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An estimate of collisional beam scattering during final focus in NDCX-II

Description: The final focus of NDCX-II contains a region with quite high plasma density. We estimate here how much collisional scatter we expect from transit through this plasma. A separate question, not explored here, is how much scatter there might be off of collective fluctuations in the neutralizing plasma, including those driven by the passage of the beam.
Date: March 23, 2010
Creator: Cohen, R.H.
Partner: UNT Libraries Government Documents Department

Improvement of electron beam quality in optical injection schemesusing negative plasma density gradients

Description: Enhanced electron trapping using plasma density down ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector was examined using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over distances greater than the plasma wavelength led to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength, e.g., {approx}5 fs), high charge (>20 pC at modest injection laser intensity 10{sup 17} W/cm{sup 2}), with a relative energy spread of a few percent at a mean energy of {approx}25 MeV, and a normalized root-mean square emittance on the order 0.5 mm mrad.
Date: July 26, 2005
Creator: Fubiani, G.; Esarey, E.; Schroeder, C.B. & Leemans, W.P.
Partner: UNT Libraries Government Documents Department

Wavefront-sensor-based electron density measurements for laser-plasma accelerators

Description: Characterization of the electron density in laser produced plasmas is presented using direct wavefront analysis of a probe laser beam. The performance of a laser-driven plasma-wakefield accelerator depends on the plasma wavelength, hence on the electron density. Density measurements using a conventional folded-wave interferometer and using a commercial wavefront sensor are compared for different regimes of the laser-plasma accelerator. It is shown that direct wavefront measurements agree with interferometric measurements and, because of the robustness of the compact commercial device, have greater phase sensitivity, straightforward analysis, improving shot-to-shot plasma-density diagnostics.
Date: February 20, 2010
Creator: Plateau, Guillaume; Matlis, Nicholas; Geddes, Cameron; Gonsalves, Anthony; Shiraishi, Satomi; Lin, Chen et al.
Partner: UNT Libraries Government Documents Department

Propagation of realistic beams in underdense plasma

Description: The effect of beam structure on propagation through underdense plasma is examined in two different examples. First, it is shown that the distribution of intensities within a laser beam affects how the beam deflects in the presence of transverse plasma flow. A detailed analysis of beam deflection shows that the rate scales linearly with intensity and plasma density, and inversely with plasma temperature. When the plasma flow is subsonic, the deflection rate is proportional to the ion damping decrement, and scales as M/(1 - M{sup 2}){sup 3/2}, where M is the transverse flow Mach number. When the plasma flow is supersonic, the deflection rate scales as 1/[M(M{sup 2} - 1){sup 1/2}]. Next, the effect of beam structure on channel formation by very intense laser beer is studied. A diffraction-limited beam with 40 TW of input power forms a channel through 4OOpm of plasma, whereas when this beam is phase aberrated, channel formation does not occur.
Date: November 10, 1997
Creator: Hinkel, D.E.; Williams, E.A.; Berger, R.L.; Powers, L.V.; Langdon, A.B. & Still, C.H.
Partner: UNT Libraries Government Documents Department

Plasma density from Cerenkov radiation, betatron oscillations, and beam steering in a plasma wakefield experiment at 30 GeV

Description: A method for using Cerenkov radiation near atomic spectral lines to measure plasma source properties for plasma wakefield applications has been discussed and experimentally verified. Because the radiation co-propagates with the electron beam, the radiation samples the source properties exactly along the path of interest with perfect temporal synchronization. Observation wavelengths were chosen with respect to the atomic resonances of the plasma source, where the relative change in the index of refraction strongly affects the Cerenkov cone angle, and permits flexible diagnostic design. The Cerenkov spatial profiles were systematically studied for a Lithium heat pipe oven as a function of oven temperature and observation wavelength. Neutral densities and plasma densities were extracted from the measurements.
Date: January 1, 2001
Creator: Catravas, P.; Chattopadhyay, S.; Esarey, E.; Leemans, W.P.; Assmann, R.; Decker, F.-J. et al.
Partner: UNT Libraries Government Documents Department

Progress in Beam Focusing and Compression for Target Heating and Warm Dense Matter Experiments

