A novel, induction type linear accelerator, the Neutralized Drift Compression eXperiment (NDCX-II), is currently being commissioned at Berkeley Lab. This accelerator is designed to deliver intense (up to 3x10 11 ions/pulse), 0.6 to ~600 ns duration pulses of 0.13 to 1.2 MeV lithium ions at a rate of about 2 pulses per minute onto 1 to 10 mm scale target areas. When focused to mm-diameter spots, the beam is predicted to volumetrically heat micrometer thick foils to temperatures of ~30,000 K. At lower beam power densities, the short excitation pulse with tunable intensity and time profile enables pump-probe type studies of defect dynamics in a broad range of materials. We briefly describe the accelerator concept and design, present results from beam pulse shaping experiments and discuss examples of pump-probe type studies of defect 2 dynamics following irradiation of materials with intense, short ion beam pulses from NDCX-II.
IntroductionIntense, short pulses of energetic ions are highly desirable for studies of warm dense matter, high energy density physics experiments with volumetrically heated targets, and for studies of defect dynamics in solids [1,2]. Irradiation with short ion beam pulses generates defects in a narrow time window and their diffusion and recombination dynamics can then be studied in time resolved "pump -probe" type experiments. Here, the short ion beam pulse acts as the "pump" which can be followed by a suitable "probe" beam, such as an x-ray pulse, which can track a selected defect signature. Combining a short pulse ion beam capability with pulsed probe beams from an x-ray free electron laser (FEL) was recently proposed for the study of radiation effects in nuclear materials by Froideval et al [1]. Suitable driver beams can be formed through longitudinal compression of space-charge dominated ion beams where short pulses have been achieved by imposing head-to-tail velocity tilts to drifting ion beams [2][3][4]. High drift compression factors are reached when ion beams traveled through a neutralizing plasma column during drift compression. Earlier, a 25 mA, 300 keV K + beam was compressed 50 to 90 fold, yielding an intense ~3 ns long pulse with a beam spot size of ~1 mm 2 [4]. In order to achieve uniform heating of micrometer thick foil targets to temperatures of 2 -3 eV, we are currently implementing this beam compression concept with increased ion beam energy (up to 1.2 MeV)for lithium ions [3,5]. The induction linac based accelerator affords a high degree of flexibility in ion beam pulse shaping. In an intensity regime well below the onset of intense target heating 3 and hydrodynamic motion, this flexibility enables the study of defect dynamics in solids and other materials (including soft matter and liquids). Here, targets can be exposed to short ion beam pulses and resulting defect structures can be probed with time-resolved in situ or with ex situ methods. This article is structured as follows: We first present results from ion beam pulse control experiments at NDCX-II in Section 2. We t...
