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Measuring the coherence properties of light emission from laser-plasma interactions. Final report

Description: Several detrimental instabilities can be excited when a high-intensity laser interacts with plasma. The temporal evolution and spectra of the scattered light emitted by many of these instabilities are used to characterize the instabilities and to benchmark theories. It has been difficult to image the emission region with sufficient resolution to make quantitative comparisons with theory. Direct measurement of the emission region would yield information on ponderomotive steepening phenomena, the true emission zone of convective instabilities, and on the saturation of absolute instabilities. The increase in laser intensity caused by the filamentation instability is conjectured to elevate the levels of parametric instabilities found in high-energy laser-plasma interactions. Because the diameter of the filaments is very small (on the order of 10 {micro}m), it is impossible to image the emission sites directly and either to prove or to disprove this conjecture. The research reported here examines an alternate method of measuring the emission region of scattered light from parametric instabilities. This report provides a brief background of coherence theory by defining the relevant parameters in Section 2. A concrete example of the effect that multiple scattering sites would have on the proposed measurement is provided in Section 3. The following section briefly describes experiments that might be able to demonstrate the proposed technique. The conclusion raises the issue of coherence and its effect on the expected angular distribution of scattering light from parametric instabilities.
Date: March 6, 1998
Creator: Batha, S.H.
Partner: UNT Libraries Government Documents Department

The roles of electric field shear and Shafranov shift in sustaining high confinement in enhanced reversed shear plasmas on the TFTR tokamak

Description: The relaxation of core transport barriers in TFTR Enhanced Reversed Shear plasmas has been studied by varying the radial electric field using different applied torques from neutral beam injection. Transport rates and fluctuations remain low over a wide range of radial electric field shear, but increase when the local E x B shearing rates are driven below a threshold comparable to the fastest linear growth rates of the dominant instabilities. Shafranov-shift-induced stabilization alone is not able to sustain enhanced confinement.
Date: February 1, 1997
Creator: Synakowski, E.J.; Beer, M.A. & Batha, S.H.
Partner: UNT Libraries Government Documents Department

Local transport barrier formation and relaxation in reverse-shear plasmas on the TFTR tokamak

Description: The roles of turbulence stabilization by sheared E x B flow and Shafranov-shift gradients are examined for TFTR. Enhanced Reverse-Shear plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of E x B and Shafranov shift effects was performed by varying the E x B shear through changes in the toroidal velocity with nearly-steady-state pressure profiles. Transport and fluctuation levels increase only when E x B shearing rates are driven below a critical value that is comparable to the fastest linear growth rates of the dominant instabilities. While a turbulence suppression criterion that involves the ratio of shearing to linear growth rates is in accord with many of these results, the existence of hidden dependencies of the criterion is suggested in experiments where the toroidal field was varied. The forward transition into the ERS regime has also been examined in strongly rotating plasmas. The power threshold is higher with unidirectional injection than with balanced injection.
Date: February 1, 1997
Creator: Synakowski, E.J.; Beer, M.A. & Batha, S.H.
Partner: UNT Libraries Government Documents Department

Supershot performance with reverse magnetic shear in TFTR

Description: Discharges with large regions of reversed magnetic shear and good energy and particle confinement have been produced in the Tokamak Fusion Test Reactor. These plasmas were created by heating the plasma during a rapid plasma current increase. The stability of these discharges is dependent on the shape of the q profile, in particular the value and location of the minimum value of q. Control of the q profile by optimizing the plasma startup, prelude start time, the neutral-beam directionality during the prelude heating phase, and the plasma current ramp rate is demonstrated. High-performance discharges, created by injecting more than 18 to 25 MW of neutral beam power into a plasma with reverse shear, are also described.
Date: August 1, 1995
Creator: Batha, S.H.; Levinton, F.M.; Zarnstorff, M.C. & Schmidt, G.L.
Partner: UNT Libraries Government Documents Department

