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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

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

Relativistic self-focusing in underdense plasma

Description: In the present paper, we discuss light self-focusing in underdense (n<n{sub c}) plasmas. We will show that ion motion is important even for picosecond pulse durations and a description of relativistic self-focusing including ion dynamics will be presented in second part of the paper. In particular, we will demonstrate the formation of empty, wide channels in underdense plasma in the wake of the laser pulse. we discuss the applicability of our results to real situations and possible consequences for the ``Fast Ignitor`` project.
Date: April 24, 1997
Creator: Feit, M.D.; Garrison, J.C.; Komashko, A.; Musher, J.L.; Rubenchik, A.M. & Turistsyn, S.K.
Partner: UNT Libraries Government Documents Department

Composite wire plasma formation and evolution

Description: The detailed understanding of the formation and evolution of plasma from rapidly heated metallic wires is a long-standing challenge in the field of plasma physics and in exploding wire engineering. This physical process is made even more complicated if the wire material is composed of a number of individual layers. The authors have successfully developed both optical and x-ray backlighting diagnostics. In particular, the x-ray backlighting technique has demonstrated the capability for quantitative determination of the plasma density over a wide range of densities. This diagnostic capability shows that the process of plasma formation is composed of two separate phases: first, current is passed through a cold wire and the wire is heated ohmically, and, second, the heated wire evolves gases that break down and forms a low-density plasma surrounding the wire.
Date: January 1, 2000
Creator: Spielman, R.B.
Partner: UNT Libraries Government Documents Department

Screening Resonances In Plasmas

Description: When it was suggested that a new recombination mechanism (Resonant Radiative Recombination (RRR)) which, based on very general physical arguments, should happen in dense plasmas and promises to provide useful information for the local temperature and density diagnostics of plasmas, they assumed the existence of screening resonances. For model potentials the existence of screening resonances has been demonstrated beyond reasonable doubt in a number of calculations. The key question, how well those potentials describe the dominant effects of a real plasma remains open. The relation of theoretical predictions to experimentally measurable effects is an important issue at the present stage of their research. In particular, RRR is expected to account for enhanced recombination rates of low energetic electrons with their ions, since the first stage is the resonant capture of a slow electron by an atom or ion. The mechanism that traps an electron is a combination of complicated many-body interactions of the ions and electrons. For clarity they start here, however, with a discussion in terms of local potential traps the shapes of which are determined predominantly and in an average way by two factors: the degree of screening present at the ionic site and the degree of short-range order in the immediate neighborhood of this ion.
Date: December 1, 1998
Creator: Winkler, P.
Partner: UNT Libraries Government Documents Department

Observation of Compressional Alfven Modes during Neutral Beam Heating on the National Spherical Torus Experiment

Description: Neutral-beam-driven compressional Alfven eigenmodes (CAE) at frequencies below the ion cyclotron frequency have been observed and identified for the first time in the National Spherical Torus Experiment (NSTX). The modes are observed as a broad spectrum of nearly equally spaced peaks in the frequency range from approximately 0.2 to approximately 1.2 omega(subscript ''ci''). The frequency has a scaling with toroidal field and plasma density consistent with Alfven waves. The modes have been observed with high bandwidth magnetic pick-up coils and with a reflectometer.
Date: October 3, 2001
Creator: Fredrickson, E.D.; Gorelenkov, N.; Cheng, C.Z.; Bell, R.; Darrow, D.; Johnson, D. et al.
Partner: UNT Libraries Government Documents Department

LLNL large-area inductively coupled plasma (ICP) source: Experiments

Description: We describe initial experiments with a large (76-cm diameter) plasma source chamber to explore the problems associated with large-area inductively coupled plasma (ICP) sources to produce high density plasmas useful for processing 400-mm semiconductor wafers. Our experiments typically use a 640-nun diameter planar ICP coil driven at 13.56 MHz. Plasma and system data are taken in Ar and N{sub 2} over the pressure range 3-50 mtorr. RF inductive power was run up to 2000W, but typically data were taken over the range 100-1000W. Diagnostics include optical emission spectroscopy, Langmuir probes, and B-dot probes as well as electrical circuit measurements. The B-dot and E-M measurements are compared with models based on commercial E-M codes. Initial indications are that uniform plasmas suitable for 400-mm processing are attainable.
Date: May 1, 1995
Creator: Richardson, R.A.; Egan, P.O. & Benjamin, R.D.
Partner: UNT Libraries Government Documents Department

Accessibillity of Electron Bernstein Modes in Over-Dense Plasma

Description: Mode-conversion between the ordinary, extraordinary and electron Bernstein modes near the plasma edge may allow signals generated by electrons in an over-dense plasma to be detected. Alternatively, high frequency power may gain accessibility to the core plasma through this mode conversion process. Many of the tools used for ion cyclotron antenna de-sign can also be applied near the electron cyclotron frequency. In this paper, we investigate the possibilities for an antenna that may couple to electron Bernstein modes inside an over-dense plasma. The optimum values for wavelengths that undergo mode-conversion are found by scanning the poloidal and toroidal response of the plasma using a warm plasma slab approximation with a sheared magnetic field. Only a very narrow region of the edge can be examined in this manner; however, ray tracing may be used to follow the mode converted power in a more general geometry. It is eventually hoped that the methods can be extended to a hot plasma representation. Using antenna design codes, some basic antenna shapes will be considered to see what types of antennas might be used to detect or launch modes that penetrate the cutoff layer in the edge plasma.
Date: April 12, 1999
Creator: Batchelor, D. B.; Bigelow, T. S. & Carter, M. D.
Partner: UNT Libraries Government Documents Department