Ultracold plasmas are formed by ionizing laser-cooled gases. The spatial disorder in the initial plasma state gives rise to rapid ion heating called disorder-induced heating. The heating rate depends on the electron temperature because lower temperature electrons more effectively shield neighbouring ion charges from one another. We illustrate the effect of electron shielding on the ion thermalization rate in ultracold plasmas.
Ultracold plasmas are formed by ionizing laser-cooled gases. The spatial disorder in the initial plasma state gives rise to rapid ion heating called disorder-induced heating. The heating rate depends on the electron temperature because lower temperature electrons more effectively shield neighbouring ion charges from one another. We illustrate the effect of electron shielding on the ion thermalization rate in ultracold plasmas.
Ultracold neutral plasmas are created by photoionizing laser-cooled atoms in a magneto-optical trap (MOT). Due to their large electrical potential energies and comparatively small kinetic energies, ultracold plasmas fall into a regime of plasma systems which are called “strongly coupled.” A priority in the field of ultracold plasmas is to generate plasmas with higher values of the strong coupling parameter Γ, which is given as the ratio of the nearest-neighbor Coulomb potential energy to the average kinetic energy. The equilibrium strong coupling in ultracold plasmas is limited by the ultrafast relaxation of the ions due to spatial disorder in the initial system. This heating mechanism is called “disorder-induced heating” (DIH) and it limits the ion strong coupling in ultracold plasmas to order unity. This thesis describes experiments that explore ways to generate higher values of the strong coupling parameter in an ultracold neutral calcium plasma.One way to increase Γ is to mitigate the effects of DIH using electron screening. This thesis describes an experiment in which the initial electron temperature was systematically changed to determine the effect that electron screening has on the ion thermalization. At lower initial electron temperatures, corresponding to a higher degree of electron shielding, it was found that the screening slows the ion thermalization and reduces the equilibrium ion temperature by as much as a factor of two. However, electron screening also reduces the ion interaction strength by the same amount, which has the net effect of leaving the effective Γ unchanged.Another method for increasing the strong coupling of an ultracold plasma is to excite the plasma ions to a higher ionization state. Simulations predict that doubly ionizing the plasma ions can increase the strong coupling in an ultracold plasma by as much as a factor of 4, with the maximum value of Γ depending on the timing of the second ionization relative to the DIH process. This thesis describes an experiment designed to test these predictions in a Ca2+ plasma. Measurements of the change in the Ca+ ion temperature as a function of the timing of the second ionization pulses were made using laser-induced fluorescence. Results of these measurements show that the heating of the Ca+ ions due to the second ionization depends on the timing of the second ionization pulses, as predicted by MD simulations.
Disorder-induced heating (DIH) is a nonequilibrium, ultrafast relaxation process that occurs when laser-cooled atoms are photoionized to make an ultracold plasma. Its effects dominate the ion motion during the first 100 ns of the plasma evolution. Using tools of atomic physics we study DIH with ns time resolution for different plasma densities and temperatures. By changing the frequency of the laser beam we use to probe the ions, we map out the time evolution of the velocity distribution. We can compare this to a fluorescence simulation in order to more clearly determine the relationship between the fluorescence signal and the velocity distribution. In this study we observe and characterize effects due to electron screening on the ions during the equilibration process.
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.
It has recently become possible to create ultracold plasmas in the microkelvin temperature regime. Unfortunately, these plasmas are created in a disordered state and the build up of Coulomb correlations leads to rapid electron and ion heating that can be understood in terms of energy conservation. We explore this disorder–induced heating for the ionization of degenerate gases. In particular, we consider the ionization of fermionic gases, which are partially ordered by the Pauli exclusion principle, and the subsequent heating of the bosonic ions. The introduction of fermionic correlations mitigates the heating of the ions. However, we find from energy conservation that the final state for typical experimental situations is a classical plasma where the bosonic character of the particles is negligible.
Disorder-induced heating (DIH) is one of the main reasons reducing the coupling strength in ultracold plasma. We propose applying an optical lattice as periodic confinement before the ultracold atomic cloud is ionized to eliminate its effect. We demonstrate a numerical simulation for the dynamics of the ultracold plasmas using classical molecular dynamics method with open boundary. DIH is reproduced in the simulation with the random Gaussian initial distribution and is absent in the results with the ordered lattice initial distribution. We further find that the collisional heating from electrons is important for ultracold plasmas with chosen spatial correlations in the optical lattice. Carefully preparing the initial condition (e.g., the ion density, initial electron temperature, and so on), collisional heating for the ions would be significantly reduced and eventually negligible. This allows a much stronger coupling in ultracold plasma to be realized.