Ultracold plasmas (UCPs) represent a marriage between the seemingly distinct fields of cold atom physics and plasma physics. In cold atom physics, dilute clouds of atoms can be trapped with laser radiation and cooled to J.1Ktemperatures. In contrast, in plasma physics the constituents generally have temperatures in the 1,000K-1O,000K range as well as much higher density. In 1999,Rolston and co-workers [1] first converted a cloud of laser-cooled atoms into a plasma by photo-ionizing the atoms just above threshold. This provides very unusual circumstances:the temperature of ions and neutrals is O.OOIK,that of electrons 10K! Such ultra-cold plasmas therefore are interesting subjects for fundamentalplasma physics studies.\nIn a recent paper [2], we argued that ultracold plasmas (UCPs) may in addition provide an entirely new way of generatinghigh-brightnesscharged-particlebeams, such as used in focused-ion beams, laser synchrotrons, and ultra-fast electron microscopy.\nHere, a low temperature T is beneficial for the intrinsic (transverse) brightness B of a\nsource since B = II(A1), where I is the current and A the cross-sectional area. I will discuss an experimental investigation of the properties of ion and electron beams extracted from ultracold plasmas, which shows that such beams indeed have low temperatures, and, as a result, high angular intensity.
Ultracold plasmas (UCPs) represent a marriage between the seemingly distinct fields of cold atom physics and plasma physics. In cold atom physics, dilute clouds of atoms can be trapped with laser radiation and cooled to J.1Ktemperatures. In contrast, in plasma physics the constituents generally have temperatures in the 1,000K-1O,000K range as well as much higher density. In 1999,Rolston and co-workers [1] first converted a cloud of laser-cooled atoms into a plasma by photo-ionizing the atoms just above threshold. This provides very unusual circumstances:the temperature of ions and neutrals is O.OOIK,that of electrons 10K! Such ultra-cold plasmas therefore are interesting subjects for fundamentalplasma physics studies.\nIn a recent paper [2], we argued that ultracold plasmas (UCPs) may in addition provide an entirely new way of generatinghigh-brightnesscharged-particlebeams, such as used in focused-ion beams, laser synchrotrons, and ultra-fast electron microscopy.\nHere, a low temperature T is beneficial for the intrinsic (transverse) brightness B of a\nsource since B = II(A1), where I is the current and A the cross-sectional area. I will discuss an experimental investigation of the properties of ion and electron beams extracted from ultracold plasmas, which shows that such beams indeed have low temperatures, and, as a result, high angular intensity.
Ultra-cold plasmas with electron temperatures of ~10 K can be created by photo-ionization just above threshold of a cloud of laser-cooled atoms. Recently it was shown 7 by GPT particle tracking simulations that an ultra-cold plasma has an enormous potential as a pulsed bright electron source. Here we discuss these results in the framework of normalized 6D brightness, which allows us to make a proper comparison both with the performance of pulsed, radio-frequency photo-emission sources and with the performance of continuous, needle-like field-emission sources. In addition we speculate on the possibility of using ultra-cold plasmas to realize quantum degenerate electron beams, constituting the ultimate limit in electron beam brightness.
By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.
In a Magneto-Optical Trap (MOT), realized for the first time in 1987, one can trap and cool neutral atoms to temperatures below a mK. The invention of this device caused a revolution in atomic physics. With an MOT collision and spectroscopy experiments could be performed with unprecedented accuracy. The main breakthrough came about 10 years later when the atom from an MOT were used to create a Bose Einstein Condensate (BEC), a source of coherent matter. All this work has been rewarded with no less than two Nobel prizes: in 1997, Chu, Cohen-Tannoudji and Phillips received the Nobel prize for the practical and theoretical development of laser cooling, and in 2001 Cornell, Ketterle and Wieman received the Nobel prize for the realization of a Bose Einstein Condensate in a dilute gas. In 1999 Killian et al. took trapped atoms in an entirely new direction when they succeeded in creating the very first Ultracold Plasma (UCP) from the atoms in an MOT. The temperature of such a plasma is more than 1000 times lower than the temperatures in conventional cold plasmas. As a result, this type of plasmas received a lot of attention, both theoretically and experimentally. So far studies of UCPs have mainly been of a fundamental kind. In this thesis we study both the fundamental aspects of atoms excited from a magneto-optical as well as the feasibility of using the electrons and ions from a UCP as a source for charged particle accelerators. The work in this thesis therefore pioneers the line between atom physics, plasma physics and accelerator physics. The first part of this thesis deals with an experimental study of the effect of an ionizing laser on cold neon atoms. Here we study the loss of atoms from a Magneto-Optical Trap caused by photionization. From these measurements we deduce a new value for the near threshold photoionization cross section 2:05±0:25x10-18 cm2 at ¿ = 351 nm and 2:05±0:25x10-18 cm2 at ¿ = 364 nm, which is a factor of four more accurate than previous measurements. The measured valuesagree with earlier measurements but differ signi¯cantly from theoretical values. The value of this cross section has a practical relevance for the realization of a continuous ion source. The second part of this thesis has been conducted at the University of Virginia. We study the role of dipole-dipole interactions in the formation of a UCP from a gas of cold Rydberg atoms, and the