Heating mechanisms in rf-driven ultracold plasmas
Straten, KWCA van der
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.
•Jan 1, 2013•0 citations