Confinement and the safety factor profile

Description: The conjecture that the safety factor profile, q(r), controls the improvement in tokamak plasmas from poor confinement in the Low (L-) mode regime to improved confinement in the supershot regime has been tested in two experiments on the Tokamak Fusion Test Reactor (TFTR). First, helium was puffed into the beam-heated phase of a supershot discharge which induced a degradation from supershot to L-mode confinement in about 100 msec, far less than the current relaxation time. The q and shear profiles measured by a motional Stark effect polarimeter showed little change during the confinement degradation. Second, rapid current ramps in supershot plasmas altered the q profile, but were observed not to change significantly the energy confinement. Thus, enhanced confinement in supershot plasmas is not due to a particular q profile which has enhanced stability or transport properties. The discharges making a continuous transition between supershot and L-mode confinement were also used to test the critical-electron-temperature-gradient transport model. It was found that this model could not reproduce the large changes in electron and ion temperature caused by the change in confinement.
Date: December 1, 1995
Creator: Batha, S.H.; Levinton, F.M. & Scott, S.D.
Partner: UNT Libraries Government Documents Department

Current profile modification during lower hybrid current drive in the Princeton Beta Experiment-Modification

Description: Current profile modification with lower hybrid waves has been demonstrated in the Princeton Beta Experiment-Modification tokamak. When the n{parallel} spectrum of the launched waves was varied, local changes in the current profile were observed according to equilibria reconstructed from motional Stark effect polarimetry measurements. Changes in the central safety factor (q) were also determined to be a function of the applied radio frequency (rf) power. These results have been modeled with the Tokamak Simulation Code/Lower Hybrid Simulation Code, which is able to duplicate the general trends seen in the data.
Date: February 1, 1996
Creator: Kaita, R.; Bell, R. & Batha, S.H.
Partner: UNT Libraries Government Documents Department

Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part I. Measurements of Magnetic Islands and Delta Prime

Description: Tearing type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from electron cyclotron emission (ECE) diagnostic and an experiment where the plasma major radius is rapidly shifted (`Jog` experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing mode eigenfunction mapped from the Jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.
Date: November 3, 1997
Creator: Chang, Z.; Fredrickson, E.D. & Batha, S.H.
Partner: UNT Libraries Government Documents Department

Experimental study of toroidicity-induced Alfven eigenmode (TAE) stability at high q(0)

Description: Experiments to destabilize the Toroidicity-induced Alfven Eigenmode (TAE) by energetic alpha particles were performed on the Tokamak Fusion Test Reactor using deuterium and tritium fuel. To decrease the alpha particle pressure instability threshold, discharges with an elevated value of q(0) > 1.5 were used. By raising q(0), the radial location of the low toroidal-mode-number TAE gaps moves toward the magnetic axis and into alignment with the region of maximum alpha pressure gradient, thereby (in theory) lowering the value of {beta}{sub {alpha}}(0) required for instability. No TAE activity was observed when the central alpha particle {beta}{sub {alpha}} reached 0.08% in a discharge with fusion power of 2.4 MW. Calculations show that the fusion power is within a factor of 1.5 to 3 of the instability threshold.
Date: July 1, 1995
Creator: Batha, S.H.; Levinton, F.M. & Spong, D.A.
Partner: UNT Libraries Government Documents Department

First evidence of collective alpha particle effect on TAE modes in the TFTR D-T experiment

Description: The alpha particle effect on the excitation of toroidal Alfven eigenmodes (TAE) was investigated in deuterium-tritium (d-t) plasmas in the Tokamak Fusion Test Reactor (TFTR). RF power was used to position the plasma near the instability threshold, and the alpha particle effect was inferred from the reduction of RF power threshold for TAE instability in d-t plasmas. Initial calculations indicate that the alpha particles contribute 10--30% of the total drive in a d-t plasma with 3 MW of peak fusion power.
Date: August 1, 1995
Creator: Wong, K.L.; Schmidt, G. & Batha, S.H.
Partner: UNT Libraries Government Documents Department

