Search Results (31 total)

The first experimental verification of electron Bernstein wave (EBW) collisional damping, and its mitigation by evaporated Li conditioning, in an overdense spherical-tokamak plasma has been observed in the National Spherical Torus Experiment (NSTX). Initial measurements of EBW emission, coupled from NSTX plasmas via double-mode conversion to O-mode waves, exhibited <10% transmission efficiencies. Simulations show 80% of the EBW energy is dissipated by collisions in the edge plasma. Li conditioning reduced the edge collision frequency by a factor of 3 and increased the fundamental EBW transmission to 60%.

A random noise-induced beam degradation that could affect intense beam transport over long propagation distances has been experimentally investigated by making use of the transverse beam dynamics equivalence between an alternating-gradient focusing system and a linear Paul trap system. For the present study, machine imperfections in the quadrupole focusing lattice are considered, which are emulated by adding small random noise on the voltage waveform of the quadrupole electrodes in the Paul trap. It is observed that externally driven noise continuously increases the rms radius, transverse emittance, and nonthermal tail of the trapped charge bunch almost linearly with the duration of the noise. The combined effects of collective modes and colored noise are also investigated and compared with numerical simulations.

A random noise-induced beam degradation that can affect intense beam transport over long propagation distances has been experimentally studied by making use of the transverse beam dynamics equivalence between an alternating-gradient (AG) focusing system and a linear Paul trap system. For the present studies, machine imperfections in the quadrupole focusing lattice are considered, which are emulated by adding small random noise on the voltage waveform of the quadrupole electrodes in the Paul trap. It is observed that externally driven noise continuously produces a nonthermal tail of trapped ions, and increases the transverse emittance almost linearly with the duration of the noise.

The Paul trap simulator experiment is a compact laboratory Paul trap that simulates a long, thin charged-particle bunch coasting through a kilometers-long magnetic alternating-gradient (AG) transport system by putting the physicist in the beam’s frame of reference. The transverse dynamics of particles in both systems are described by similar equations, including all nonlinear space-charge effects. The time-dependent quadrupolar electric fields created by the confinement electrodes of a linear Paul trap correspond to the axially dependent magnetic fields applied in the AG system. Results are presented for experiments in which the lattice period and strength are changed over the course of the experiment to transversely compress a beam with an initial depressed tune of 0.9. Instantaneous and smooth changes are considered. Emphasis is placed on determining the conditions that minimize the emittance growth and the number of halo particles produced by the beam compression process. Both the results of particle-in-cell simulations performed with the warp code and envelope equation solutions agree well with the experimental data.

The transverse compression of a long charge bunch is investigated in the Paul trap simulator experiment (PTSX), which is a linear Paul trap that simulates the nonlinear transverse dynamics of an intense charged particle beam propagating through an equivalent kilometers-long magnetic alternating-gradient (AG) focusing system. Changing the voltage amplitude at fixed focusing frequency in the PTSX device corresponds to changing the field gradient of the quadrupole magnets with fixed axial periodicity in the AG transport system. In this work, we present experimental results on transverse compression of the charge bunch in which the amplitude of the applied oscillatory focusing voltage is changed instantaneously, and adiabatically. The experimental data are also compared with analytical estimates and 2D WARP particle-in-cell simulations.

The Paul Trap Simulator Experiment (PTSX) is a linear Paul trap whose purpose is to simulate the nonlinear transverse dynamics of intense charged particle beam propagation in periodic-focusing quadrupole magnetic transport systems. Externally created cesium ions are injected and trapped in the long central electrodes of the PTSX device. In order to have well-matched one-component plasma equilibria for various beam physics experiments, it is important to optimize the ion injection. From the experimental studies reported in this paper, it is found that the injection process can be optimized by minimizing the beam mismatch between the source and the focusing lattice, and by minimizing the number of particles present in the vicinity of the injection electrodes when the injection electrodes are switched from the fully oscillating voltage waveform to their static trapping voltage.

Heavy ion drivers for heavy ion fusion and high energy density physics applications use space-charge-dominated ion beams which must undergo longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The Neutralized Drift Compression Experiment-1A (NDCX-1A) at Lawrence Berkeley National Laboratory is used to determine the effective limits of neutralized drift compression, which occurs due to an imposed longitudinal velocity tilt on the drifting beam and subsequent neutralization of the beam’s space charge with background plasma. The accurate and temporally resolved measurement of the ion beam’s current and pulse length, which has been longitudinally compressed to a few nanoseconds duration at its focal plane, is a critical diagnostic. This paper describes the design and experimental results for a fast and accurate ion beam probe, which reliably measures the absolute beam current in the presence of high density plasma at the focal plane as a function of time. A particle-in-cell code has been used to model the propagation of the intense ion beam and to design the diagnostic probe.

