Ionization efficiency of a COMIC ion source equipped with a quartz plasma chamber

Increased ionization efficiencies of light noble gases and molecules are required for new physics experiments in present and future radioactive ion beam facilities. In order to improve these beams, a new COMIC-type ion source with fully quartz made plasma chamber was tested. The beam current stability is typically better than 1 % and beams are easily reproducible. The highest efficiency for xenon is about 15 %. However, the main goal is produce molecular beam including radioactive carbon (in CO or CO2), in which case the efficiency was measured to be only about 0.2 %. This paper describes the experimental prototype and its performance and provides ideas for future development. INTRODUCTION The ISOLDE facility at CERN produces a wide range of radioactive ion beams due to a long history on target and ion source development. Because the radioactive isotope production is very limited, the most important ion source parameters are high ionization efficiency, selectivity and reliable operation under intense radiation. Currently used ion sources (mainly laser (RILIS [1]) and arc discharge –type ion sources (VADIS [2])) do not efficiently ionize light noble gases, such as helium, and molecules, such as CO, CO2, N2 and NO. These beams were previously planned to be produced with 1+ ECR ion sources operating at 2.45 GHz (for example MINIMONO [3]). However, due to new and more efficient RF coupling of COMIC-type ion sources [4], we expect to advance in 2.45 GHz ECRIS utilization for radioactive beam production. Q-COMIC The new COMIC source (Fig. 1) designed by LPSC/Grenoble incorporates special features such as a plasma chamber fully made of quartz (Q-COMIC), which should provide chemically favourable conditions for molecular ion beam production, especially for CO2. The beam is mainly formed between plasma (hole diameter 3.1 mm) and intermediate electrodes, which have 1.5 – 3 kV potential difference over 10 mm gap. The intermediate electrode is important in minimizing the effect of using different operation voltages to the beam formation and shape. Comprehensive emittance measurements will be performed in near future. Results are expected to be similar to those of standard COMIC [4]. Figure 1: Schematic of Q-COMIC. Figure 2: Q-COMIC setup and gas injection system The source is placed inside a standard ISOLDE target base (Fig. 2), in vacuum. Consequently, a water cooling system is necessary to protect the NdFeB -permanent magnets from overheating. In this prototype unit there is no target container (between 1 kA current feedthroughs, Fig. 2) and the gas of interest (simulating a radioactive gas from target) is injected through a calibrated leak of 3.3E-6 mbarl/s (value corresponding air). The buffer gas is injected into the system by using a Pfeiffer EVR116 gas dosing valve operated by a RVC300 controller unit. Gas injection system calibration was verified with a calibrated helium leak detector. The microwave power generator is Kuhne Electronic GmbH “KU SG 2.45-30A” operating at 2.45 GHz and capable of injecting up to 30 W microwave power. The plasma ignites typically at the pressure level of about 5E5 mbar (at the extraction) when employing the full microwave power. However, at higher pressure of to 1E-2 Proceedings of ECRIS2010, Grenoble, France MOPOT006 02 New Development 51 mbar the ign the plasma is 2.7 W still m were perform using 30 k measured spe Figure 3: Spe the source w Experimen gas and b) (simulating r for igniting a A. Injectio In order to the gas effi buffer gas achieved wa over 1.2 bar) 57 %). Comp and nitrogen nitrogen, it s of N2 (atomi molecular N power). ition can be a ignited the p aintaining th ed at ISOL V accelerati ctrum is show ctrum when ith krypton as GAS EF ts were perfo injecting C adioactive CO nd maintainin n of one ga measure the ciency, kryp dosing valve s 6 % when i of natural kr arable measu . Results are hould be note c nitrogen be 2 beam with chieved alrea ower can be e plasma. A DE off-line m on voltage. n in Fig. 3. injecting 11.3 a buffer gas. FICIENC rmed by a) O2 through 2 from targe g plasma. s influence of ton was inje . The highe njecting 62 p ypton (where rement was d shown in Fig d that this is am current is 28 W of fo dy at 10 W. decreased dow ll the experim ass spectrom An exampl