of nonlinear ”hole” and ”clump” phase space structures and the nonlinear modes supported by the structures in the presence of dissipation due to the background plasma. A system consisting of a single mode driven by a weakly destabilizing distribution function in a dissipative medium close to the threshold of linear instability exhibits explosive instability. This instability results in the formation of the phase space structures and the corresponding modes. The holes and clumps were expected to persist for an appropriate collisional time scale. However, for certain initial conditions Fokker-Planck calculations for the nonlinear system abruptly break down in the course of the calculation. We find that this is because an adiabatic description of phase space structures at zero collisionality does not necessarily lead to continual adiabatic frequency sweeping. For a class of initial distribution functions criteria are found that detervii mine whether adiabatic frequency sweeping will continue indefinitely or suddenly terminate. For certain other initial distribution functions that describe the predominantly deeply trapped particles, critical points may be encountered where the adiabatic analysis fails to yield a unique solution. Except for establishing boundary conditions, the contribution of passing particles is found to be unimportant in the dynamics of the phase space structures within the framework of the adiabatic description. We derived a self-consistent dispersion relation for the perturbed eigenmodes of the system and benchmarked the result with the dispersion relation obtained earlier in Ref. [53] and demonstrated their agreement. We analyzed this dispersion relation and demonstrated that the critical points of the adiabatic theory occur exactly where linear instability is triggered. Numerical runs were performed to test both the adiabatic theory and the instability analysis of a BGK (Bernstein-Greene-Kruskal) mode for a model problem where the distribution function of passing particles has zero slope with respect to the action variable. This problem has same essential features as the problem where the slope of the passing particle distribution function is constant. In particular, linear instability of the same nature is also predicted to arise whenever adiabatic analysis predicts termination of frequency sweeping. This procedure has the virtue of enabling a precise comparison of the theory with the simulations and indeed it does so until instability sets in. The model problem was also used to demonstrate the agreement between the numerical growth rate and the growth rate predicted in the instability analysis. Then a passing particle distribution function was used that has a constant slope with respect to the action variable, and it too showed agreement with the theory for the evolution of the adiabatic phase, for where the onset of the instability was predicted to occur, and for where the persistence of the phase space structures after the instability relaxation. Both cases showed that after the instability dies away, smaller phase space structures still persist and the frequency sweeping continues at a slower rate. The numerical simulations demonstrated the additional effect that several generations of the nonlinear phase space structures are often produced. The numerical data shows that the mode amplitude is reduced when there are neighboring modes. We considered two possible mechanisms that may account for such reduction of the primary mode amplitude. In one mechanism, the particle orbits remain regular and the mode amplitude reduction is caused by the accumulation (in the case of a clump) [or depletion in the case of a hole] of the passing particle distribution of one of the modes, because it is part of the trapped particle region of the other mode. The other mechanism is the chaotic erosion of trapped particles near the separatrix.
A self-consistent model of helicon discharge
(2008-08) Chen, Guangye, 1976-; Raja, Laxminarayan L.; Breizman, Boris N.
We developed a self-consistent model of helicon discharges, motivated by a number of applications. One example is a plasma-based space propulsion system that employs a helicon discharge as its plasma source. Our study of helicon discharges involves two steps. An electro-magnetic wave solver is first developed to study wave phenomena and power deposition. In this work, we model a resonant response of the discharge observed in a recent experiment. The radially localized helicon (RLH) wave is identified as the primary mechanism of rf-power deposition into the plasma. The second step is to take into account electron heat transfer and ion transport so that a self-consistent simulation can be performed. As a case study of validating the model, we simulated one of Boswell’s early experiment in which a jump of plasma density in a scan of external magnetic field is observed. Calculation shows that a classical heat transport is unable to sustain the plasma density profile observed in the experiment. Solutions comparable to the experiment are obtained only when extra heat conductivity is used. The density profiles and excited wave-lengths are in good agreement with the experiment. Especially, the dual-stable solution of the simulation supports the observed plasma density jump.