effect of dipole-dipole interactions on the broadening of microwave transitions. We find that the broadening scales linearly with the density en scales with the fourth power of n, the principal quantum number. These results show the great importance of dipole-dipole interactions in gas of cold Rydberg atoms and are also relevant for the possible realization of a quantum computer from Rydberg atoms. In a third part we studied the potential of using a UCP as a source for electron accelerators. This was done based upon elaborate simulations performed with the General Particle Tracer Code. The main result of these simulations is that, based upon realistic parameters, the brightness, i.e., the figure of merit for charged particle sources defined as the current divided by the emittance squared, of such a source can be up to two orders of magnitude larger than the brightness of conventional sources, which is a major advance. The emittance is a measure for the the focusability of the bunch, and scales as p T, with T the electron temperature. The increase in brightness is mainly attributed to the low electron temperatures of approximately 10 K in an UCP, resulting in an initial emittance of 0.01 mm mrad, compared to 1 mm mrad in a conventional source. With experimentally proven techniques it is possible to extract 10 nC/s from the source, which is a competitive repetition rate. In the next part we describe the design, construction and testing of the first setup with which we want to build an electron accelerator based upon cold atoms. For this we first built a vapor-cell magneto-optical trap, in which we can trap over 109 rubidium atoms with a temperature of 200 ¹K, straight from a background vapor, we also designed an atomic beam that will be implemented in the near future and can provide an increased repetition rate. In the last chapter we show how Rydberg atoms excited from an MOT, evolve into an UCP spontaneously. We extracted a minimum of 1 pC from the plasma. As a main result, we were one of the ¯rst to show how the ions and electrons from a UCP can be imaged on a phosphor screen. From these images we can deduct an upper value for the electron and the ion temperature, we extract for both the electrons and the ions an upper temperature of 40 K, when we translate this temperature to an initial emittance, we obtain a value of 0.01 mm mrad, proving the potential of the electrons extracted from atoms excited out of an MOT as a low emittance source. Our measured current of 1 ¹A is not yet competitive and results from the small electric field, but is expected to increase dramatically in the near future. We will implement a 30 kV 10 ns risetime accelerator with which it should be possible to get a current of about 50 mA. Furthermore we will implement the proper diagnostics to measure the proper charge, current and emittance. In the very last chapter we speculate charged particle sources from cold atoms might advance the state of the art in achievable brightness for both electron and ion beams.
Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1-1000 K and the ion temperature is around 1 K. The density can approach 1011 cm−3. Fundamental interest stems from the possibility of creating strongly-coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+2S1/2−>2P1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.
A novel way of creating low-temperature electron and ion beams is demonstrated. The beams are generated by converting a laser-cooled atom cloud to a highly excited Rydberg gas, which subsequently develops into an ultracold plasma. Charged particles are extracted from the Rydberg gas and the plasma by a pulsed electric field. The properties of the resulting electron and ion pulses are experimentally studied. Pulses of a few hundred ns duration containing a few pC of charge were observed. Upper limits for the temperature of such beams (60K for ions and 500K for electrons) are obtained, and the beams are shown to have low emittance. Further development of the method may lead to the generation of high-brightness charged-particle beams from ultracold plasmas.
Electron microscopy and diffraction have long been used to study the structure of materials. A recent development is the achievement of ultrafast time resolution using high-repetition rate single electron sources and single-shot many-electron sources. Achieving large coherence lengths under these conditions is challenging due to intrinsic heating mechanisms including inter-particle Coulombic forces. Near-threshold, pulsed photoionization of atoms trapped and cooled with lasers is being investigated as a means to produce intense electron pulses with large intrinsic coherence length due to a low electron temperature. Photo-ionizaton sources are, of course, not an entirely new idea but recent implementations using trapped atoms offer new possibilities. Several properties of electron beams extracted from such ultracold sources have already been investigated, showing in particular that temperatures of a few Kelvin and emittances in the nanometer-radian range can be achieved with tens of thousands electrons per pulse for nanosecond pulse lengths. This can be combined with delicate shaping of the charge distribution to counteract emittance growth due to non-linear self fields. Using ultrafast lasers for the ionization step, shorter electron pulses can be achieved, but an interesting question is whether the concomitant spread in photon energy still allows for low electron temperature. Recent experiments have shown that few-Kelvin temperatures combined with picosecond pulse lengths are indeed possible. It turns out that this can be understood with a classical model for electron trajectories resulting from photoionization. In addition, experimentally observed polarization effects are in general agreement with model results. --------------------------------------------------------------------------------