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

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4. THE PERTURBED AXIAL FIELD 59<br />

where we introduced the new error parameter<br />

N<br />

<br />

L εp + εR p<br />

ε ∗ p =<br />

p=1<br />

2<br />

+ (−1) p εR p − ε L p<br />

2<br />

<br />

(4.53)<br />

In order to evaluate the frequency error <str<strong>on</strong>g>and</str<strong>on</strong>g> remembering the equati<strong>on</strong><br />

(4.11), <strong>on</strong>e obtains<br />

<br />

<br />

1<br />

∆ω = ω0 √ − 1 ∼ K<br />

= −jω0 sinh ∆x (4.54)<br />

1 − jK sinh ∆x 2<br />

Since ∆x is <strong>of</strong> the same order <strong>of</strong> magnitude <strong>of</strong> all the ε L,R<br />

p , it is accept-<br />

able the following approximati<strong>on</strong><br />

K<br />

∆ω = −jω0<br />

2 sinh<br />

<br />

j 1<br />

N<br />

N<br />

p=1<br />

ε ∗ p<br />

<br />

∼ = ω0K<br />

2N<br />

N<br />

p=1<br />

ε ∗ p<br />

(4.55)<br />

Note that the quantity ε ∗ p is equal to ε R p for even p, <str<strong>on</strong>g>and</str<strong>on</strong>g> for odd p it is<br />

equal to ε L p . Then, ∆ω is proporti<strong>on</strong>al to the following sum<br />

ε L 1 + ε R 2 + ε L 3 + ε R 4 + ε L 5 + . . . (4.56)<br />

which is a sum over the odd cavities.<br />

This is a general result since it states that the frequency error <strong>of</strong><br />

the whole chain depends <strong>on</strong>ly <strong>on</strong> the errors in the accelerating cavities,<br />

which are the charged <strong>on</strong>es for the mode π/2 mode. It is worth noting<br />

that this result is obtained under the hypothesis <strong>of</strong> negligible n<strong>on</strong>adjacent<br />

cavities coupling, since in the other case the stop-b<str<strong>on</strong>g>and</str<strong>on</strong>g> c<strong>on</strong>cept<br />

is involved <str<strong>on</strong>g>and</str<strong>on</strong>g> the errors in the coupling cavities become important.<br />

Furthermore, the expressi<strong>on</strong> (4.55) does not c<strong>on</strong>tain product <strong>of</strong> different<br />

cavity errors, as we arrested the expansi<strong>on</strong> to the first order, namely we<br />

neglect the correlati<strong>on</strong> am<strong>on</strong>g the errors. Finally, under the previous<br />

hypothesis, it is also clear that the frequency error is independent <strong>of</strong> the<br />

relative positi<strong>on</strong>s <strong>of</strong> the cavities <str<strong>on</strong>g>and</str<strong>on</strong>g> that the variance <strong>of</strong> the frequency<br />

error is N times the <strong>on</strong>e <strong>of</strong> the variables ε L,R<br />

p .<br />

4. The perturbed axial field<br />

In the circuit model, the voltage <strong>on</strong> the capacitance <strong>of</strong> a generic<br />

cell is equal to the accelerating voltage. Then, except for the term<br />

1/(jω2Ci) (1/(jωCi) for the lateral half cells), we look for the current<br />

Ii which is<br />

<br />

N<br />

Ii =<br />

<br />

, for i = 1, 2, . . . , N (4.57)<br />

l=i<br />

Tl<br />

22<br />

Note that such an expressi<strong>on</strong> comes from the hypothesis IN+1 = 1<br />

which means that the last half cell is short-circuited <str<strong>on</strong>g>and</str<strong>on</strong>g> we arbitrarily<br />

fixed the current to <strong>on</strong>e. In the following, we always use this hypothesis,<br />

<str<strong>on</strong>g>and</str<strong>on</strong>g> i = 1, 2, . . . , N.

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