JET Experimental Campaigns launched

After a long shutdown and restart, JET experimental campaign C15 started on 24 April and ended on 25 May. An intervention to repair a water leak that occurred on one of the Neutral Beam systems has been completed, with the start of the JET Campaigns C16-C17 on 3 July 2006.

The C15 campaign was devoted to studies of plasmas with ITER-like configurations. In this Bulletin, some of the achievements of the C15 campaign are highlighted.

The 2006 JET experimental campaigns aim at taking advantage of the latest enhancements of the JET facilities to address ITER high priority issues.

The objectives of the 2006 EFDA-JET Workprogramme, with experiments spanning the period 24 April 2006 to 20 October 2006, are to study issues which could impact on the detailed design of ITER components, such as the first wall, heating & current drive systems and diagnostics; to optimise ITER operating scenarios, including the ELMy H-Mode, the Advanced Scenario and the Hybrid Scenario; and to study ITER-relevant physics such as burning plasma physics, transport, MHD and the effects of toroidal magnetic field ripple.

In order to conduct these studies with the most ITER-like plasmas, during the 2004/5 shutdown, the auxiliary heating power available on JET was increased, and a new divertor (see Figure 1) was installed, which allows high power operation with an ITER-like, higher triangularity plasma shape (see, e.g., Figure 2). In addition a set of 15 new or upgraded diagnostics were installed, allowing the necessary measurements to achieve the goals of the programme. During the early part of operations in 2006, high priority was given to bringing these enhancements to full performance.

Power upgrades

A 3dB coupler was installed on a pair of ion cyclotron resonance heating (ICRH) antennas to improve coupling to the plasma in the presence of Type I edge localised modes (ELMs). This has been tested successfully at low power, and requires progress with arc detection for a demonstration at high power. In addition, a septum was installed in the neutraliser of the neutral beam (NB) system, to increase the neutralisation efficiency. This resulted in an increase in NB power by ~10%. Subsequently, an ELMy H-mode was achieved with a peak of 29MW of additional heating power (21.5MW NB power and 7.5MW ICRH power), and delivery of 115MJ of energy to the plasma, in Pulse No. 66424.

New or upgraded diagnostics

New or upgraded diagnostics to study interaction of the plasma with the first wall include a divertor bolometer, a wide-angle IR camera and quartz microbalances (QMBs) in the divertor. As examples of preliminary observations with these: the bolometer shows radiating structures in the divertor region (see Figure 4); the IR camera shows power loss to the main chamber during ELMs and disruptions; and the QMBs show a correlation between strike point position and erosion/deposition in the private flux region. A new charge exchange recombination spectroscopy (CXRS) system, with a time resolution of 10ms, was installed to provide core data for scenario optimisation and physics studies. This system has been used in unique transport studies to measure ion temperature modulation with modulated ICRH, applied at up to 20Hz (see examples in Figure 3). This is expected to yield a quantitative estimate of the ion temperature profile stiffness. Optimisation of the Advanced Scenario will also benefit from an X-mode reflectometer, which can localise Alfvén Cascades to determine the location of zero magnetic shear in reversed-shear discharges (see Figure 5).  This reflectometer makes use of new, low-loss (~0.005 dB/m) waveguides, which are necessary to obtain an adequate signal-to-noise ratio. New diagnostics for burning plasma studies include a time-of-flight neutron spectrometer for 2.45MeV neutrons, and a scintillator probe and Faraday cups to measure fast particle losses. The neutron spectrometer clearly shows the high-energy deuterium tail during ICRH, and the scintillator probe and Faraday cups show losses of high-energy particles correlated with magnetohydrodynamic (MHD) activity (see Figure 6). In addition, on one channel, the data acquisition rate for gamma-ray spectroscopy has been successfully upgraded from a few tens of kHz to 450kHz.

Scenario optimisation with the new divertor

The task of demonstrating the performance of the new divertor is integrated with the scenario optimisation activity. Studies are underway to characterise the H-mode and the Advanced Scenarios in high-triangularity configurations, which are possible with the new divertor. Confi nement in the ELMy H-mode is being investigated in two high-triangularity configurations, including the ITER-like plasma shape. This shape has been successfully tested up to a plasma current of 2.5MA and heating power of 20MW, with initial studies focussing on the effects of the new divertor geometry on performance, and on ELM type. This is the first step in a programme which will continue into 2007, to develop a 4MA, high power plasma in the ITER shape, which will be closer in absolute parameters to ITER than previously possible. It will also be possible to discriminate the effects of the lower and upper triangularities on the behaviour of ELMs more finely than was possible previously. The search for mild, Type II ELMs, as seen on ASDEX Upgrade, is also continuing in a quasi-double-null (QDN) configuration, which can now be matched more closely to the QDN configuration on ASDEX Upgrade, in which Type II ELMs are observed.

Initial experiments have also been performed to investigate potential Advanced Tokamak regimes in an ITER-like plasma shape, and in a configuration with high lower triantularity, but low upper triangularity. First results suggest improvements in performance in the latter shape, with respect to previous high-triangularity Advanced Scenario operation. Studies in both configurations are ongoing.

Reducing the probability of disruptions in ITER scenarios remains a high priority. The intrinsic error fi eld on JET has been determined in the ITER-like configuration, enabling the Error Field Correction Coils to be used for this purpose. In the ELMy H-mode scenario measurements have been made to make the vertical speed measurement more reliable during ELMs, to reduce the risk of disruptions due to vertical displacement events. Studies are also underway to reduce the risk of disruptions due to neoclassical tearing modes (NTMs), by reducing the size of sawteeth using ion cyclotron current drive (ICCD) or by improving NTM stability using lower hybrid current drive.