The electron distribution function and diffusion coefficient in energy space have been calculated for the first time for a weakly coupled ultracold plasma in a magnetic field in the range of magnetic fields B = 100−50000 G for various temperatures. The dependence of these characteristics on the magnetic field is analyzed and the distribution function is shown to depend on the electron energy shift in a magnetic field. The position of the “bottleneck” of the distribution function has been found to be shifted toward negative energies with increasing magnetic field. The electron velocity autocorrelators as a function of the magnetic field have been calculated; their behavior suggests that the frequency of collisions between charged particles decreases significantly with increasing magnetic field. The collisional recombination coefficient α B has been calculated in the diffusion approximation for a weakly coupled ultracold plasma in a magnetic field. An increase in magnetic field is shown to lead to a decrease in α B and this decrease can be several orders of magnitude.
The electron distribution function and diffusion coefficient in energy space have been calculated for the first time for a weakly coupled ultracold plasma in a magnetic field in the range of magnetic fields B = 100−50000 G for various temperatures. The dependence of these characteristics on the magnetic field is analyzed and the distribution function is shown to depend on the electron energy shift in a magnetic field. The position of the “bottleneck” of the distribution function has been found to be shifted toward negative energies with increasing magnetic field. The electron velocity autocorrelators as a function of the magnetic field have been calculated; their behavior suggests that the frequency of collisions between charged particles decreases significantly with increasing magnetic field. The collisional recombination coefficient α B has been calculated in the diffusion approximation for a weakly coupled ultracold plasma in a magnetic field. An increase in magnetic field is shown to lead to a decrease in α B and this decrease can be several orders of magnitude.
The distribution functions and diffusion coefficient of electrons in the Rydberg energy space in an ultracold plasma are obtained as functions of temperature by numerically solving a system of kinetic balance equations. As the initial conditions, the results obtained in previous papers by the molecular dynamics method are used. From calculation of Rydberg electron fluxes, the temperature dependence of the recombination coefficient is obtained, in good agreement with the recombination coefficient calculated previously by the molecular dynamics method.
Plasmas, often called the fourth state of matter and the most common one in the universe, have parameters varying by many orders of magnitude, from temperature of a few hundred kelvin in the Earth's ionosphere to 10 16 K in the magnetosphere of a pulsar. Ultracold plasmas, produced by photoionizing a sample of laser-cooled and trapped atoms near the ionization limit, have extended traditional neutral plasma parameters by many orders of magnitude, to electron temperatures below 1 K and ion temperatures in the tens of &mu K to a few Kelvin, and densities of 10 5 cm -3 to 10 10 cm -3 . These plasmas thus provide a testing ground to study basic plasma theory in a clean and simple system with or without a magnetic field. Previous studies of ultracold plasmas have primarily concentrated on temperature measurements, collective modes and expansion dynamics in the absence of magnetic fields. This thesis presents the first study of ultracold plasma dynamics in a magnetic field. The presence of a magnetic field during the expansion can initiate various phenomena, such as plasma confinement and plasma instabilities. While the electron temperatures are very low in ultracold plasmas, we need only tens of Gauss of magnetic field to observe significant effects on the expansion dynamics. To probe the ultraocold plasma dynamics in a magnetic field, we developed a new diagnostic - projection imaging, which images the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (< 20 &mu s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G),and observe that the transverse expansion velocity scales as B &minus1/2 , explained by a nonlinear ambipolar diffusion model that involes anisotropic diffusion in two different directions. We also present the first observation of a plasma instability in an expanding ultracold plasma. We observe periodic emission of electrons from an ultracold plasma in weak, crossed magnetic and electric fields, and a strong perturbed electron density distribution in electron time-of-flight projection images. We identify this instability as a high-frequency electron drift instability due to the coupling between the electron drift wave and electron cyclotron harmonic, which has large wavenumbers corresponding to wavelengths close to the electron gyroradius.
Electrons in an expanding ultracold plasma are expected to be in quasiequilibrium since the collision times are short compared to the plasma lifetime, yet electron evaporation is observed as the ion density decreases during expansion. A small electric field that shifts the electron cloud with respect to the ions increases the evaporation rate. Treating the electrons as a zero-temperature fluid, their spatial distribution is calculated as a function of the applied field strength and the ion density. The zero-temperature approximation gives the maximum number of electrons the plasma can hold in the absence of evaporation. By applying this calculation at all times during expansion, the flux of cold electrons from the plasma is predicted and found to be in good agreement with the observed electron signal.
