Feasibility of Short Wavelength, Short Pulse Laser Ion Source for the LHC Injector

Results of experimental investigations of characteristics of ion streams generated from laser plasma after focusing the laser beam either with an aspheric lens or with a parabolic mirror (allowing an observation of the ion emission in the direction of the target normal) are presented. The photodissociation iodine laser PERUN operating with = 1:315 m and delivering energy up to 50J was exploited in the experiment. In this contribution we restricted ourselves to the results of ion emission measurements from Ta , Pb and Bi plasma. Introduction Present studies of the emission of ions from the laser produced plasmas are mainly motivated by a growing interest in the physics of heavy ion accelerators. In the application of the laser plasma as a source of multiply charged heavy ions high current densities are required. From this viewpoint, as numerous experiments show, the laser plasma sources seem to be very promising. In comparison with the electron cyclotron resonance ion sources, which are employed for heavy ion injectors at present, higher current densities of highly charged ions are expected. Thus the charge state of ions, the ion velocity (or the ion energy) and the ion current density were the basic parameters of interest. However, it is evident that whilst the ECR sources are nowadays highly developed as far as their reliability and simplicity of operation is concerned, the laser sources still face major technological and even scientific problems. PERUN Experiment and Results Ion emission experiments were mainly performed with the photodissociation iodine laser system PERUN [1]. Ion collectors (IC), a cylindrical electrostatic energy analyzer (IEA) and a Thomson mass spectrometer (TS) were applied to monitoring the emission of the ions [2]. The ion species, their energy, abundance and/or velocity distribution were explored in dependence on the laser power density, focus setting with respect to the target surface and the changing the angle of observation. The collectors are based purely on the time-of-flight effect, the spectrometers combine time-of-flight with the action of electric or magnetic field on the ions. The collectors first separate the electron component and then they measure the ion current. The outcome is, however, influenced by the secondary emission, which is adding to the net current. Since the secondary emission coefficient, which is specific for any cathode material, may be energy and charge dependent, it introduces a certain degree of uncertainty in the results. This is the main source of error in the absolute estimates of the ion number. In the following it was assumed that for each ion charge unit impinging on the collector cathode one extra secondary electron is struck out. The analyser devices use either an electric field alone to separate the ion species as in the IEA or the combined electric and magnetic field in the TS. The geometry of IEA is that of a cylindrical capacitor segment, where the radial electrostatic field separates the ions entering through a slit. The sensor is a vacuum windowless electron multiplier. An IEA requires a repetitive laser operation (typically 20 shots) to determine the charge energy spectrum. 1.5 1.6 1.7 1.8 1.9 2.0 -0.15 -0.10 -0.05 0.00 E i /z = 50 keV L IEA = 240 cm