Theory, Design and Tests on a Prototype Module of a Compact ...

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82 5. RADIOFREQUENCY MEASUREMENT All this means that it is required a RF measurement able to acquire the relative electric field distribution inside the cavities <strong>and</strong> this is possible using the bead pulling technique. Bead pulling is a perturbation technique based on Slater’s theorem [36, 37]. With the help of a perturbing object (the bead) which travels along the axis of the multi-cell structure, the electric field distribution is determined in terms of phase variation introduced by the bead. The multi-cell structure is driven at a fixed frequency that is the accelerating mode frequency. The Slater perturbation theorem can be written as follows: f 2 = f 2 ∆τ 0 1 + α (µH2 − εE2 ) dτ V (µH2 + εE2 (5.12) ) dv where α is a constant related to the particular perturbing object, the numerator represents the integration over the volume of the perturbing object <strong>and</strong> the denominator is equal to twice the average energy stored in the cavity U. If perturbation is small the (5.12) can be simplified to ∆f f0 = α 4U ∆τ (µH 2 − εE 2 ) dτ. (5.13) This important relation states that the change in frequency depends upon the integral ∆τ (µH2 − εE2 ) dτ. And, if the perturbing object acts in a zone where only the electric field is present in a significant way, the previous relation simplifies again. Then, a procedure to measure the relative level of electric field in a multi-cell structure can be set up as follows: (1) A nylon wire is stretched along the longitudinal axis of the structure <strong>and</strong> some mechanical tools <strong>and</strong> a motor allow the movement of the wire. Note that the wire itself modifies the resonant frequency, since it is a dielectric 9 . Of course, a good alignment is needed between the wire <strong>and</strong> the axis of the cavities in order to have the same perturbation in all the cavities. (2) A perturbing object (the bead) is put on the wire. The dimensions should be little enough to perturb as little as possible the field distribution, but sufficient to give a good signal noise ratio. Using a cylindrical shape (a needle for example), the perturbation acts on the axis <strong>and</strong> only the longitudinal component of the electric field is perturbed. (3) The network analyzer drives the structure at the resonant frequency. The bead perturbs the electric field distribution along the axis <strong>and</strong> this implies a change in frequency 10 <strong>and</strong> then, in the phase of the signal detected by the network analyzer. 9 The nylon wire lowers the frequency since it acts in a zone where electric field is strong <strong>and</strong>, as in a capacitor, it canalizes the field lines. 10 Strictly speaking, the bead grows the frequency.
5. A TUNING PROCEDURE FOR THE LIBO MODULE 83 Therefore, around resonances, the relative change of electric field is proportional to the change in phase shown by the network analyzer 11 ∆E2 ∆Φ ∝ , for each cavity, E2 Φ (4) The measurement is made in the time domain. The motor moves the wire <strong>and</strong> the bead through the cavities <strong>and</strong> the network analyzer shows the relative electric field level for each cavity. The longer is the measurement time, the more precise the measurement is. At high resonant frequencies, very long times need a temperature controlled laboratory, since even little changes in temperature imply changes of resonant frequency <strong>and</strong> this implies a linear slope in the output graph. The figure 5.12 shows a schematic view of a bead pulling apparatus <strong>and</strong> the figure 5.13 shows a particular realization on the LIBO module. In a next paragraph several examples of bead pulling will be given, since the bead pulling measurement is a fundamental pawn to tune the LIBO module to the correct distribution field. Figure 5.12. Sketch of a bead pulling measurement apparatus. 5. A tuning procedure for the LIBO module In this section it is presented the procedure used to tune the first LIBO module. The bead pulling measurements reported on the following were made before the final brazing where the four thanks <strong>and</strong> the three bridge couplers were brazed together. The goal of the measurement was to fix the position of the rods tuners for the 102 cavities in the four tanks. The facts are reported in a chronological order, <strong>and</strong> through the presentation of the followed steps, whereas it is convenient, some explications are reported on the connected theory <strong>and</strong> on the technological aspect of the LIBO module. 11 It is better to use a driven frequency which is slightly different from the resonant frequency, because in this case the change in phase is always in the same direction, whereas at the exact resonant frequency some noise in the measurement can give results with a bigger error.
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UNIVERSITÀ DEGLI STUDI DI NAPOLI F
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ii CONTENTS List of Figures 105 Ack
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iv INTRODUCTION In the state of art
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2 1. INTRODUCTION TO THE HADRONTHER
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8 2. LINAC AND SCL ACCELERATORS par
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14 2. LINAC AND SCL ACCELERATORS 2.
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16 2. LINAC AND SCL ACCELERATORS f
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20 2. LINAC AND SCL ACCELERATORS re
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CHAPTER 3 Design T
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1. GLOBAL PARAMETERS OF THE ACCELER
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2. ANALYSIS OF COUPLED CAVITIES 27
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3. SINGLE CAVITY DESIGN 29 Spice ha
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Page 107: CONCLUSIONS AND OUTLOOK 101 Finally
Page 110 and 111: 104 BIBLIOGRAPHY [23] U. Amaldi, B.
Page 112 and 113: 106 LIST OF FIGURES 2.12 Drawing of
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