Longitudinal compression of a velocity-tailored, intense neutralized K⁺ beam at 300 keV, 25 mA has been demonstrated. The compression takes place in a 1–2 m drift section filled with plasma to provide space-charge neutralization. An induction cell produces a head-to-tail velocity ramp that longitudinally compresses the neutralized beam, enhancing the beam peak current by a factor of 50 and producing a pulse duration of about 3 ns. This measurement has been confirmed independently with two different diagnostic systems.

In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final-focus magnet system through the fusion chamber to hit spots on the target with radii of about 2 mm. For the heavy-ion-fusion power-plant scenarios presently favored in the U.S., a substantial fraction of the ion-beam space charge must be neutralized during this final transport. The most effective neutralization technique found in numerical simulations is to pass each beam through a low-density plasma after the final focusing. To provide quantitative comparisons of these theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the neutralized-transport experiment. The experiment consists of three main sections, each with its own physics issues. The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam, while still allowing variable perveance by a beam aperturing technique. The magnetic-focusing section, consisting of four pulsed quadrupoles, permits the study of magnet tuning, as well as the effects of phase-space dilution due to higher-order nonlinear fields. In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment.

The results presented here demonstrate that the Paul trap simulator experiment (PTSX) simulates the propagation of intense charged particle beams over distances of many kilometers through magnetic alternating-gradient (AG) transport systems by making use of the similarity between the transverse dynamics of particles in the two systems. Plasmas have been trapped that correspond to normalized intensity parameters s-^ =ω_p²(0)/2ω_q²≤0.8, where ω_p(r) is the plasma frequency and ω_q is the average transverse focusing frequency in the smooth-focusing approximation. The measured rms radius of the beam is consistent with a model, equally applicable to both PTSX and AG systems. The PTSX device confines one-component cesium ion plasmas for hundreds of milliseconds, which is equivalent to over 10 km of beam propagation.

In the CDX-U spherical torus, agreement between radiation temperature and Thomson scattering electron temperature profiles indicates ∼100% conversion of thermally emitted electron Bernstein waves to the X mode. This has been achieved by controlling the electron density scale length (L_n) in the conversion region with a local limiter outside the last closed flux surface, shortening L_n to the theoretically required value for optimal conversion. From symmetry of the conversion process, prospects for efficient coupling in heating and current drive scenarios are strongly supported.

Alpha-particle-driven toroidal Alfvén eigenmodes (TAEs) have been observed for the first time in deuterium-tritium (D-T) plasmas on the tokamak fusion test reactor (TFTR). These modes are observed 100–200 ms following the end of neutral beam injection in plasmas with reduced central magnetic shear and elevated central safety factor [q(0)>1]. Mode activity is localized to the central region of the discharge (r/a<0.5) with magnetic fluctuation level B̃_⊥/B_∥∼10^-5 and toroidal mode numbers in the range n  =  2–4, consistent with theoretical calculations of α-TAE stability in TFTR.

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×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.

Helium ash production and transport have been measured in TFTR deuterium-tritium plasmas using charge-exchange recombination spectroscopy. The helium ash confinement time, including recycling effects, is 6-10 times the energy confinement time and is compatible with sustained ignition in a reactor. The ash confinement time is dominated by edge pumping rates rather than core transport. The measured evolution of the local thermal ash density is consistent with modeling based on previously measured helium transport coefficients and classical slowing down of the alpha particles.

The t(d,n)α and d(d,n)³He neutron emissivity profiles are measured in a deuterium neutral-beam–heated plasma where a small amount of tritium (T) gas has been puffed. The tritium density is inferred from the neutron emissivities, and transport coefficients ( D,V) are determined. The particle diffusivities of T and ⁴He and the thermal diffusivity are similar in magnitude and profile shape. The convective velocity is small for r/a<0.6, and is anomalous for r/a>0.6. These are the first measurements of D and V for a hydrogen isotope in a tokamak plasma.