Ultracold plasmas (UCPs) are created by photo-ionization of a cloud of laser-cooled atoms, and have initial electron temperatures in the range 1-100 K and initial ion temperatures in the range 0.001-1 K. As a consequence UCPs can be in the strongly coupled regime, where the typical Coulomb interaction between the particles exceeds the thermal energy of the particles; a clear distinction with conventional plasmas. UCPs are not stable systems and the electron and ion temperature will rise during their evolution. The introduction of a disturbing rf-field to the plasma is expected to speed up the heating of the plasma.In this thesis the intrinsic electron heating mechanisms are studied as well as the heating mechanisms induced by an external rf field. Numerical simulations were performed with the General Particle Tracer code and compared to analytical theories.Two intrinsic heating mechanisms were studied: disorder-induced heating (DIH) and heating by three-body recombination (TBR). DIH arises due the random initial positions of the electrons. An excess of potential energy exists in the electron distribution which is rapidly converted into thermal energy. The time scale of DIH was found to be on the order of the inverse Mie-frequency, confirming analytical theories. TBR was identified in the simulations and the TBR heating rate was found to agree well with analytical models. Two rf-induced heating mechanisms were studied: collisionless energy absorption and collisional absorption. The first of these processes was found to depend strongly on the ratio between the rf field frequency and the oscillation frequency of the electrons in the plasma. Collisional absorption was studied in a regime that collisionless absorption is negligible. Absorption arises because the electrons, which are oscillating at the rf field frequency, are deflected by the Coulomb fields of the ions. The amount of collisional absorption was found to depend strongly on the amplitude of the rf field. It was found that the rf field effectively suppresses the electron-ion collision frequency as a function of increasing field amplitude, confirming analytical theories. For low to moderate amplitudes the amount of energy absorption increases, but less than one might intuitively expect due to the decrease in collision frequency. For very strong field amplitudes the energy absorption even decreases as a function of field amplitude.The understanding achieved, provides new roads to control these type of plasmas experimentally. Ultracold plasmas (UCPs) are created by photo-ionization of a cloud of laser-cooled atoms, and have initial electron temperatures in the range 1-100 K and initial ion temperatures in the range 0.001-1 K. As a consequence UCPs can be in the strongly coupled regime, where the typical Coulomb interaction between the particles exceeds the thermal energy of the particles; a clear distinction with conventional plasmas. UCPs are not stable systems and the electron and ion temperature will rise during their evolution. The introduction of a disturbing rf-field to the plasma is expected to speed up the heating of the plasma.In this thesis the intrinsic electron heating mechanisms are studied as well as the heating mechanisms induced by an external rf field. Numerical simulations were performed with the General Particle Tracer code and compared to analytical theories.Two intrinsic heating mechanisms were studied: disorder-induced heating (DIH) and heating by three-body recombination (TBR). DIH arises due the random initial positions of the electrons. An excess of potential energy exists in the electron distribution which is rapidly converted into thermal energy. The time scale of DIH was found to be on the order of the inverse Mie-frequency, confirming analytical theories. TBR was identified in the simulations and the TBR heating rate was found to agree well with analytical models. Two rf-induced heating mechanisms were studied: collisionless energy absorption and collisional absorption. The first of these processes was found to depend strongly on the ratio between the rf field frequency and the oscillation frequency of the electrons in the plasma. Collisional absorption was studied in a regime that collisionless absorption is negligible. Absorption arises because the electrons, which are oscillating at the rf field frequency, are deflected by the Coulomb fields of the ions. The amount of collisional absorption was found to depend strongly on the amplitude of the rf field. It was found that the rf field effectively suppresses the electron-ion collision frequency as a function of increasing field amplitude, confirming analytical theories. For low to moderate amplitudes the amount of energy absorption increases, but less than one might intuitively expect due to the decrease in collision frequency. For very strong field amplitudes the energy absorption even decreases as a function of field amplitude.The understanding achieved, provides new roads to control these type of plasmas experimentally.
Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼10 10 cm −3 ) are investigated. The measured dependence on initial ion density is N 1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.