An Overview of the CLIC Testing Program W. Wuensch CLIC ACE 16-2-2008
Outline • Key issues. • High-power RF testing objectives • Testing with beam • Specialized tests
Key Issues The two main high-power effects which limit gradient are RF breakdown and pulsed surface heating. Dark current capture may one day limit gradient but some (limited) analysis indicates that it should not be a significant problem. Pulsed surface heating RF breakdown
Are CLIC parameters within reach? We have fully designed CLIC PETS and CLIC_G which are based on our latest high-power constraints (Alexej’s talk). The constraints are in turn based on quite a number of tests, mainly from the NLC/JLC program, and a reasonably consistent theory. Consequently the structures should work. From another perspective, we have two (more and less) direct tests that that indicate that 100 MV/m is feasible for certain geometries of accelerating structures.
100 MV/m within reach? Old CERN X-band structure tested at SLAC, 153 MV/m (first cell), 150 ns stable over minutes. CLIC goal
136 MW within reach? Judging the likelihood of feasibility from previous results from PETS in CTF2 is very difficult – different geometry, short pulse, power loss with damping material installed, no breakdown rate measurement. On the other hand the CTF3 mid-linac PETS has produced up to 100 MW in a 9 mm aperture at 30 GHz. However it doesn’t have any higher-order mode damping and no breakdown rate measurement has been done (yet). What makes us most confident is that all high-power parameters are lower in the PETS than the accelerating structure except total power.
Plans Demonstrating feasibility should thus simply consist of building and testing the CLIC PETS and the nominal CLIC accelerating structure. Two milestones and everybody is happy. We will do this. This is ‘Plan A’. Special note on PETS: CTF3 can be used to test nominal geometry but the structure needs to be longer to compensate for the lower current than in CLIC. PETS test program will be described in detail by Igor. We will also test other structures to cover a number of uncertainties, a number of things that could go wrong, provide data for further optimization and help with basic understanding and work towards higher efficiency. This is ‘Plan B’. I will discuss this next. Details of the testing accelerating structure program will be presented in Riccardo’s and Steffen’s talks. Test structure fabrication by Germana.
Subjects and questions for Plan B The damping features may directly or indirectly affect gradient. The test results shown were for undamped structures. The main indirect effect of damping waveguides is to lower vg and R/Q for the same iris aperture. P/C and Sc (Alexej’s talk) change relatively little but if there is a direct vg dependence this could be important. Potential direct effects include pulse-surface-heating-induced-breakdown (NLC/JLC couplers) and slots in the high electric field region directly result in field enhancements. ACTION – test structures with and without damping features. Dependence on gradient on iris diameter. Smaller diameter is clearly better for gradient (experiment and theory) BUT worse for beam dynamics and consequently efficiency. Optimized CLIC structures are generally smaller aperture than NLC/JLC structures so we are obliged to make some extrapolation of the dependence. ACTION – we have an iris scan experiment.
Plan B continued Effect of tapering. Optimized CLIC structures generally have stronger tapering than NLC/JLC structures since we are not constrained by precise dipole mode detuning. We have some evidence that a particular geometry reacts differently to breakdown at the beginning and end of the structure (Igor). ACTION – we test more and less tapered structures. Fabrication and preparation. For the old CLIC parameters, 30 GHz and 150 MV/m, we focused on milled quadrant manufacture which has given mixed and hard to interpret high-power results so far. We need a feasibility demonstration as soon as possible so it is best to take advantage of all the NLC/JLC expertise. ACTION – order complete test structures, about 13 in pipeline, from SLAC and KEK and resurrect old CLIC disk technology. Details in Germana’s talk. Statistics. We do not have clear idea of the reproducibility of gradient (Toshi and Chris have high statistics mentioning this point). ACTION – fabricate at least two of every structure type. Is this nearly enough? Couplers. We know this will be an issue but we already have a full test program. ACTION – use proven mode launcher couplers for the moment even if they are too long to use in CLIC.
Plan B, last but NOT least Soft copper. We already have indications from rf pulsed heating and other tests that soft copper may be unacceptable for CLIC structures (Sami, talk of Samuli and Alexej). We may end up demonstrating gradient with an unacceptable lifetime. ACTION – continue development and testing of quadrants so that copper alloys can be used (Mauro and Samuli) and continue/strengthen pulsed surface heating testing. In addition, the only option we can see for the PETS are milled octants (with no fundamental difference between octants and quadrants expected) so we are stuck with the technology anyway. PETS tests occur very late. The first tests of PETS in CTF3 will occur in the middle of the year at the latest. Experience with quadrants in accelerating structures very relevant but clearly not enough. ACTION - we will also test the PETS at SLAC in ‘waveguide mode’ to get an indication of performance sooner. The field profile of a PETS with the power fed into the structure is probably much less favorable, we have a high-power first-cell rather than last-cell.
Test Facilities High-power RF: 30 GHz mid linac test stand in CTF3 – 1 port source but with limited availability since CTF3 has many users and objectives. SLAC – 4 to 6 port source but other users to compete with. Are the resources (still) there to run everything? KEK – 1 port source, dedicated. Starting up again in new location. 12 GHz power source at CERN – 1 port source. On line in 2009? Complete presentation of RF test facilities in talk by Steffen. Two-beam test stand: Beam-based source. Primary objectives are: high-power PETS operation with beam, accelerating structure and PETS breakdown kick, accelerating structure and PETS wakefield monitor, inter-structure alignment, breakdown rate as a function of beam loading, beam loading energy spread compensation, CLIC module installation and other non-structure work like stabilization. Can also be used as a high power source for accelerating structures but this will not be easy to use regularly. Roger will describe all this. TBL: Beam based source. Primary objectives are beam dynamics experiments on deceleration but these may lead to feedback on the PETS design. Many PETS will be operated at high-power so much useful information expected. TBL review by Steffen.
Specialized Test Facilities Pulsed surface heating at SLAC: Medium-power source feeding special standing wave cavity is being used to test interchangeable circular samples. CLIC collaborates by supplying samples of materials which are also tested with ultrasound. EXTREMELY interesting and important preliminary results. Samuli will describe all the fatigue experiments. Pulsed surface heating at Dubna: 33 GHz FEM powers traveling wave cell so gives correct pulse length. Slow progress but low cost. Ultrasonic fatigue experiments: For understanding the extension of RF data to CLIC lifetime and a simple and high-availability test set-up to test many materials and material states. Laser fatigue: Started a few years ago to gain experience with short-pulse-length low-depth fatigue and to start while waiting for RF experiments to come on-line. dc spark: The dc spark experiments are used to provide fundamental breakdown data on materials, preparation, effect of/on vacuum, spectra, etc. Simple set-up and samples allows high throughput. Measurements taken with RF have been established for dc spark including breakdown rate. Sergio will describe some dc spark results.