Improved confinement with reversed magnetic shear in TFTR

Description: Highly peaked density and pressure profiles in a new operating regime have been observed on the Tokamak Fusion Test Reactor (TFTR). The q-profile has a region of reversed magnetic shear extending from the magnetic axis to r/a {approximately}0.3-0.4. The central electron density rises from 0.45 x 10{sup 20} m{sup {minus}3} to nearly 1.2 x 10{sup 20} m{sup {minus}3} during neutral beam injection. The electron particle diffusivity drops precipitously in the plasma core with the onset of the improved confinement mode and can be reduced by a factor of {approximately}50 to near the neoclassical particle diffusivity level.
Date: July 1, 1995
Creator: Levinton, F.M.; Batha, S.H. & Zarnstorff, M.C.
Partner: UNT Libraries Government Documents Department

Alpha particle loss in TFTR deuterium-tritium plasmas with reversed magnetic shear

Description: The confinement and loss of fusion alpha particles are examined for reversed magnetic shear plasmas in TFTR. Such plasmas, with high central q and non-monotonic q profiles can exhibit remarkably reduced energy and particle transport of the thermal ions. However, these same conditions are theoretically predicted to produce high levels of stochastic ripple loss of suprathermal particles, which may reduce the efficiency of plasma heating by the alpha particles and other heating schemes involving fast ions. This paper presents calculations of guiding-center code alpha particle orbit loss from deuterium-tritium (DT) simulations of TFTR deuterium-only experiments. They are compared to results of measurements made in DT reversed shear plasmas of both the confined alpha particle distribution and the alpha particles lost from the plasma. Large fast particle losses have also been found in reversed shear ITER simulations (up to 20%) and from measurements of triton burnup in reversed shear experiments on JT-60U (12%).
Date: June 1, 1997
Creator: Redi, M.H.; Batha, S.H. & Budny, R.V.
Partner: UNT Libraries Government Documents Department

High performance deuterium-tritium plasmas in TFTR

Description: Plasmas composed of nominally equal concentrations of deuterium and tritium (DT) have been created in TFTR with the goals of producing significant levels of fusion power and of examining the effects of DT fusion alpha particles. Conditioning of the limiter by the injection of lithium pellets has led to an approximate doubling of the energy confinement time, {tau}{sub E}, in supershot plasmas at high plasma current (I{sub p} {le} 2.5 MA) and high heating power (P{sub b} {le} 33 MW). Operation with DT typically results in an additional 20% increase in {tau}{sub E}. In the high poloidal beta, advanced tokamak regime in TFTR, confinement enhancement H {triple_bond} {tau}{sub E}/{tau}{sub E ITER-89P} > 4 has been obtained in a limiter H-mode configuration at moderate plasma current I{sub p} = 0.85 {minus} 1.5 MA. By peaking the plasma current profile, {beta}{sub N dia} {triple_bond} 10{sup 8} < {beta}{sub t{perpendicular}} > aB{sub 0}/I{sub p} = 3 has been obtained in these plasmas, exceeding the {beta}{sub N} limit for TFTR plasmas with lower internal inductance, l{sub i}. Confinement of alpha particles appears to be classical and losses due to collective effects have not been observed. While small fluctuations in fusion product loss were observed during ELMs, no large loss was detected in DT plasmas.
Date: March 1, 1995
Creator: Sabbagh, S.A.; Batha, S.H. & Bell, M.G.
Partner: UNT Libraries Government Documents Department

The effect of E{sub r} on MSE measurements of q, a new technique for measuring E{sub r}, and a test of the neoclassical electric field

Description: Previous analysis of motional-Stark Effect (MSE) data to measure the q-profile ignored contributions from the plasma electric field. The MSE measurements are shown to be sensitive to the electric field and require significant corrections for plasmas with large rotation velocities or pressure gradients. MSE measurements from rotating plasmas on the Tokamak Fusion Test Reactor (TFTR) confirm the significance of these corrections and verify their magnitude. Several attractive configurations are considered for future MSE-based diagnostics for measuring the plasma radial electric field. MSE data from TFTR is analyzed to determine the change in the radial electric field between two plasmas. The measured electric field quantitatively agrees with the predictions of neoclassical theory. These results confirm the utility of a MSE electric field measurement.
Date: October 1, 1996
Creator: Zarnstorff, M.C.; Synakowski, E.J.; Levinton, F.M. & Batha, S.H.
Partner: UNT Libraries Government Documents Department

