Novel permanent magnet quadrupoles for LINAC4
by Alessandra Lombardi (CERN)
PMQ for tank2 , provided by Elytt, 60 mm in diameter and 80 mm in length. Image credit: CERN
The Drift Tube Linac (DTL) of LINAC4 is to be equipped with Permanent Magnet Quadrupoles (PMQ). The DTL is a 352MHz Radio Frequency Structure composed of 3 tanks that accelerates the beam from 3 MeV to 50MeV. In LINAC2 Electromagnetic Quadrupoles are used presently. Amongst the main reasons to use PMQs in LINAC 4 were compactness, cost efficiency considerations and simplification of the operation.
Permanent magnets have been chosen as the best practical way to provide the required high gradient within the small volume available inside the high-frequency accelerating structure. The PMQs are electron-beam welded inside the copper drift tubes and alignment is provided by the outer cylindrical surface and radial pins. Additional advantages include simple fixed-optics operation and cost savings as power supplies and cables are not needed. This choice relied on the correctness of the beam dynamics calculations as the focusing inside the Drift Tube Linac cannot be changed once the quadrupoles are produced.
The procurement of the 113 PMQs started in 2009. It was split amongst two companies: ASTER(USA) and Elytt (Spain) to avoid single-source and especially with the aim of bringing the PMQs technology to European firms. The quadrupoless of tank1, which were needed first, were ordered from ASTER; whereas the one of tank2-3, needed later in time were ordered from Elytt. At the same time a small quantity of quadrupoles was made in house with the purpose of demonstrating the PMQ technology internally, the goal of which was fully achieved. CERN has acted as a catalyser to bring the PMQs technology to Europe in a continuous collaborative spirit with the selected companies. As a result of this success, it was decided to equip the inter-tanks of the next accelerating structure (a Cell Coupled Drift Tube LINAC (CCDTL), up to 100MeV) with PMQs as well.
PMQ for tank1 , provided by ASTER, 60 mm in diameter and 45 mm in length. Image credit: CERN
All the quadrupoles have been received and accepted after magnetic measurements at CERN by November 2012. In August 2014 the first beam accelerated through the tank1 to 12 MeV showed excellent success with no losses along the structure thanks-also- to the perfect focusing of the PMQs.
