ACCELERATION OF INTENSE BEAMS OF HIGHLY-CHARGED IONS USING DIRECT PLASMA INJECTION SCHEME*

Laser Ion Source (LIS) is the most intense source of highly-charged ions of different elements capable of being providing beams with current 10-100 mA and pulse durations 1-10 μs [1]. Such parameters well meet the requirements of single turn injection into synchrotron and FFAG rings. A few years ago the Direct Plasma Injection Scheme (DPIS) was proposed to extract and accelerate intense beams of highly-charged ions from laser produced plasma [2]. By this approach extraction of ions from plasma happens almost inside the first cell of an RFQ, eliminating severe space charge problems in a lowenergy-beam-transport (LEBT) and the LEBT itself. About 35 mA/1.5 μs (6·10 ions/pulse) of C ions were accelerated by RFQ up to 100 keV/u using DPIS with amplitude of total current up to 60 mA [3]. 17 mA/0.4 μs (6·10 ions/pulse) of C ions has been accelerated up to 100 keV/u as well [4]. The results obtained well meet the requirements of small scale synchrotron or FFAG accelerators for carbon cancer therapy. The latest results on Al ions acceleration using DPIS are presented. The results obtained show that DPIS is, probably, the best choice as high current injector of highly-charged ions for FFAG accelerator. DIRECT PLASMA INJECTION SCHEME Laser-produced plasma is a very intense source of highly-charged ions of almost all elements of the periodic table. As far as the time of plasma generation is much shorter than time-of-flight of ions up to the extraction system and the initial size of the plasma is much less than distance L to extraction system, ion pulse duration τ and current I are defined by following relations [5]: L ∝ τ , 3 − ∝ L I . (1), (2) Possible recombination losses of highly-charged ions during plasma expansion into vacuum are neglected here. Ion current and pulse duration strongly depend on the parameters of the laser and elements used. As an example, C ion current into 1 cm aperture and corresponding ion pulse duration measured at 3.7 m from the target and scaled according to relations (1) and (2) to other distances are presented in Fig. 1 for the case of a 3J/30ns Nd-glass laser and 10 W/cm power density on the target surface. One can see that ion current can reach 1 A at the short distances from the target; this is much higher than the #presently in RIKEN, e-mail: skondrashev@riken.jp current limit of the subsequent RFQ. In a conventional LIS with LEBT, the distance from the target to the extraction system is defined by the required ion pulse duration τ according to relation (1). Very often the required τ is more than few μs and that brings the total extracted ion to the value of about 100 mA which is of the order of current limit of the subsequent RFQ. In this case, a LEBT can not be avoided if it is desired to inject as many ions as possible into the RFQ acceptance. However, it is not possible to fully avoid losses as space charge is very high and time variations of current are very big. The best value of extracted ion beam transmission into RFQ acceptance is, typically, less than 50% [6]. Figure 1: Dependences of C ion current into 1 cm and corresponding ion pulse duration on the distance from the target. If the required ion pulse duration required does not exceed 1-2 μs (small synchrotron or FFAG rings), there is a different possibility for injecting highly-charged ions from LIS to the subsequent RFQ. The extracted ion current is so high (typically, much more than 100 mA) that even without matching of beam emittance to RFQ acceptance (without LEBT), the RFQ can be filled by ions up to space charge limit and, thus, RFQ output current will reach its maximal possible value. DPIS realizes such an approach. AL IONS ACCELERATION USING DPIS