Local tests of parallel electrical resistivity in the Tokamak Fusion Test Reactor

Description: The motional Stark effect (MSE) polarimeter measures the local magnetic field pitch angle, proportional to the ratio of the poloidal to toroidal magnetic fields, in the Tokamak Fusion Test Reactor (TFTR). The authors have used the polarimeter to measure the temporal evolution of the local value of the magnetic field pitch angle during large changes in the current profile such as during a current ramp or discharge initiation. The measured evolution is compared to the evolution predicted by classical and neoclassical resistivity models. The neoclassical resistivity model is a better predictor of the local pitch angle temporal evolution than the classical model.
Date: January 1, 1997
Creator: Batha, S.H.; Levinton, F.M.; Ramsey, A.T.; Schmidt, G.L. & Zarnstorff, M.C.
Partner: UNT Libraries Government Documents Department

Turbulent fluctuations in the main core of TFTR plasmas with negative magnetic shear

Description: Turbulent fluctuations in plasmas with reversed magnetic shear have been investigated in TFTR. Under intense auxiliary heating, these plasmas are observed to bifurcate into two states with different transport properties. In the state with better confinement, it has been found that the level of fluctuations is very small throughout most of the region with negative shear. By contrast, the state with lower confinement is characterized by large bursts of fluctuations which suggest a competition between the driving and the suppression of turbulence. These results are consistent with the suppression of turbulence by the E x B velocity shear.
Date: September 1997
Creator: Mazzucato, E.; Beer, M. A.; Bell, M. G. & Batha, S. H.
Partner: UNT Libraries Government Documents Department

Calculations of alpha particle loss for reversed magnetic shear in the Tokamak Fusion Test Reactor

Description: Hamiltonian coordinate, guiding center code calculations of the toroidal field ripple loss of alpha particles from a reversed shear plasma predict both total alpha losses and ripple diffusion losses to be greater than those from a comparable non-reversed magnetic shear plasma in the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. High central q is found to increase alpha ripple losses as well as first orbit losses of alphas in the reversed shear simulations. A simple ripple loss model, benchmarked against the guiding center code, is found to work satisfactorily in transport analysis modelling of reversed and monotonic shear scenarios. Alpha ripple transport on TFTR affects ions within r/a=0.5, not at the plasma edge. The entire plasma is above threshold for stochastic ripple loss of alpha particles at birth energy in the reversed shear case simulated, so that all trapped 3.5 MeV alphas are lost stochastically or through prompt losses. The 40% alpha particle loss predictions for TFTR suggest that reduction of toroidal field ripple will be a critical issue in the design of a reversed shear fusion reactor.
Date: March 1, 1997
Creator: Redi, M.H.; White, R.B.; Batha, S.H.; Levinton, F.M. & McCune, D.C.
Partner: UNT Libraries Government Documents Department

Neoclassical Simulations of Fusion Alpha Particles in Pellet Charge Exchange Experiments on the Tokamak Fusion Test Reactor

Description: Neoclassical simulations of alpha particle density profiles in high fusion power plasmas on the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 5 (1998) 1577] are found to be in good agreement with measurements of the alpha distribution function made with a sensitive active neutral particle diagnostic. The calculations are carried out in Hamiltonian magnetic coordinates with a fast, particle-following Monte Carlo code which includes the neoclassical transport processes, a recent first-principles model for stochastic ripple loss and collisional effects. New global loss and confinement domain calculations allow an estimate of the actual alpha particle densities measured with the pellet charge exchange diagnostic.
Date: February 1, 1999
Creator: Batha, S.H.; Budny, R.V.; Darrow, D.S.; Levinton, F.M.; Redi, M.H. & al, et
Partner: UNT Libraries Government Documents Department

Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part 1. Measurements of Magnetic Islands and Delta Prime.

