A new local field quantity is presented which gives the high gradient performance limit of accelerating structures due to vacuum rf breakdown. The new field quantity, a modified Poynting vector ${S}_{c}$, is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity ${S}_{c}$ takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both traveling wave and standing wave, and the value of ${S}_{c}$ achieved in the experiments agrees well with analytical estimates.

/pdf/new-local-field-quantity-describing-the-high-gradient-limit-3hyoz4vsgu.pdf

New local field quantity describing the high gradient limit of accelerating structures

Beam Dynamics Newsletter

The designs for a next-generation linear collider based on normal-conducting rf structures require operation at gradients much higher than those in existing linacs. For the NLC/JLC 11.4-GHz structures, the design unloaded gradient is 65 MV/m, which is about four times that of the 2.9-GHz SLAC Linac. The CLIC proposal using 30-GHz structures requires an even higher gradient, 170 MV/m. Both the CLIC and NLC/JLC groups are aggressively pursuing programs to develop structures that operate reliably at these gradients and also have acceptable efficiencies and transverse wakefields. Much progress has been made in the past few years, and this paper reviews the programs, test facilities and results from this research.

/pdf/normal-conducting-rf-structure-test-facilities-and-results-3dq4v1tasy.pdf

Normal-conducting rf structure test facilities and results

A high-efficiency, high-power magnicon at 34.272 GHz has been designed and built as a microwave source to develop radio frequency (RF) technology for future multi-TeV electron-positron linear colliders. To develop this technology, this new RF source is being perfected for necessary tests of accelerating structures, RF pulse compressors, RF components, and to determine limits of breakdown and metal fatigue. The design of this high-power amplifier tube, as well as the first experimental results are presented.

Commissioning of the 34-GHz, 45-MW pulsed magnicon

The Compact LInear Collider (CLIC) study at the European Organization for Nuclear Research (CERN) is developing the design of a 3 TeV ee linear collider. The discovery reach of this machine depends on obtaining a luminosity of 10 cms, which will be done by colliding beams with transverse spot sizes in the nanometre range (≈ 60 × 0.7 nm). Tolerances on fast mechanical stability of the focusing quadrupoles reach the 0.2 nm level. The serious concern of magnet stabilization for future linear colliders has been addressed by building a CERN test stand on magnet stability, bringing together state-of-the-art stabilization technology, latest equipment for vibration measurements and realistic magnet prototypes. For the first time an accelerator magnet was successfully stabilized to the sub-nanometre level, reducing its vibrations level by one order of magnitude with respect to the supporting ground. The best measurements indicate transverse RMS vibration amplitudes (above 4 Hz) of (0.79±0.08) nm horizontally and (0.43±0.04) nm vertically, maintained to a maximum of less than (1.47±0.15) nm and (1.00±0.10) nm, respectively, over a period of several days. Detailed simulations of time-dependent luminosity, which use a model for magnet displacements based on measured vibration spectra, show that approximately 70% of the CLIC goal luminosity can be achieved with the demonstrated performance in the CERN test stand. This indicates the basic feasibility of colliding nanometre-size beams in CLIC. Geneva, Switzerland 1 June 2004 PhD thesis presented at the University of Lausanne (CH), High Energy Physics Institute (IPHE), in December 2003. Work was carried out at CERN from January 2001 to December 2003, in the framework of the CLIC Stability Study Team.

Stabilization of Nanometre-Size Particle Beams in the Final Focus System of the Compact LInear Collider (CLIC)

One priority of the CLIC (Compact Linear Collider) accelerating-structure development program has been to investigate ways to achieve accelerating gradients above 150 MV/m. Two main concepts to achieve such high gradients have emerged: reduced surface field geometries and the use of alternative materials. An experimental demonstration of these two concepts has been made in CTFII (CLIC Test Facility) using three 30 GHz accelerating structures: one made entirely from copper, one with copper cavity walls and tungsten irises and one with copper cavity walls and molybdenum irises. A peak accelerating gradient of over 190 MV/m was achieved using the molybdenum-iris structure. The effect of pulse length on achievable gradient was investigated using a novel 'pulse stretcher'.

/pdf/demonstration-of-high-gradient-acceleration-23ccpogb7k.pdf

Demonstration of high-gradient acceleration

One of the major objectives of CTF3 (CLIC Test Facility) is the production of 30 GHz power for the high-gradient testing of CLIC accelerating structures. To this end a dedicated beam line, power generating structure and power transfer line have been designed, installed and commissioned. 52 MW of 30 GHz power with a pulse length of 74 ns and a repetition rate of 16 Hz were delivered to the high-gradient test area. This will allow operation of test accelerating structures in the first CTF3 run of 2005 up to the nominal CLIC accelerating gradient of 150 MV/m and beyond the nominal pulse length. The system is described and the performances of the CTF3 linac, beam line and the rf components are reviewed.

/pdf/30-ghz-power-production-in-ctf3-1kxowxrocr.pdf

30 GHz Power Production in CTF3

The fabrication of a prototype 30-GHz accelerating section for CERN linear collider studies

/pdf/the-fabrication-of-a-prototype-30-ghz-accelerating-section-2qudqdvlzm.pdf

The fabrication of a prototype 30 GHz accelerating section for CERN linear collider studies

/pdf/a-30-ghz-beam-driven-high-gradient-single-cell-cavity-test-jbesaz6lzy.pdf

A 30 ghz beam driven high gradient single cell cavity test

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