Description: The Heavy-Ion Fusion Sciences Virtual National Laboratory is pursuing an approach to target heating experiments in the warm dense matter regime, using space-charge-dominated ion beams that are simultaneously longitudinally bunched and transversely focused. Longitudinal beam compression by large factors has been demonstrated in the Neutralized Drift Compression Experiment (NDCX) with controlled ramps and forced neutralization. Using an injected 30 mA K{sup +} ion beam with initial kinetic energy 0.3 MeV, axial compression leading to {approx}50X current amplification and simultaneous radial focusing to a few mm have led to encouraging energy deposition approaching the intensities required for eV-range target heating experiments. We discuss the status of several improvements to NDCX to reach the necessary higher beam intensities, including: beam diagnostics, greater axial compression via a longer velocity ramp; and plasma injection improvements to establish a plasma density always greater than the beam density, expected to be > 10{sup 13} cm{sup -3}.
Date: April 17, 2009
Creator: Seidl, Peter; Anders, A.; Bieniosek, F.M.; Barnard, J.J.; Cohen, R.H.; Coleman, J.E. et al.
Partner: UNT Libraries Government Documents Department

Betatron radiation from density tailored plasmas

Description: In laser wakefield accelerators, electron motion is driven by intense forces that depend on the plasma density. Transverse oscillations in the accelerated electron orbits produce betatron radiation. The electron motion and the resulting betatron radiation spectrum can therefore be controlled by shaping the plasma density along the orbit of the electrons. Here, a method based on the use of a plasma with a longitudinal density variation (density depression or step) is proposed to increase the transverse oscillation amplitude and the energy of the electrons accelerated in a wakefield cavity. For fixed laser parameters, by appropriately tailoring the plasma profile, the betatron radiation emitted by these electrons is significantly increased in both flux and energy.
Date: April 11, 2009
Creator: Ta Phuoc, Kim; Esarey, E.; Leurent, V.; Cormier-Michel, E.; Geddes, C.G.R.; Schroeder, C.B. et al.
Partner: UNT Libraries Government Documents Department

Amplification of an ultra short pulse laser by stimulated Raman scattering of a 1ns pulse in a low density plasma

Description: Experiments are described in which a 1mJ, 1ps, 1200 nm seed laser beam is amplified by interaction with an intersecting 350 J, 1ns, 1054 nm pump beam in a low density (1 x 10{sup 19}/cm{sup 3}) plasma. The transmission of the seed beam is observed to be enhanced by > {approx} 25 x when the plasma is near the resonant density for stimulated Raman scattering (SRS), compared to measured transmissions at wavelengths just below the resonant value. The amplification is observed to increase rapidly with increases in both pump intensity and plasma density.
Date: October 8, 2007
Creator: Kirkwood, R K; Dewald, E; Niemann, C; Meezan, N; Wilks, S C; Price, D W et al.
Partner: UNT Libraries Government Documents Department

ON THE FEASIBILITY OF POLARIZED HEAVY IONS IN RHIC.

Description: Heavy nonspherical ions such as uranium have been proposed for collisions in RHIC[1]. When two such ions collide with their long axes aligned parallel to the beams (large helicities), then the plasma density might be as much as 60% higher. Since the collisions might have any orientation of the two nuclei, the alignment of the nuclei must be inferred from a complicated unfolding of multiplicity distributions. Instead, if it would be possible to polarize the ions and control the orientation in RHIC, then a much better sensitivity might be obtained. This paper investigates the manipulation of such polarized ions with highly distorted shapes in RHIC. A number of ion species are considered as possibilities with either full or partial Siberian snakes in RHIC.
Date: June 23, 2006
Creator: MACKAY, W.W.
Partner: UNT Libraries Government Documents Department

Plasma in Saturn's nightside magnetosphere and the implications for global circulation

Description: We present a bulk ion flow map from the nightside equatorial region of Saturn's magnetosphere derived from the Cassini CAPS ion mass spectrometer data. The map clearly demonstrates the dominance of corotation flow over radial flow and suggests that the flux tubes sampled are still closed and attached to the planet up to distances of 50 R{sub s}. The plasma characteristics in the near-midnight region are described and indicate a transition between the region of the magnetosphere containing plasma on closed drift paths and that containing flux tubes which may not complete a full rotation around the planet. Data from the electron spectrometer reveal two plasma states of high and low density. These are attributed either to the sampling of mass-loaded and depleted flux tubes, respectively, or to the latitudinal structure of the plasma sheet Depleted, returning flux tubes are not, in general, directly observed in the ions, although the electron observations suggest that such a process must take place in order to produce the low density population. Flux tube content is conserved below a limIt defined by the mass-loading and magnetic field strength and indicates that the flux tubes sampled may survive their passage through the tail. The conditions for mass release are evaluated using measured densities, angular velocities and magnetic field strength, The results suggest that for the relatively dense ion populations detectable by IMS, the condition for flux-tube breakage has not yet been exceeded, However, the low-density regimes observed in the electron data suggest that loaded flux tubes at greater distances do exceed the threshold for mass loss and subsequently return to the inner magnetosphere significantly depleted of plasma.
Date: January 1, 2009
Creator: Mcandrews, Hazel J; Wilson, R J; Henderson, M G; Tokar, R L; Jackman, C M; Khurana, K K et al.
Partner: UNT Libraries Government Documents Department

