Showing papers in "Physics of Plasmas in 1996"

Short‐pulse laser harmonics from oscillating plasma surfaces driven at relativistic intensity

Abstract: The generation of harmonics by interaction of an ultrashort laser pulse with a step boundary of a plane overdense plasma layer is studied at intensities Iλ2=1017–1019 W cm−2 μm2 for normal and oblique incidence and different polarizations. Fully relativistic one‐dimensional particle‐in‐cell (PIC) simulations are performed with high spectral resolution. Harmonic emission increases with intensity and also when lowering the plasma density. The simulations reveal strong oscillations of the critical surface driven by the normal component of the laser field and by the ponderomotive force. It is shown that the generation of harmonics can be understood as reflection from the oscillating surface, taking full account of retardation. Describing the oscillations by one or more Fourier components with adjustable amplitudes, model spectra are obtained that well reproduce the PIC spectra. The model is based on relativistic cold plasma equations for oblique incidence. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.

Lattice waves in dust plasma crystals

Abstract: Techniques previously known from solid state physics are used to look at linear and weak non‐linear wave propagation in dust lattices. These expansion techniques include only electrostatic interactions between neighbor particles in addition to assuming small vibrations in the dust lattice. As a simple model for the dust lattice, a one‐dimensional Bravais lattice is considered. For this particular lattice, expressions for the linear phase velocity are compared to a quasi‐particle simulation. The word quasi here means that only the dust particles are represented as diffuse objects, while the plasma is treated as a fluid. The simulation is also used to study the breakdown of the analytical theory and to investigate non‐linear dust lattice waves. A very good agreement is found between the analytical expressions and the particle simulations, for cases where the average dust separation a is of the order of or larger than the plasma Debye length λD. This is a condition which very often applies to dust crystal in...

Solitary potentials in dusty plasmas

Abstract: It is found that a dusty plasma with inertial dust fluid and Boltzmann distributed ions admits only negative solitary potentials associated with nonlinear dust‐acoustic waves. The dynamics of small‐amplitude disturbances is governed by the Korteweg–de Vries (KdV) equation, the stationary solution of which assumes the inverted bell‐shaped secant hyperbolic squared profile. The associated dust and ion density perturbations are, on the other hand, positive. The solitary potentials can be identified as nonlinear structures in low‐temperature dusty plasmas such as those in laboratory and astrophysical environments.

Threshold for neoclassical magnetic islands in a low collision frequency tokamak

Abstract: A kinetic theory for magnetic islands in a low collision frequency tokamak plasma is presented. Self‐consistent equations for the islands’ width, w, and propagation frequency, ω, are derived. These include contributions from the perturbed bootstrap current and the toroidally enhanced ion polarization drift. The bootstrap current is independent of the island propagation frequency and provides a drive for the island in tokamak plasmas when the pressure decreases with an increasing safety factor. The polarization drift is frequency dependent, and therefore its effect on the island stability cannot be deduced unless ω is known. This frequency is determined by the dominant dissipation mechanism, which for low effective collision frequency, νeff=ν/e<ω, is governed by the electrons close to the trapped/passing boundary. The islands are found to propagate in the electron diamagnetic direction in which case the polarization drift is stabilizing and results in a threshold width for island growth, which is of the order of the ion banana width. At larger island widths the polarization current term becomes small and the island evolution is determined by the bootstrap current drive and Δ′ alone, where Δ′ is a measure of the magnetic free energy.

The structure of three‐dimensional magnetic neutral points

Abstract: The local configurations of three‐dimensional magnetic neutral points are investigated by a linear analysis about the null. It is found that the number of free parameters determining the arrangement of field lines is four. The configurations are first classified as either potential or non‐potential. Then the non‐potential cases are subdivided into three cases depending on whether the component of current parallel to the spine is less than, equal to or greater than a threshold current; therefore there are three types of linear non‐potential null configurations (a radial null, a critical spiral and a spiral). The effect of the four free parameters on the system is examined and it is found that only one parameter categorizes the potential configurations, whilst two parameters are required if current is parallel to the spine. However, all four parameters are needed if there is current both parallel and perpendicular to the spine axis. The magnitude of the current parallel to the spine determines whether the null has spiral, critical spiral or radial field lines whilst the current perpendicular to the spine affects the inclination of the fan plane to the spine. A simple method is given to determine the basic structure of a null given M the matrix which describes the local linear structure about a null point.

