Session PI3: Beams, Traps, and Radiation

Antihydrogen formation by autoresonant excitation of antiproton plasmas

In efforts to trap antihydrogen, a key problem is the vast disparity between the neutral trap energy scale ($\sim 50\,\mu\mathrm{eV}$), and the energy scales associated with plasma confinement and space charge ($\sim 1\,\mathrm{eV}$). In order to merge charged particle species for direct recombination, the larger energy scale must be overcome in a manner that minimizes the initial antihydrogen kinetic energy. This issue motivated the development of a novel injection technique utilizing the nonlinear nature of particle oscillations in our traps. We demonstrated controllable excitation of the center-of-mass longitudinal motion of a cold, dense antiproton plasma using a swept-frequency autoresonant drive. Antihydrogen was produced and trapped by using this technique to drive antiprotons into a positron plasma, thereby initiating atomic recombination. The nature of this injection overcomes some of the difficulties associated with matching the energies of the charged species used to produce antihydrogen. We present measurements and simulations that probe the dynamics of this mixing process as well as adjustments to the drive aimed at improving the likelihood of producing trapped antihydrogen.   

The Creation and Diagnosis of Hot Solid-Density Plasmas with an X-ray Free-Electron Laser

We report on the first experimental investigation of the detailed interaction process of an intense short-pulse X-ray beam with solid density matter, conducted on the Linac Coherent Light Source X-ray laser at SLAC National Accelerator Laboratory. In the experiment, we have focused the X-ray beam to micron-size spots on a thin aluminum foil, thereby heating it to temperatures up to 200 eV. This is achieved on the time scale of the excitation pulse, of order 80 fs FWHM, far quicker than the time necessary for electron-ion coupling and hydrodynamic expansion, so that a hot plasma is created at solid densities. We have studied this system by X-ray K-alpha spectroscopy for a range of excitation photon energies above the cold aluminum K-edge. Detailed simulations of the interaction process were conducted with the radiative-collisional code SCFLY, illustrating a good overall agreement with the experimental spectra, and, importantly, also providing insight into the evolution of the charge state distribution within the sample, the electron density and temperature, and the ionization potential depression. In particular, we note that the spectra observed in emission and the fundamental properties of the system are not trivially related, but are rather complicated by the intense FEL pulse, which acts both as a heating source as well as a low-bandwidth probe. We discuss our experimental results in light of these observations and their implications for the study of X-ray laser-generated dense plasmas.   

Precision Laser and Linac Technologies for Nuclear Photonics Gamma-Ray Sources

Tunable, high-precision gamma-ray sources are under development to enable nuclear photonics, an emerging field of research. This presentation focuses on the theoretical and technological challenges related to precision Compton scattering gamma-ray sources. In this scheme, incident laser photons are scattered and Doppler upshifted by a high-brightness electron beam to generate tunable and highly collimated gamma-ray pulses. The electron and laser beam parameters can be optimized to achieve the spectral brightness and narrow bandwidth required by nuclear photonics applications. detailed presentation of the Compton scattering mechanism and theoretical modeling will be followed by a description of the design of the next generation precision gamma-ray source currently under construction at LLNL. Within this context, high-gradient X-band technology used in conjunction with fiber-based photocathode drive laser and diode pumped solid-state interaction laser technologies, will be shown to offer optimal performance.   

Enhanced collective focusing of intense neutralized ion beam pulses in the presence of weak solenoidal magnetic fields

The design of ion drivers for warm dense matter and high energy density physics applications and heavy ion fusion involves transverse focusing and longitudinal compression of intense ion beams to a small spot size on the target. To facilitate the process, the compression occurs in a long drift section filled with a dense background plasma, which neutralizes the intense beam self-fields. Typically, the ion bunch charge is better neutralized than its current, and as a result a net self-pinching (magnetic) force is produced. The self-pinching effect is of particular practical importance, and is used in various ion driver designs in order to control the transverse beam envelope. In the present work we demonstrate that this radial self-focusing force can be significantly enhanced if a weak ($\sim $100 G) solenoidal magnetic field is applied inside the neutralized drift section, thus allowing for substantially improved transport. It is shown that in contrast to the magnetic self-pinching, the enhanced collective self-focusing has a radial electric field component and occurs as a result of the overcompensation of the beam charge by plasma electrons, whereas the beam current becomes well-neutralized. As the beam leaves the neutralizing drift section, additional transverse focusing can be applied. For instance, in the Neutralized Drift Compression Experiments (NDCX) a strong (several Tesla) final focus solenoid is used for this purpose. In the present analysis we propose that the tight final focus in the NDCX experiments can possibly be achieved by using a much weaker (few hundred Gauss) magnetic lens, provided the ion beam carries an equal amount of co-moving neutralizing electrons from the preceding drift section into the lens. In this case the enhanced focusing is provided by the collective electron dynamics strongly affected by a weak applied magnetic field.