Abstract
The axial electric field of Alvarez drift tube linacs (DTLs) is known to be susceptible to variations due to static and dynamic effects like manufacturing tolerances and beam loading. Post-couplers are used to stabilize the accelerating fields of DTLs against tuning errors. Tilt sensitivity and its slope have been introduced as measures for the stability right from the invention of post-couplers but since then the actual stabilization has mostly been done by tedious iteration. In the present article, the local tilt-sensitivity slope TS 0 is established as the principal measure for stabilization instead of tilt sensitivity or some visual slope, and its significance is developed on the basis of an equivalent-circuit diagram of the DTL. Experimental and 3D simulation results are used to analyze its behavior and to define a technique for stabilization that allows finding the best post-coupler settings with just four tilt-sensitivity measurements. CERN’s Linac4 DTL Tank 2 and Tank 3 have been stabilized successfully using this technique. The final tilt-sensitivity error has been reduced from � 100%=MHz down to � 3%=MHz for Tank 2 and down to � 1%/MHz for Tank 3. Finally, an accurate procedure for tuning the structure using slug tuners is discussed.
Highlights
In manufacturing and operation of long accelerating structures, stabilization and tuning is essential as various factors such as manufacturing tolerances, movable tuner movement, and beam loading contribute to tuning errors
Tilt sensitivity and its slope have been introduced as measures for the stability right from the invention of post-couplers but since the actual stabilization has mostly been done by tedious iteration
The derivation of the technique goes beyond previous approaches in that it (a) establishes the local tilt-sensitivity slope TS0n as the principal measure for stabilization, (b) shows that this measure is proportional to the coupling admittance of post-couplers in the equivalent-circuit model, (c) describes a method to extract the individual post-coupler behavior, (d) strictly applies the condition of resonance to the circuit elements of an individual post-coupler for stabilization, and most importantly (e) shows that all the post-couplers of a structure can be adjusted to their individual optimum by only four tilt-sensitivity measurements
Summary
In manufacturing and operation of long accelerating structures, stabilization and tuning is essential as various factors such as manufacturing tolerances, movable tuner movement, and beam loading contribute to tuning errors. A new straightforward and accurate technique to find optimum post-coupler lengths is discussed It is based on a combination of results of earlier publications in that it (a) analyzes tilt-sensitivity slope [2,10], (b) using an equivalent-circuit model [13,16], and (c) focusing on the conditions of resonance on the postcoupler [5,16]. The derivation of the technique goes beyond previous approaches in that it (a) establishes the local tilt-sensitivity slope TS0n as the principal measure for stabilization, (b) shows that this measure is proportional to the coupling admittance of post-couplers in the equivalent-circuit model, (c) describes a method to extract the individual post-coupler behavior, (d) strictly applies the condition of resonance to the circuit elements of an individual post-coupler for stabilization, and most importantly (e) shows that all the post-couplers of a structure can be adjusted to their individual optimum by only four tilt-sensitivity measurements.
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