Effects of vortex‐like and non‐thermal ion distributions on non‐linear dust‐acoustic waves

Abstract: The effects of vortex‐like and non‐thermal ion distributions are incorporated in the study of nonlinear dust‐acoustic waves in an unmagnetized dusty plasma. It is found that owing to the departure from the Boltzmann ion distribution to a vortex‐like phase space distribution, the dynamics of small but finite amplitude dust‐acoustic waves is governed by a modified Kortweg–de Vries equation. The latter admits a stationary dust‐acoustic solitary wave solution, which has larger amplitude, smaller width, and higher propagation velocity than that involving adiabatic ions. On the other hand, consideration of a non‐thermal ion distribution provides the possibility of coexistence of large amplitude rarefactive as well as compressive dust‐acoustic solitary waves, whereas these structures appear independently when the wave amplitudes become infinitely small. The present investigation should help us to understand the salient features of the non‐linear dust‐acoustic waves that have been observed in a recent numerical simulation study.

The dynamics of marginality and self-organized criticality as a paradigm for turbulent transport

Abstract: A general paradigm, based on the concept of self‐organized criticality (SOC), for turbulent transport in magnetically confined plasmas, has been recently suggested as an explanation for some of the apparent discrepancies between most theoretical models of turbulent transport and experimental observations of the transport in magnetically confined plasmas. This model describes the dynamics of the transport without relying on the underlying local fluctuation mechanisms. Computations based on a cellular automata realization of such a model have found that noise‐driven SOC systems can maintain average profiles that are linearly stable (submarginal) and yet are able to sustain active transport dynamics. It is also found that the dominant scales in the transport dynamics in the absence of sheared flow are system scales rather than the underlying local fluctuation scales. The addition of sheared flow into the dynamics leads to a large reduction of the system‐scale transport events and a commensurate increase in the fluctuation‐scale transport events needed to maintain the constant flux. The dynamics of these models and the potential ramifications for transport studies are discussed.

The evolution of ultra‐intense, short‐pulse lasers in underdense plasmas

Abstract: The propagation of short‐pulse lasers through underdense plasmas at ultra‐high intensities (I≥1019 W/cm) is examined. The pulse evolution is found to be significantly different than it is for moderate intensities. The pulse breakup is dominated by leading edge erosion and plasma wave wake formation rather than from Raman forward scattering type instabilities. A differential equation which describes local pump depletion is derived and used to analyze the formation and evolution of the erosion. Pulse erosion is demonstrated with one dimensional particle in cell (PIC) simulations. In addition, two dimensional simulations are presented which show pulse erosion along with other effects such as channeling and diffraction. Possible applications for plasma based accelerators and light sources are discussed.

Experimental observation of very low-frequency macroscopic modes in a dusty plasma

Abstract: Images of a cloud of grains in a dusty plasma reveal a pair of very low‐frequency modes, termed here the filamentary and great void modes. The plasma was a radio‐frequency discharge formed between parallel‐plate graphite electrodes. A cloud of 100 nm carbon particles was produced by accretion of carbon atoms produced by sputtering the graphite. The cloud was illuminated with a laser sheet and imaged with a video camera. The great void mode was a spoke‐shaped region of the cloud that was free of dust and rotated azimuthally in the discharge. The filamentary mode had the appearance of turbulent striations, with a smaller amplitude than the great void. The filamentary mode sometimes appeared as a distinctive vortex, curling in the poloidal direction. Both modes had a very low frequency, on the order of 10 Hz. Two possible causes of the modes are discussed. The low phase velocity of the modes may be consistent with a dust‐acoustic wave. Alternatively, the great void may be an ionization wave that moved the dust about, since a modulation in the glow was seen moving at the same speed as the void. It is argued that existing theories of waves in dusty plasmas assume weakly collisional plasmas, which may be unsuitable for explaining experimental results in laboratory dusty plasmas, since they are often strongly coupled.

