First contactless electrical resistivity measurements for HEDP experiments with heavy ion beams

have been based either on a differential two point method or the well known four point method [1, 2]. In this report we present the first steps in the development of a contactless, radio-frequency measurement method similar to those discussed in [3, 4]. This method is expected to provide for better spatial resolution and signal to noise ratio. A possible target design for such measurements involves a dielectric capillary surrounded by one or two coils (in this case a coil acts as transmitter and the other as receiver) and a wire shaped target along its axis. The wire target is heated by the ion beam and expands to finally hit the inner wall of the capillary. The conductivity of the layer which builds up at the capillary is then measured through induced eddy currents. To better understand such target assemblies and optimize the experimental setup, several simulations have been performed with the CST Microwave Studio code. The simulated distribution of the electrical current density induced in a conductive layer at the inner wall of the capillary is shown in Fig. 1. The good localisation of the eddy currents in the proximity of the coil is revealed in this image. The simulations also showed that the reflection at the transmitter coil delivers better signals than the receiver coil. Therefore the first tests have been performed with a single coil. The first proof of principle experiment has been performed during the december 2006 HHT beam time. The experimental setup consisted of a signal generator which delivered a 4 ns FWHM quasi-gaussian pulse, a signal splitter, a 300 MHz oscilloscope and the target assembly, all connected by 50 Ω coaxial cables. The target assembly included a 3 mm long quartz capillary with a diameter of 1 mm and a wall thickness of 0.1 mm, and a fivewinding coil made of 0.1 mm copper wire. The target has been a 4 mm long lead wire with a diameter of 0.25 mm. The 2·10 ions intense uranium beam has been delivered in short pulses of about 100 ns FWHM duration and focused down to about 0.5 mm x 0.4 mm at the target. One of the recorded signals is presented in Fig. 2 together with the fourier transforms of the signals reflected from the coil before and during the experimental shot. The