The nominal parameters for the compact linear collider (CLIC) foresee acceleration of the electron and positron bunches to an energy of 1.5 TeV by 30 GHz normal-conducting accelerating structures operating at an average gradient of 150 MV/m. Such a high gradient is desirable to limit the length, and in consequence the cost of the linacs, but it is very ambitious when compared with present-day accelerators, which run typically at gradients of around 25 MV/m. The confidence that such high gradients could be achieved was shaken three years ago, when substantial damage of the copper surfaces of prototype 30 GHz accelerating structures was discovered after operating them in the CLIC Test Facility (CTF2) at gradients of only 70 MV/m for pulse lengths of only 15 ns – the CLIC nominal pulse length is 130 ns and the achievable gradient is expected to decrease with pulse length.
Following this disturbing discovery, a vigorous programme of R&D began, which aimed to understand the origin of the problem, and if possible propose and implement solutions. Three years later, the hard work has paid off. At the end of May, a novel 30-cell test structure reached an average accelerating gradient of 125 MV/m with a peak gradient in the first cell of 150 MV/m when powered with 15 ns pulses. On inspection, the structure was found to be undamaged.
The novel features that led to the dramatically higher gradient of this structure were three-fold. First, tungsten was used to make the parts of the structure where the surface electric field is highest, and where damage was observed in copper structures. Tungsten was chosen for its high melting point and low vapour pressure, and because it is renowned for its resistance to damage from arcing.
Second, a new type of power coupler – a so-called mode launcher coupler – was used for the first time to bring in and take out the power. Third, the geometry of the structure was revised to reduce the peak surface electric fields where breakdown occurs. News of this CLIC success was announced at EPAC2002, where it was very well received.
In November, a similar structure, but this time with molybdenum, did even better and achieved a peak accelerating gradient of 195 MV/m in the first cell, and an average accelerating field of 150 MV/m – this is the nominal CLIC loaded gradient. Again, on inspection the structure was found to be undamaged.
These are very important steps forward for the CLIC team and indeed for the whole linear collider field, although it is not yet the end of the road, because the gradient must be demonstrated at the CLIC nominal pulse length of 130 ns. Still, this latest CLIC high-gradient test result is a fitting way to end the very successful CTF2 programme. CTF2 was closed definitively at the end of 2002 to make way for CTF3. The next step in the high-gradient development programme will be the testing of an 11 GHz tungsten or molybdenum iris structure in the NLCTA at SLAC next June with 200 ns long pulses. From 2004, CTF3 will be used to make high-gradient testing at 30 GHz possible again at CERN with 130 ns long pulses.