A model realization of self‐organized criticality for plasma confinement

Abstract: A model for plasma transport near marginal stability is presented. The model is based on subcri‐ tical resistive pressure‐gradient‐driven turbulence. Three‐dimensional nonlinear calculations based on this model show effective transport for subcritical mean profiles. This model exhibits some of the characteristic properties of self‐organized criticality. Perturbative transport techniques are used to elucidate the transport properties. Propagation of positive and negative pulses is studied. The observed results suggest a possible explanation of the apparent nonlocal effects ob‐ served with perturbative experiments in tokamaks.

Direct‐drive laser‐fusion experiments with the OMEGA, 60‐beam, >40 kJ, ultraviolet laser system

Abstract: OMEGA, a 60‐beam, 351 nm, Nd:glass laser with an on‐target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct‐ and indirect‐drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct‐drive capsules with shaped laser pulses (dynamic range ≳400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct‐drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high‐gain, direct‐drive capsules. Details are presented of the OMEGA direct‐drive experimental program and initial data from direct‐drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficienc...

Toroidal gyrofluid equations for simulations of tokamak turbulence

Abstract: A set of nonlinear gyrofluid equations for simulations of tokamak turbulence are derived by taking moments of the nonlinear toroidal gyrokinetic equation The moment hierarchy is closed with approximations that model the kinetic effects of parallel Landau damping, toroidal drift resonances, and finite Larmor radius effects These equations generalize the work of Dorland and Hammett [Phys Fluids B 5, 812 (1993)] to toroidal geometry by including essential toroidal effects The closures for phase mixing from toroidal ∇B and curvature drifts take the basic form presented in Waltz et al [Phys Fluids B 4, 3138 (1992)], but here a more rigorous procedure is used, including an extension to higher moments, which provides significantly improved accuracy In addition, trapped ion effects and collisions are incorporated This reduced set of nonlinear equations accurately models most of the physics considered important for ion dynamics in core tokamak turbulence, and is simple enough to be used in high resolution

Enhancement of Tokamak Fusion Test Reactor performance by lithium conditioning

Abstract: Wall conditioning in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] by injection of lithium pellets into the plasma has resulted in large improvements in deuterium–tritium fusion power production (up to 10.7 MW), the Lawson triple product (up to 1021 m−3 s keV), and energy confinement time (up to 330 ms). The maximum plasma current for access to high‐performance supershots has been increased from 1.9 to 2.7 MA, leading to stable operation at plasma stored energy values greater than 5 MJ. The amount of lithium on the limiter and the effectiveness of its action are maximized through (1) distributing the Li over the limiter surface by injection of four Li pellets into Ohmic plasmas of increasing major and minor radius, and (2) injection of four Li pellets into the Ohmic phase of supershot discharges before neutral‐beam heating is begun.

Shock waves in plasmas containing variable‐charge impurities

Abstract: Shock structures in plasmas containing variable‐charge macro particles are shown to exist because of an effective dissipation associated with charging of the latter.

The Nike KrF laser facility: Performance and initial target experiments

Abstract: Krypton‐fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability (≳THz) desired for rapid beam smoothing and the short laser wavelength (1/4 μm) needed for good laser–target coupling. Nike is a recently completed 56‐beam KrF laser and target facility at the Naval Research Laboratory. Because of its bandwidth of 1 THz FWHM (full width at half‐maximum), Nike produces more uniform focal distributions than any other high‐energy ultraviolet laser. Nike was designed to study the hydrodynamic instability of ablatively accelerated planar targets. First results show that Nike has spatially uniform ablation pressures (Δp/p<2%). Targets have been accelerated for distances sufficient to study hydrodynamic instability while maintaining good planarity. In this review we present the performance of the Nike laser in producing uniform illumination, and its performance in correspondingly uniform acceleration of targets.