On Nonlinear Self-interaction of Geodesic Acoustic Mode Driven By Energetic Particles

Description: It is shown that nonlinear self-interaction of energetic particle-driven Geodesic Acoustic Mode does not generate a second harmonic in radial electric field using the fluid model. However, kinetic effects of energetic particles can induce a second harmonic in the radial electric field. A formula for the second order plasma density perturbation is derived. It is shown that a second harmonic of plasma density perturbation is generated by the convective nonlinearity of both thermal plasma and energetic particles. Near the midplane of a tokamak, the second order plasma density perturbation (the sum of second harmonic and zero frequency sideband) is negative on the low field side with its size comparable to the main harmonic at low fluctuation level. These analytic predictions are consistent with the recent experimental observation in DIII-D.
Date: October 1, 2010
Creator: Fu, G.Y.
Partner: UNT Libraries Government Documents Department

PRIMARY TESTS OF LASER / E BEAM INTERACTION IN A PLASMA CHANNEL.

Description: A high-energy CO{sub 2} laser is channeled in a capillary discharge. Plasma dynamic simulations confirm occurrence of guiding conditions at the relatively low axial plasma density 1 {divided_by} 4 x 10{sup 17} cm{sup -3}. A relativistic electron beam transmitted through the capillary changes its properties depending upon the plasma density. We observe focusing, defocusing or steering of the e-beam. Counter-propagation of the electron and laser beams in the plasma channel results in generation of intense picosecond x-ray pulses.
Date: June 23, 2002
Creator: POGORELSKY,I.V.; BEN ZVI,I.; HIROSE,T.; YAKIMENKO,V.; KUSCHE,K.; SIDDONS,P. et al.
Partner: UNT Libraries Government Documents Department

Engineering design of a radiative divertor for DIII-D

Description: A new divertor configuration is being developed for the DIII-D tokamak. This divertor will operate in the radiative mode. Experiments and modeling form the basis for the new design. The Radiative Divertor reduces the heat flux on the divertor plates by dispersing the power with radiation in the divertor region. In addition, the Radiative Divertor structure will allow density control in plasma shapes required for advanced tokamak operation. The divertor structure allows for operation in either double-null or single-null plasma configurations. Four independently controlled divertor cryopumps will enable pumping at either the inboard (upper and lower) or the outboard (upper and lower) divertor plates. An upgrade to the DIII-D cryogenic system is part of this project. The increased capabilities of the cryogenic system will allow delivery of liquid helium and nitrogen to the three new cryopumps. The Radiative Divertor design is very flexible, and will allow physics studies of the effects of slot width and length. Radiative Divertor diagnostics are being designed in parallel to provide comprehensive measurements for diagnosing the divertor. The Radiative divertor installation is scheduled for late 1996. Engineering experience gained in the DIII-D Advanced Divertor program form a foundation for the design work on the Radiative Divertor.
Date: October 1, 1995
Creator: Smith, J.P.; Baxi, C.B. & Bozek, A.S.
Partner: UNT Libraries Government Documents Department

RF heating experiments in CHS and RF development for LHD

Description: The Lame Helical Device, LHD, is in its final construction phase. It is a 1=2, m=10 Heliotron/Torsatron type helical system with a major radius of 4 m. The compact helical system, CHS, is a 1=2, m=8 helical system of the same type with a major radius of 1m. CHS has been used for supporting experiments to clarify physics issues of helical systems and to examine the key ideas which will be applied to LHD. This paper summarizes the experimental results of those supporting experiments in CHS and how this knowledge is incorporated in the design of RF heating in LHD. ICRF Heating results in CHS by use of loop antennas are described in section II. The results of using a Nagoya type-III coil is described in Section III. Two types of antennas used in the initial phase of the LHD ICRF Heating: a loop antenna designed for steady state heating and a folded wave guide antenna designed for EBW, are described in section IV.
Date: April 1, 1997
Creator: Watari, T.; Kumazawa, R. & Nishimura, K.
Partner: UNT Libraries Government Documents Department

