Experiments on neutralized beam extraction from a laser ion source

Particle loss due to space charge effects is a severe problem in the low energy section of accelerators. The extraction system and beam transport sections are the most sensitive parts. Especially for the case of a laser-ion source (LIS) the extraction of ions from very dense plasma can lead to a high current density in the extraction gap. To make use of this high current density (i.e. to maximize the extracted current ) without increasing the extraction voltage one can compensate the space charge of the extracted beam. There are three possibilities of space charge compensation: compensation by residual gas ionization, neutralization of space charge in a plasma and compensation by free electrons. For the case of compensation by residual gas ionization two conditions have to be fulfilled: the density of the residual gas ions has to be sufficient and the time required for the gas ions to leave the potential well of the ion beam has to be short compared to the bunch length of the ion beam. Considering the positive ion beams the ions collide with residual gas molecules leading to ionization. The electrons are trapped in the ion beam potential well, whereas the molecular ions are ejected slowly (on the ion mobility time scale). As a result the ion beam space charge is reduced. However, it takes time in the order of 10 to 100 μs to create a sufficient electron density to compensate the beam, while the typical LIS bunch length is below 10 μs. Space charge compensation of an ion beam in a plasma works on the short electron mobility time scale, since it involves the redistribution of the plasma electron density. The use of free electrons is preferred for the reduction of space charge effects in positive ion beams. For this purpose thermal electrons cannot be used, as the resulting high electron density and the high beam ion charge-state lead to a high recombination rate which can result in a significant beam loss [1]. Recombination is less sever in the case of hot electrons in a preformed discharge plasma because the electrons have a high velocity relative to the beam ions. However, this neutralization scheme cannot be applied to the extraction gap of an ion source as the high voltage of the gap is short-circuited by the plasma electrons. It is possible to neutralize the extraction gap by electron beams without the above mentioned problems. An active compensation where the electrons are injected in the ion beam was studied in Munich [2]. Here a heated donor wire supplying the electrons is shielded from the extraction system by a suppressor electrode and an anode grid serves for the velocity adjustment of the electrons. The measured current increase was found to be of about a factor 5. In the CERN experiments [3] an electron beam was send in the opposite direction of the ion beam through the extraction gap into the source equipped with an Al target. The ferroelectrically generated pulsed hollow electron beam had a pulse length of about 50 ns FWHM, kinetic energies of a few keV, a 1-5 A current amplitude and a 10-30 A/cm current density. For Al or Al ions the totally extracted current density measured immediately after the neutralization section was doubled. The enhancement factors were greater in the case of higher ion charge state up to a factor 4. An experiment for the investigation of a neutralized beam extraction from a LIS was started at GSI following the experimental work at CERN. This GSI experiment consists of two parts: The first part is the development and investigation of an electron source with high current density, high beam quality and long pulses. A carbon fiber bundle source (FBS), supplied by the Weizmann Institute (as shown in Fig.1), is a field emission enhanced source with about 100A current and about 250ns pulse duration. The advantage of this source is the possible use of a carbon fiber bundle for an arbitrary cathode geometry. Fig. 1: Electron source mounted on a CF100 flange