Laser driven electron acceleration in vacuum, gases and plasmas

Abstract: In this paper we discuss some of the important issues pertaining to laser acceleration in vacuum, neutral gases, and plasmas. The limitations of laser vacuum acceleration as they relate to electron slippage, laser diffraction, material damage, and electron aperture effects, are discussed. An inverse Cherenkov laser acceleration configuration is presented in which a laser beam is self‐guided in a partially ionized gas. Optical self‐guiding is the result of a balance between the nonlinear self‐focusing properties of neutral gases and the diffraction effects of ionization. The stability of self‐guided beams is analyzed and discussed. In addition, aspects of the laser wakefield accelerator are presented and laser‐driven accelerator experiments are briefly discussed.

The effect of the ion temperature on large amplitude ion‐acoustic waves in an electron–positron–ion plasma

Abstract: The effect of the ion temperature on large amplitude ion‐acoustic waves is studied in an electron–positron–ion plasma. The region of the existence of the ion‐acoustic waves is presented, by the analysis of the structure of the pseudopotential. It is found that the ion temperature increases the maximum Mach number and decreases the amplitude of the ion‐acoustic waves. The region of the existence sensitively depends on the ion temperature and the positron density and temperature. It is shown that the region of the existence of the ion‐acoustic wave spreads as the ion temperature decreases. The present theory is applicable to analyzing large amplitude ion‐acoustic waves in an electron–positron–ion plasma associated with the ion temperature which may occur in space.

Fluctuation-induced flux at the plasma edge in toroidal devices

Abstract: Fluctuation‐induced fluxes have a bursty character. As a consequence, a significant part of the total particle flux is carried out by sporadic, large transport bursts. The local flux distribution function is consistent with a near‐Gaussian character of the fluctuations. The radial dependence of the statistical properties of plasma fluctuations and induced fluxes have been investigated in the plasma boundary region of the TJ‐I tokamak [I. Garcia‐Cortes et al., Phys. Fluids B 4, 4007 (1992)] and the TJ‐IU torsatron [E. Ascasibar et al., in Plasma Physics and Controlled Fusion Research, Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville (International Atomic Energy Agency, Vienna, in press)]. There is a striking similarity between the statistical properties of turbulent transport in both devices.

Energy transfer between crossing laser beams

Abstract: Induced scattering between crossing laser beams is considered, including the effects of long‐wavelength modulations in the plasma. This fundamental process can impact the choice of beam‐smoothing techniques for laser‐driven hohlraums. Study of this process is an ideal way to quantify stimulated scattering instabilities, since one can independently vary the intensity, polarization, and frequency separation of the crossing beams.

Current sheet formation along three‐dimensional magnetic separators

Abstract: It was shown by Syrovatskii [Sov. Phys. JETP 33, 933 (1971)] that a perfectly conducting two‐dimensional magnetic field stressed externally develops a current sheet at an X point. A generalization of this analysis shows that a generic three‐dimensional magnetic field develops a closed ribbon of current along separators already existing in the field. A separator is a field line that begins and ends at different magnetic null points. A current ribbon will occur if there are two distinct separators beginning and ending at the same two nulls, thus forming a loop. The magnetic field containing the current ribbon is in force‐free equilibrium. An expression is found for the net current induced in the ribbon, and the magnetic energy stored by it, in terms of the amount of external stress. An electric field parallel to the current will facilitate reconnection and will dissipate the current and release the stored energy.

Physics of helicon discharges

Abstract: Because of their high density, low‐pressure discharges ionized by helicon waves are being studied for their possible use in cluster tools for the fabrication of next‐generation computer chips. How helicon waves are related to whistler waves and waves in a plasma‐filled waveguide is explained, and the mystery of the high ionization efficiency is outlined. Experimental data on the waves and the equilibrium properties of the discharge are shown, and the status of our current understanding of the physical processes therein is summarized. The importance of kinetic effects and of a short‐wavelength mode arising at low magnetic fields is evaluated. Applications of helicon discharges to such diverse fields as plasma accelerators, microwave generators, and tokamak physics are illustrated. Low‐temperature plasma physics is often considered a discipline so different from high‐temperature plasma physics that there is little overlap, but these studies show that the techniques developed in fusion and space plasma physics can be applied to technological plasmas as well.

