V.G. Vaccaro

Successful high power test of a proton linac booster (LIBO) prototype for hadrontherapy

The linac booster (LIBO) project aims to build a 3 GHz proton linac to give the beam from 50-70 MeV cyclotrons, which exist in several laboratories and hospitals, a final energy of 200 MeV. This will allow the treatment of deep-seated tumours. A prototype of the first LIBO module was designed, constructed and RF tested by a collaboration of CERN, University and INFN of Milan, University and INFN of Naples, and the TERA Foundation. Low power RF measurements have shown good field uniformity and stability along the axis of the four tanks composing the LIBO module. In December 2000, full power RF measurements at a repetition rate of 100 Hz have been performed at CERN. After a very short conditioning period, an accelerating gradient approaching 30 MV/m has been easily achieved in the tanks, well above the nominal 15.8 MV/m. The particularities of the design and the reasons for the successful performance are discussed.

Study of the Dynamics in a Linac Booster for Proton Therapy in the 30-62 MeV Energy Range

Recent results in accelerator physics have shown the feasibility of a coupling scheme between a cyclotron and a linac for proton acceleration. Cyclotrons with energies up to 30 MeV, mainly devoted to radioisotopes production, are available in a large number of medical centres. These two evidences have suggested the idea to study and design a linac booster able to increase the initial proton energy up to the values required for the treatment of tumors, like the ocular ones. One of the main challenges in such a project is related to meet the requirement of having sufficient mean current for therapy from a given injection current coming from the cyclotron. In this paper we will review the rationale of the project in order to optimize the transmittance and to minimize the duty-cycle. In this frame we will discuss the basic design of a compact 3GHz linac, from 30 to 62 MeV, with a new approach to the cavities used in a SCL (Side Coupled Linac) structure (PALME project).

A New Tuning Method for Resonant Coupling Structures

In order to have efficient particle acceleration it is fundamental that the particles experience, in the accelerating gap, field amplitudes as uniform and as high as possible from gap to gap. Because of the unavoidable fabrication errors, an accelerating structure, when assembled, exhibits field values lower than the nominal ones and/or not uniform. All the usual procedures developed in order to adjust the parameter deviations responsible of the malfunctioning of these structures, are based on field amplitude measurements, by using the bead pull technique, which is a very invasive technique. In this paper the philosophy is reversed: it is assumed that all the information can be got by Sounding the Modes of the whole System (SMS) and correct the deviation of each frequency mode from its nominal value by means of an appropriate tuning of the cavities: resorting to a perturbative technique applied to a circuit model representing this kind of structures, it is possible to calculate the amount of tuning to give to the cavities. It will be shown that a very good equalization and maximization of the fields in the cavities can be achieved by using this technique.

High precision measurements of LINAC coupled cells

For an assembled structure (module, tank) of a linac, the single cells, when coupled, loose their individuality and in cooperation contribute to the generation of the structure modes (resonant frequencies) Fm. On the other end these modes are the only measurable quantities. The system of the coupled cells can be modelled, in a narrow frequency band, as a lumped constant circuit. The modes are solution of an equation obtained equating to zero the determinant relevant to the lumped circuit representation. This is an algebraic equation of the same order as the number N of cells. A plausible question can be posed: is it possible from a manipulation of the measurable quantities (Fm) to draw the lumped circuit parameters, namely coupling constants and single cell resonant frequencies? The answer is positive if a certain degree of symmetry is satisfied. The coefficients of above mentioned equation can be easily related to the measured modes Fm. By varying, by means of tuners, the tune of a single cell of a small unknown amount, any couple of equation coefficient moves on a straight line. Therefore, we have N(N-1)/2 known straight line coefficients which may give the unknowns with extremely high accuracy.