When hydrogen plasma interacts in JET with the carbon divertor target plates, amorphous hydrocarbon films are deposited, particularly in the pump duct region of the inner divertor. These films can trap large quantities of tritium and are therefore a key area of research in support of ITER, due to safety related restrictions on the quantities of tritium which can be retained in the ITER torus.

The traditional method for measuring film growth has been to analyze tiles removed from the JET torus during a major shutdown. Surface analysis provides valuable data on long term evolution and composition of surface layers but it is difficult to tell which type of discharges contribute most to the measured deposits.

The quartz micro-balance (QMB) device has been developed through an EFDA-JET enhancement project and preliminary results show that it can measure deposition on a shot to shot basis with a resolution of around one monolayer (~10-10m). This novel device is based on a quartz crystal oscillator circuit originally developed to measure the pressure in oil wells. At its heart is a high temperature application-specific integrated circuit (ASIC) which is mounted close to the measuring crystal and must survive the harsh conditions inside the JET torus. The hardware was largely designed and assembled at JET by G. Neill and D. Wilson (UKAEA) and calibrated and tested at Forschungszentrum Jülich (Germany) by G. Esser. The system was successfully commissioned at the start of JET Campaign 5 (C5, 18 March – 31 May 2002) and is now being used as part of the physics programme by the Exhaust Physics Task Force.

The QMB assembly was mounted in JET during the last shutdown. The shutter, which normally protects the quartz crystal, is shown in the open position. Before a JET pulse the resonant frequency of the crystal is measured with respect to an internal reference. During the pulse the shutter is opened to expose the crystal during a specific part of the discharge and after the pulse the frequency is again measured. This resonant frequency decreases as the mass of the deposit increases.