Capacitive, inductive and helicon‐wave modes of operation of a helicon plasma source

Abstract: Vector‐rf‐B‐field measurements in the near‐field of a helicon plasma source taken throughout the volume of the source are reported. Three distinct modes of operation of the helicon plasma source, capacitive, inductive, and helicon‐wave, are identified by the structure of the plasma‐wave‐fields. Results are reported for a double‐half‐turn antenna, which is believed to be the first reporting for such an antenna structure in application to helicon‐wave plasma sources. Comparison is made to a double‐saddle‐coil antenna which also demonstrates the distinct inductive and helicon‐wave modes.

Three-dimensional simulations of Nova high growth factor capsule implosion experiments

Abstract: Capsule implosion experiments carried out on the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] are simulated with the three‐dimensional HYDRA radiation hydrodynamics code [NTIS Document No. DE‐96004569 (M. M. Marinak et al. in UCRL‐LR‐105821‐95‐3)]. Simulations of ordered, near single mode perturbations indicate that structures which evolve into round spikes can penetrate farthest into the hot spot. Bubble‐shaped perturbations can burn through the capsule shell fastest, in which case they cause even more damage. A simulation of a capsule with a multimode perturbation of moderate amplitude shows spike amplitudes evolving in good agreement with a saturation model during the deceleration phase. The presence of sizable low mode asymmetry, caused either by drive asymmetry or perturbations in the capsule shell, can dramatically affect the manner in which spikes approach the center of the hot spot. Three‐dimensional coupling between the low mode shell perturbations intrinsic to Nova capsules and the drive asymmetry is found to be important, bringing the simulated neutron yields into closer agreement with the experimental values.

Self‐consistent stability analysis of ablation fronts with large Froude numbers

Abstract: The linear growth rate of the Rayleigh–Taylor instability is calculated for accelerated ablation fronts with small Froude numbers (Fr≪1) The derivation is carried out self‐consistently by including the effects of finite thermal conductivity (κ∼Tν) and density gradient scale length (L) It is shown that long‐wavelength modes with wave numbers kL0≪1 [L0=νν/(ν+1)ν+1 min(L)] have a growth rate γ≂√ATkg−βkVa, where Va is the ablation velocity, g is the acceleration, AT=1+O[(kL0)1/ν], and 1<β(ν)<2 Short‐wavelength modes are stabilized by ablative convection, finite density gradient, and thermal smoothing The growth rate is γ=√αg/L0+c20k4L20V2a−c0k2L0Va for 1≪kL0≪Fr−1/3, and γ=c1g/(Vak2L20)−c2kVa for the wave numbers near the cutoff kc The parameters α and c0−2 mainly depend on the power index ν; and the cutoff kc of the unstable spectrum occurs for kcL0∼Fr−1/3≫1 Furthermore, an asymptotic formula reproducing the growth rate at small and large Froude numbers is derived and compared with numerical results

Plasma production from helicon waves

Abstract: Experimental measurements taken in a large magnetoplasma show that a simple double half‐turn antenna will excite m=1 helicon waves with wavelengths from 10–60 cm. Increased ionization in the center of the downstream plasma is measured when the axial wavelength of the helicon wave becomes less than the characteristic length of the system, typically 50–100 cm. A sharp maximum in the plasma density downstream from the source is measured for a magnetic field of 50 G, where the helicon wave phase velocity is about 3×108 cm s−1. Transport of energy away from the source to the downstream region must occur to create the hot electrons needed for the increased ionization. A simple model shows that electrons in a Maxwellian distribution most likely to ionize for these experimental conditions also have a velocity of around 3×108 cm s−1. This strong correlation suggests that the helicon wave is trapping electrons in the Maxwellian distribution with velocities somewhat slower than the wave and accelerating them into a quasibeam with velocity somewhat faster than the wave. The nonlinear increase in central density downstream as the power is increased for helicon waves with phase velocities close to the optimum electron velocity for ionization lends support to this idea.

