Search Results (20 total)

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.

Extensive lithium wall coatings and liquid lithium plasma-limiting surfaces reduce recycling, with dramatic improvements in Ohmic plasma discharges in the Current Drive Experiment-Upgrade. Global energy confinement times increase by up to 6 times. These results exceed confinement scalings such as ITER98P(y,1) by 2–3 times, and represent the largest increase in energy confinement ever observed for an Ohmic tokamak plasma. Measurements of D_α emission indicate that global recycling coefficients decrease to approximately 0.3, the lowest documented for a magnetically confined hydrogen plasma.

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.

A correlation is explored between the presence of energetic particle modes (EPM) and long-period sawtooth oscillations in tokamak plasmas heated by rf waves. The eventual crash of these sawteeth is explained in terms of the loss of the stabilizing fast particles due to the EPM. The absence of long-period sawteeth in high q_a discharges is explained in terms of ion loss due to toroidal Alfvén eigenmodes.

Shearing of the plasma poloidal rotation velocity was observed during application of ion Bernstein wave power in the Tokamak Fusion Test Reactor. The first evidence of corroboration between measured poloidal velocity shearing and actively induced Reynolds stress effects is presented. A model reproduces salient experimental features: The observed sheared flow occurs near the tritium fifth harmonic cyclotron resonance layer and depends strongly on the tritium density in agreement with the model. Furthermore, the model reproduces the observed insensitivity of the induced rotation to the tritium density in the region between the third deuterium harmonic layer and the fifth tritium harmonic layer.

Global energy confinement time in plasmas heated with waves in the ion cyclotron range of frequencies in the Tokamak Fusion Test Reactor is 8%–11% higher in deuterium-tritium (D-T) than in deuterium (D) only plasmas. Kinetic analysis based on local equilibrium profile measurements indicates that the increase in stored energy is almost entirely in the electron component and that the electron thermal diffusivity χ_e is smaller in D-T than in D plasmas. The implied scaling of χ_e with normalized ion gyro radius ρ_* is opposite to that observed in Joint European Torus (JET) and DIII-D tokamaks when ρ_* was varied by changing the magnetic field strength.

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 alpha particle effect on the excitation of toroidal Alfvén eigenmodes (TAE) was investigated in deuterium-titrium (D-T) plasmas in the Tokamak Fusion Test Reactor. rf power was used to position the plasma near the instability threshold, and the alpha particle effect was inferred from the reduction of rf power threshold for TAE instability in D-T plasmas. Initial calculations indicate that the alpha particles contribute 10%–30% of the total drive in a D-T plasma with 3 MW of peak fusion power.

The first experimental demonstration that mode conversion from the fast magnetosonic wave to an ion Bernstein wave can efficiently heat electrons and drive current with low field side antennas in a tokamak plasma is reported. Up to 130 kA of current was noninductively driven, on and off axis, and the resultant current profiles were measured in the Tokamak Fusion Test Reactor. In heating experiments, 10 keV peak electron temperatures were produced with 3.3 MW of radio-frequency heating power.

Experiments have been performed on the TFTR to study rf wave heating of a D-T plasma by way of the second-harmonic tritium cyclotron resonance. The addition of tritium ions to a deuterium plasma allows for absorption of the rf waves at the tritium cyclotron harmonics and by electron damping of a mode converted ion Bernstein wave. Competing mechanisms include direct electron damping and damping at the fundamental cyclotron resonance of deuterium, α particles, and ³He ions. The contribution of each is estimated from a series of plasma discharges where various plasma parameters are varied. The majority of the rf power is found to damp on the tritium ions.

An approach to obtaining efficient single-pass-mode conversion at high parallel wave number from the fast magnetosonic wave to the slow ion Bernstein wave, in a two-ion species tokamak plasma, is described. The intent is to produce localized electron heating or current drive via the mode-converted slow wave. In particular, this technique can be adapted to off-axis current drive for current profile control. Modeling for the case of deuterium-tritium plasmas in TFTR is presented.

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.

It is shown that wave eigenmode fields can provide rf stabilization of the interchange instability. The excitation level of an m=+1 fast magnetosonic wave is controlled by tailoring of the k_? spectrum of an axial ion-cyclotron radio-frequency antenna array in the Phaedrus-B tandem mirror system. When the k_? spectrum is chosen to excite the wave strongly, density-fluctuation levels in the plasma are greatly reduced.

Suppression of the drift-cyclotron loss-cone instability (DCLC) in an axisymmetric mirror plasma has been observed for frequencies in the vicinity of the bounce frequency of electrostatically trapped thermal electrons. The location and width of the frequency region of suppression is in good agreement with calculations of bounce-resonance Landau damping.

The current-driven electrostatic ion-cyclotron instability in a Q machine operated in the low-density regime (ω_pe / ω_ce)²≪1 is stabilized by externally generated rf electrostatic fields in the lower-hybrid range. Stabilization is due to a resonant ponderomotive force, which reduces the instability frequency, thus increasing the ion-cyclotron damping. The experimental results are in good agreement with an analysis which describes the effect of rf power on the instability frequency shift and growth rate.