Abstract
Frequency jumps in an ion linac use to be made in order to provide a large transverse acceptance in the low-energy part and a high accelerating gradient in the high-energy part. This frequency jump may induce a discontinuity in the average longitudinal force per focusing period and shrink the longitudinal acceptance of the linac if this transition is not performed carefully. In this paper, three techniques are developed which produce a ``certain'' continuity of the channel at the transition between. The continuity type is discussed. It is demonstrated that the longitudinal acceptance can be preserved whatever the frequencies of the cavities in the linac. This point is very important when comparisons between different cavity types are made (spoke and elliptical cavities for, instance). A few examples are shown to illustrate the performances of the three techniques.
Highlights
Frequency jumps in ion linacs use to be made in order to provide a large transverse acceptance in the low-energy part and a high accelerating gradient and/or a better shunt impedance in the high-energy part
It is explained that as the frequency is doubled in the coupled cavity drift tube linac (CCDTL), a conservative synchronous phase of ÿ60 degrees is required at the beginning of the structure to capture all particles
A comparison of the 3 accelerating rate (AR) reduction is shown in Fig. 1 for k 2 and a synchronous phase of the low frequency section varying from ÿ45 to 0 degrees
Summary
Frequency jumps in ion linacs use to be made in order to provide a large transverse acceptance (physical aperture) in the low-energy part and a high accelerating gradient and/or a better shunt impedance in the high-energy part. Smaller cavities help to reduce the cryogenic load and induce a cheaper fabrication This frequency jump may induce a discontinuity in the average longitudinal force per focusing period and shrink the longitudinal acceptance of the linac if this transition is not performed carefully. The concept of the continuity of the phase advance per unit length is discussed for the transverse plane in order to provide a current and emittance independent design This continuity simplifies significantly the matching at the transition. A third technique which is a mix of the first two has been proposed In this paper, these three techniques are developed and compared to the classical method which is a matching at the transition (tuning of the focusing elements to maintain a smooth evolution of the phase advance per meter ) keeping a high accelerating efficiency
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