Trapping of dust and dust acoustic waves in laboratory plasmas

Abstract: Trapping of negatively charged dust particles is observed in a hot cathode plasma discharge when a layer of dust is exposed to the plasma. The particles are visible in the scattered He–Ne laser light. The trajectories of individual particles have been photographed. The dust particles are excluded from the sheath region of any object in the plasma. The intensity of scattered light as well as the potential on a floating Langmuir probe show coherent fluctuations in the frequency range 1–15 Hz. After several hours of exposure to the plasma, the dust layer develops striations similar to those on sand dunes. Trapping of dust particles by the plasma and the possible identification of the observed low‐frequency fluctuations with dust acoustic waves are discussed.

Transport processes and entropy production in toroidal plasmas with gyrokinetic electromagnetic turbulence

Abstract: Transport processes and resultant entropy production in magnetically confined plasmas are studied in detail for toroidal systems with gyrokinetic electromagnetic turbulence. The kinetic equation including the turbulent fluctuations are double averaged over the ensemble and the gyrophase. The entropy balance equation is derived from the double‐averaged kinetic equation with the nonlinear gyrokinetic equation for the fluctuating distribution function. The result clarifies the spatial transport and local production of the entropy due to the classical, neoclassical and anomalous transport processes, respectively. For the anomalous transport process due to the electromagnetic turbulence as well as the classical and neoclassical processes, the kinetic form of the entropy production is rewritten as the thermodynamic form, from which the conjugate pairs of the thermodynamic forces and the transport fluxes are identified. The Onsager symmetry for the anomalous transport equations is shown to be valid within the quasilinear framework. The complete energy balance equation, which takes account of the anomalous transport and exchange of energy due to the fluctuations, is derived from the ensemble‐averaged kinetic equation. The intrinsic ambipolarity of the anomalous particle fluxes is shown to hold for the self‐consistent turbulent electromagnetic fields satisfying Poisson’s equation and Ampere’s law.

Acceleration in femtosecond laser-produced plasmas

Abstract: The influence of the acceleration of a femtosecond laser‐generated plasma on the reflected spectrum of the plasma‐producing pulse is analyzed quantitatively, and compared to experimental results. It is shown that the spectral positions of the reflected laser light are complicated functions of the temporally varying optical properties of the plasma and the hydrodynamic motion. The linewidths, however, depend only on the acceleration of the plasma mirror and the chirp of the laser pulse and can consequently be used to measure the acceleration in a laser‐produced plasma. Plasma accelerations on the order of 1018 m s−2≊1017 g directed both away from the solid target at intensities of I≤1017 W cm−2 and into the target for I≥3×1017 W cm−2 are obtained from an analysis of the experiments. The results demonstrate that during the short subpicosecond laser pulse the plasma motion is actually dominated by acceleration rather than by a constant expansion velocity. The measured accelerations are among the highest acce...

Rotational and magnetic shear stabilization of magnetohydrodynamic modes and turbulence in DIII‐D high performance discharges

Abstract: The confinement and the stability properties of the DIII-D tokamak high performance discharges are evaluated in terms of rotational and magnetic shear with emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS region accompanied by a reduction in core fluctuation amplitudes. Increasing negative magnetic shear contributes to the formation of this core transport barrier, but by itself is not sufficient to fully stabilize the toroidal drift mode (trapped- electron-{eta}{sub i}mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth rate of the {eta}{sub i} mode suggests that the large core {bold E x B} flow shear can stabilize this mode and broaden the region of reduced core transport . Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by the resistive interchange mode to low {Beta}{sub N} {lt} 2.3. This mode is insensitive to the details of the rotational and the magnetic shear profiles. A new class of discharges which has a broad region of weak or slightly negative magnetic shear (WNS) is described. The WNS discharges have broader pressure profiles and higher values than the NCS discharges together with high confinement and high fusion reactivity.

Laser–plasma interactions in ignition‐scale hohlraum plasmas

Abstract: Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low‐Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high‐Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).