The Tomamak Fusion Test reactor has performed initial high-power experiments with the plasma fueled with nominally equal densities of deuterium and tritium. Compared to pure deuterium plasmas, the energy stored in the electron and ions increased by ∼20%. These increases indicate improvements in confinement associated with the use of tritium and possibly heating of electrons by α particles created by the D-T fusion reactions.

Peak fusion power production of 6.2±0.4 MW has been achieved in TFTR plasmas heated by deuterium and tritium neutral beams at a total power of 29.5 MW. These plasmas have an inferred central fusion alpha particle density of 1.2×10¹⁷ m^-3 without the appearance of either disruptive magnetohydrodynamics events or detectable changes in Alfvén wave activity. The measured loss rate of energetic alpha particles agreed with the approximately 5% losses expected from alpha particles which are born on unconfined orbits.

Local particle and heat transport coefficients have been measured in a temperature scan of neutral-beam–heated plasmas with n, I_p, and B_cphi held constant. The electron transport is ascertained from a flux analysis of a small density perturbation, and the heat transport is obtained from the equilibrium power balance. The transport coefficients vary as T_e^α, where α=1.5–2.5. The observed temperature dependence is predicted by numerical calculations of anomalous transport due to trapped-particle drift-type microinstabilities.

Measurements of the spatial structure of the transport coefficients of He²⁺ on the Tokamak Fusion Test Reactor is reported. He²⁺ profiles were measured using charge-exchange spectroscopy after a helium puff into corotating L-mode plasmas. Modeling shows that the He²⁺ diffusivity is about 10 m²/s near the plasma edge and drops to below 1 m²/s inside of the q=1 surface. The convective velocity ranges from 1–3 m/s near q=1 to 20–40 m/s near the edge. The helium diffusivity is on the order of the ion momentum and thermal diffusivity and is greater than the electron thermal diffusivity.

Circular-limiter H modes are obtained on the TFTR tokamak during high-power neutral-beam heating. The transition is usually from the supershot to the H mode rather than the usual L to H transition, and thus is obtained in a low-recycling environment with core fueling mainly from the heating beams. As a result, the density and pressure profiles are highly peaked at the center. The global confinement time τ_E is enhanced over L-mode scaling by up to ≊2.5 times. The onset of edge-localized MHD modes shortly after the H-mode transition appears to limit τ_E. Limiter H modes of up to 1.5 s duration have been realized.

A coupled set of equations, one describing the time evolution of the ordinary-mode wave energy and the other describing the time evolution of the electron distribution function, is presented. The wave damping is mainly determined by T_?, while the radiative equilibrium is mainly an equipartition with T_⊥. The time rate of change of T_⊥, T_?, particle density (N₀), and current density (J_?) are examined for finite-k_? electron-cyclotron-resonance heating of plasmas. The effects of collisional broadening and collisional damping are also examined. For blackbody absorbing conditions it is shown that the increase of T_⊥ with time in electron-cyclotron-resonance heating is exponential and not linear. From the quasilinear theory it is found that the Ohkawa steady-state current drive efficiency criterion is really a consequence of the conservation laws of energy, momentum, particle density, and the collisional relaxation of the current density.

The Gaussian broadening of absorption lines due to the Doppler effect is well known. However, it is shown here for the first time that Doppler splitting occurs for the ordinary-mode electron-cyclotron-resonance absorption in plasmas. Although this absorption is due to the finite size of the electron Larmor orbits it is mainly determined by T_? and is only weakly dependent on T_⊥ via cyclotron-overstability-type terms. This is in contrast to intuitive expectations which would suggest that finite-Larmor-radius effects should depend strongly on T_⊥.

Neutral-beam heating of plasmas in the Tokamak Fusion Test Reactor at low preinjection densities [n_e(0)≃10¹⁹ m^-3] were characterized by T_e(0)=6.5 keV, T_i(0)=20 keV, n_e(0)=7×10¹⁹ m^-3, τ_E=170 msec, β_theta=2, and a d(d,n)³He neutron emission rate of 10¹⁶ sec^-1. The ion temperature and the deuterium-fusion neutron yields were significantly higher than for previous tokamak experiments. The low initial densities were achieved by operation of the Tokamak Fusion Test Reactor with low plasma currents (≤1 MA) and by extensive limiter conditioning.

Tangentially coinjected deuterium beam ions were accelerated from 82 up to 150 keV during a major-radius compression experiment in the tokamak fusion test reactor. The ion energy spectra and the variation in fusion yield were in good agreement with Fokker-Planck code simulations. In addition, the plasma rotation velocity was observed to rise during compression.