Description: Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.
Date: November 1, 1997
Creator: Fredrickson, E.D.; Bell, M.B.; Budny, R.V.; Batha, S.H.; Chang, Z. & al, et
Partner: UNT Libraries Government Documents Department

Core Poloidal Rotation and Internal Trnasport Barrier Formation in TFTR

Description: Impurity poloidal rotation velocities have been measured in the core of TFTR plasmas using a new spectroscopic diagnostic. Two types of transitions to enhanced confinement in reversed shear plasmas are examined. A bifurcation in carbon poloidal rotation is observed to occur before the transition to enhanced confinement for one of these types, while other measured plasmas parameters remain constant. A narrow radial region with reversed poloidal rotation and rotational shear is established 60-100 ms before the transition, and is associated with a large negative radial electric field.
Date: January 1, 1998
Creator: Synakowski, E.J.; Levinton, F.M.; Zarnstorff, M.C.; Bell, R.E.; Batha, S.H. & al, et
Partner: UNT Libraries Government Documents Department

Toroidal Alfvén Eigenmodes in TFTR Deuterium-Tritium Plasmas

Description: Purely alpha-particle-driven Toroidal Alfvén Eigenmodes (TAEs) with toroidal mode numbers n=1-6 have been observed in Deuterium-Tritium (D-T) plasmas on the Tokamak Fusion Test Reactor [D.J. Grove and D.M. Meade, Nucl. Fusion 25, 1167 (1985)]. The appearance of mode activity following termination of neutral beam injection in plasmas with q(0)>1 is generally consistent with theoretical predictions of TAE stability [G.Y. Fu et al., Phys. Plasmas 3, 4036 (1996]. Internal reflectometer measurements of TAE activity is compared with theoretical calculations of the radial mode structure. Core localization of the modes to the region of reduced central magnetic shear is confirmed, however the mode structure can deviate significantly from theoretical estimates. The peak measured TAE amplitude of delta n/n~10(superscript -4) at r/a~0.3-0.4 corresponds to delta B/B~10-5, while dB/B~10(superscript -8) is measured at the plasma edge. Enhanced alpha particle loss associated with TAE activity has not been observed.
Date: January 1, 1998
Creator: Fu, G.Y.; Berk, H.; Nazikian, R.; Batha, S.H.; Chang, Z. & al, et
Partner: UNT Libraries Government Documents Department

RAGE simulations of single-mode Richtmyer-Meshkov growth in a convergent geometry

Description: The Richtmyer-Meshkov (RM) instability is initiated by a shock accelerating an interface between two materials. Small perturbations of the interface grow into bubble and spike structures causing mixing of the materials that lie on either side of the interface. Recent Los Alamos National Laboratory experiments have focused on RM initiated mix in a compressible, miscible, convergent geometry. Motivated by the lack of a generally accepted model for this physical regime, cylindrical implosion experiments of single-mode, nonlinear RM growth and saturation are undeway at the OMEGA laser facility. Initial targets consist of an m=28 perturbation with an initial amplitude of 2.5 microns machined onto an aluminum marker layer embedded 55 {micro}m from the target surface. Initial perturbations of varying amplitudes and wavelengths are being studied using the RAGE code.
Date: January 1, 2002
Creator: Scott, J. M. (John M.); Balkey, M. M. (Matthew M.); Batha, S. H. (Steven H.); Barnes, C. W. (Christopher, W.); Holmes, R. L. (Richard L.); Lanier, N. E. (Nicholas E.) et al.
Partner: UNT Libraries Government Documents Department