Microwave Reflectometry for Magnetically Confined Plasmas

Description: This paper is about microwave reflectometry -- a radar technique for plasma density measurements using the reflection of electromagnetic waves by a plasma cutoff. Both the theoretical foundations of reflectometry and its practical application to the study of magnetically confined plasmas are reviewed in this paper. In particular, the role of short-scale density fluctuations is discussed at length, both as a unique diagnostic tool for turbulence studies in thermonuclear plasmas and for the deleterious effects that fluctuations may have on the measurement of the average plasma density with microwave reflectometry.
Date: February 1, 1998
Creator: Mazzucato, E.
Partner: UNT Libraries Government Documents Department

Influence of Surface Material on the BCl Density in Inductively Coupled Discharges

Description: The relative density of BCl radicals has been measured in a modified Applied Materials DPS metal etch chamber using laser-induced fluorescence. In plasmas containing mixtures of BCl{sub 3} with Cl{sub 2}, Ar and/or N{sub 2}, the relative BCl density was measured as a function of source and bias power, pressure, flow rate, BCl{sub 3}/Cl{sub 2} ratio and argon addition. To determine the influence of surface materials on the bulk plasma properties, the relative BCl density was measured using four different substrate types; aluminum, alumina, photoresist, and photoresist-patterned aluminum. In most cases, the relative BCl density was highest above photoresist-coated wafers and lowest above blanket aluminum wafers. The BCl density increased with increasing source power and the ratio of BCl{sub 3} to Cl{sub 2}, while the addition of N{sub 2} to a BCl{sub 3}/Cl{sub 2} plasma resulted in a decrease in BCl density. The BCl density was relatively insensitive to changes in the other plasma parameters.
Date: March 15, 1999
Creator: Blain, M.G.; Hamilton, T.W. & Hebner, G.A.
Partner: UNT Libraries Government Documents Department

The magnetospheric trough

Description: The authors review the history of the concepts of the magnetospheric cold-ion trough and hot-electron trough and conclude that the two regions are actually essentially the same. The magnetospheric trough may be viewed as a temporal state in the evolution of convecting flux tubes. These flux tubes are in contact with the earth`s upper atmosphere which acts both as a sink for precipitating hot plasma sheet electrons and as a source for the cold ionospheric plasma leading to progressive depletion of the plasma sheet and refilling with cold plasma. Geosynchronous plasma observations show that the rate of loss of plasma-sheet electron energy density is commensurate with the precipitating electron flux at the low-latitude edge of the diffuse aurora. The rate at which geosynchronous flux tubes fill with cold ionospheric plasma is found to be consistent with previous estimates of early-time refilling. Geosynchronous observations further indicate that both Coulomb collisions and wave-particle effects probably play a role in trapping ionospheric material in the magnetosphere.
Date: March 4, 1997
Creator: Thomsen, M. F.; McComas, D. J.; Elphic, R. C. & Borovsky, J. E.
Partner: UNT Libraries Government Documents Department

Effects of 2D and Finite Density Fluctuations on O-X Correlation Reflectometry

Description: The correlation between O-mode and X-mode reflectometer signals is studied with a 1D and 2D reflectometer model in order to explore its feasibilities as a q-profile diagnostic. It was found that 2D effects and finite fluctuation levels both decrease the O-X correlation. At very low fluctuation levels, which are usually present in the plasma core, there is good possibility to determine the local magnetic field strength and use that as a constraint for the equilibrium reconstruction.
Date: July 5, 2001
Creator: Kramer, G.J.; Nazikian, R. & Valeo, E.
Partner: UNT Libraries Government Documents Department

Experimental Investigation of Short Scalelength Density Fluctuations in Laser-Produced Plasmas

Description: The technique of near forward laser. scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas. Intrinsic fluctuations are studied in long scalelength plasmas where the fluctuations exhibit scale sizes related to the size of the intensity variations in the plasma forming and interaction beams. Stimulated Brillouin forward scattering and filamentation appear to be the primary mechanism through which these fluctuations originate. The beam spray which results from these fluctuations is important to understand since it can affect symmetry in an inertial confinement fusion (ICF) experiment. Controlled fluctuations are studied in foam and exploding foil targets. Forward scattered light from foam targets shows evidence that the initial target inhomogeneities remain after the target is laser heated. Forward scattered light from an exploding foil plasma shows that a regular intensity pattern can be used to produce a spatially correlated density fluctuation pattern. These results provide data which can be used to benchmark numerical models of beam spray.
Date: November 1, 1999
Creator: Moody, J.D.; MacGowan, B.J.; Glenzer, S.H.; Kirkwood, R.K.; Kruer, W.L.; Williams, E.A. et al.
Partner: UNT Libraries Government Documents Department

Experimental Investigation of Short Scalelength Density Fluctuations in Laser-Produced Plasmas

Description: The technique of near forward laser scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas. Intrinsic fluctuations are studied in long-scale length plasmas where we find that the fluctuations exhibit scale sizes related to the intensity variation scales in the plasma-forming and interaction beams. Stimulated Brillouin forward scattering and filamentation appear to be the primary mechanism through which these fluctuations originate. The beam spray resulting from these fluctuations is important to understand since it can affect symmetry in an inertial confinement fusion (ICF) experiment. Controlled fluctuations are studied in foam and exploding foil targets. Forward scattered light from foam targets shows evidence that the initial target inhomogeneities remain after the target is laser heated. Forward scattered light from an exploding foil plasma shows that a regular intensity pattern can be used to produce a spatially correlated density fluctuation pattern. These results provide data which are being used to benchmark numerical models of beam spray.
Date: January 5, 2000
Creator: Moody, J.D.; MacGowan, B.J.; Glenzer, S.H.; Kirkwood, R.K.; Kruer, W.L.; Montgomery, D.S. et al.
Partner: UNT Libraries Government Documents Department

A Tutorial on Basic Principles of Microwave Reflectometry Applied to Fluctuation Measurements in Fusion Plasmas

Description: Microwave reflectometry is now routinely used for probing the structure of magnetohydrodynamic and turbulent fluctuations in fusion plasmas. Conditions specific to the core of tokamak plasmas, such as small amplitude of density irregularities and the uniformity of the background plasma, have enabled progress in the quantitative interpretation of reflectometer signals. In particular, the extent of applicability of the 1-D [one-dimensional] geometric optics description of the reflected field is investigated by direct comparison to 1-D full wave analysis. Significant advances in laboratory experiments are discussed which are paving the way towards a thorough understanding of this important measurement technique. Data is presented from the Tokamak Fusion Test Reactor [R. Hawryluk, Plasma Physics and Controlled Fusion 33 (1991) 1509] identifying the validity of the geometric optics description of the scattered field and demonstrating the feasibility of imaging turbulent fluctuations in fusion scale devices.
Date: February 16, 2001
Creator: Nazikian, R.; Kramer, G.J. & Valeo, E.
Partner: UNT Libraries Government Documents Department

Bifurcation Mode of Relativistic and Charge-Displacement Self-Channeling

Description: Stable self-channeling of ultra-powerful (P{sub 0} - 1 TW -1 PW) laser pulses in dense plasmas is a key process for many applications requiring the controlled compression of power at high levels. Theoretical computations predict that the transition zone between the stable and highly unstable regimes of relativistic/charge-displacement self-channeling is well characterized by a form of weakly unstable behavior that involves bifurcation of the propagating energy into two powerful channels. Recent observations of channel instability with femtosecond 248 nm pulses reveal a mode of bifurcation that corresponds well to these theoretical predictions. It is further experimentally shown that the use of a suitable longitudinal gradient in the plasma density can eliminate this unstable behavior and restore the efficient formation of stable channels.
Date: July 20, 2000
Creator: BORISOV,A.B.; CAMERON,STEWART M.; LUK,TING S.; NELSON,THOMAS R.; VAN TASSLE,A.J.; SANTORO,J. et al.
Partner: UNT Libraries Government Documents Department

Visible imaging of edge turbulence in NSTX

Description: Edge plasma turbulence in tokamaks and stellarators is believed to cause the radical heat and particle flux across the separatrix and into the scrape-off-layers of these devices. This paper describes initial measurements of 2-D space-time structure of the edge density turbulence made using a visible imaging diagnostic in the National Spherical Torus Experiment (NSTX). The structure of the edge turbulence is most clearly visible using a method of gas puff imaging to locally illuminate the edge density turbulence.
Date: June 13, 2000
Creator: Zweben, S.; Maqueda, R.; Hill, K.; Johnson, D. & al, et
Partner: UNT Libraries Government Documents Department