Transport Physics in Reversed Shear Plasmas

Description: Reversed magnetic shear is considered a good candidate for improving the tokamak concept because it has the potential to stabilize MHD instabilities and reduce particle and energy transport. With reduced transport the high pressure gradient would generate a strong off-axis bootstrap current and could sustain a hollow current density profile. Such a combination of favorable conditions could lead to an attractive steady-state tokamak configuration. Indeed, a new tokamak confinement regime with reversed magnetic shear has been observed on the Tokamak Fusion Test Reactor (TFTR) where the particle, momentum, and ion thermal diffusivities drop precipitously, by over an order of magnitude. The particle diffusivity drops to the neoclassical level and the ion thermal diffusivity drops to much less than the neoclassical value in the region with reversed shear. This enhanced reversed shear (ERS) confinement mode is characterized by an abrupt transition with a large rate of rise of the density in the reversed shear region during neutral beam injection, resulting in nearly a factor of three increase in the central density to 1.2 X 10(exp 20) cube m. At the same time the density fluctuation level in the reversed shear region dramatically decreases. The ion and electron temperatures, which are about 20 keV and 7 keV respectively, change little during the ERS mode. The transport and transition into and out of the ERS mode have been studied on TFTR with plasma currents in the range 0.9-2.2 MA, with a toroidal magnetic field of 2.7-4.6 T, and the radius of the q(r) minimum, q{sub min}, has been varied from r/a = 0.35 to 0.55. Toroidal field and co/counter neutral beam injection toroidal rotation variations have been used to elucidate the underlying physics of the transition mechanism and power threshold of the ERS mode.
Date: December 31, 1997
Creator: Levinton, F.M.; Batha, S.H.; Beer, M.A.; Bell, M.G.; Budny, R.V.; Efthimion, P.C. et al.
Partner: UNT Libraries Government Documents Department

RECENT RESULTS OF RADIATION HYDRODYNAMICS AND TURBULENCE EXPERIMENTS IN CYLINDRICAL GEOMETRY.

Description: Cylindrical implosion experiments at the University of Rochester laser facility, OMEGA, were performed to study radiation hydrodynamics and compressible turbulence in convergent geometry. Laser beams were used to directly drive a cylinder with either a gold (AU) or dichloropolystyrene (C6H8CL2) marker layer placed between a solid CH ablator and a foam cushion. When the cylinder is imploded the Richtmyer-Meshkov instability and convergence cause the marker layer to increase in thickness. Marker thickness measurements were made by x-ray backlighting along the cylinder axis. Experimental results of the effect of surface roughness will be presented. Computational results with an AMR code are in good agreement with the experimental results from targets with the roughest surface. Computational results suggest that marker layer 'end effects' and bowing increase the effective thickness of the marker layer at lower levels of roughness.
Date: January 1, 2001
Creator: R.), Magelssen G. R. (Glenn; Scott, J. M. (John M.); Batha, S. H. (Steven H.); Holmes, R. L. (Richard L.); Lanier, N. E. (Nicholas E.); Tubbs, D. L. (David L.) et al.
Partner: UNT Libraries Government Documents Department

Convergent, compressible Richtmyer-Meshkov experiment-zero order hydrodynamics

Description: Cylindrical experiments were performed on the OMEGA laser at the University of Rochester to study unstable interfaces in single and double shells. For single shells a marker layer of more opaque and higher density material is placed between foam and an outside ablator. The marker is either smooth or with a well defined surface roughness. For double shells an inner cylinder is placed along the outer cylinder axis. The outer cylinder is irradiated directly with 50 laser beams which produces a strong shock (mach number 5-15) that passes through the unstable marker interf'ace creating a Richtmyer-Meshkov (RM) instability. For double shells this shock bounces off the inner cylinder back to the incoming marker layer causing it to decelerate. We present comparisons of the measured smooth marker layer hydrodynamics with computer simulations using both Lagrangian and Eulerian codes.
Date: January 1, 2002
Creator: R.), Magelssen G. R. (Glenn; Scott, J. M. (John M.); Lanier, N. E. (Nicholas E.); Batha, S. H. (Steven H.); Balkey, M. M. (Matthew M.); Barnes, C. W. (Christopher, W.) et al.
Partner: UNT Libraries Government Documents Department