The Spallation Neutron Source project includes a superconducting linac section in the energy range from 186 MeV to 1000 MeV. For this energy range two types of cavities are needed with geometrical /spl beta/ values of /spl beta/=0.61 and /spl beta/=0.81. An aggressive cavity prototyping program is being pursued at Jefferson Lab, which calls for fabricating and testing four /spl beta/=0.61 cavities and two /spl beta/=0.81 cavities. Both types consist of six cells made from high purity niobium and feature one HOM coupler of the TESLA type on each beam pipe and a port for a high power coaxial input coupler. Three of the four /spl beta/=0.61 cavities will be used for a cryomodule test at the end of 2001. Two cavities of each type have been fabricated and the first tests on both cavities exceeded the design values for gradient and Q value: E/sub acc/=10.1 MV/m and Q=5/spl times/10/sup 9/ at 2.1 K for the /spl beta/=0.61 and E/sub acc/=12.5 MV/m and Q=5/spl times/10/sup 9/ at 2.1 K for the /spl beta/=0.81.

/pdf/superconducting-prototype-cavities-for-the-spallation-4t9svbznbd.pdf

Superconducting prototype cavities for the Spallation Neutron Source (SNS) project

RF superconductivity has become a major subfield of accelerator science. There has been an explosion in the number of accelerator applications and in the number of laboratories engaged. The first lecture at this meeting of the CAS presented a review of fundamental design principles to develop cavity geometries to accelerate velocity-of-light particles (β = v/c ~ 1), moving on to the corresponding design principles for medium-velocity (medium-β) and low-velocity (low-β) structures. The lecture included mechanical design topics. The second lecture dealt with input couplers, higher-order mode extraction couplers with absorbers, and tuners of both the slow and fast varieties.

/pdf/design-topics-for-superconducting-rf-cavities-and-1mep7eqv52.pdf

Design Topics for Superconducting RF Cavities and Ancillaries

The small bandwidth of superconducting cavities makes the study of dynamic Lorentz force detuning and its compensation indispensable in case of pulsed mode operation of high gradient accelerators. In this paper, we present the study of this detuning and also propose an optimized design for five cell 650 MHz β g =0.9 elliptic superconducting cavities, which will be used in the high energy section of the 1 GeV H − linear accelerator for the proposed Indian spallation neutron source project, by suitably inserting the inter-cell stiffeners. The paper presents a sequential design methodology which starts with study of static Lorentz force detuning and tunability; and progresses to find out the structural modes and related dynamic detuning values by performing transient structural dynamics calculations. The developed methodology is general in nature and can be used for a three dimensional model of any geometry. The work will be useful for optimizing the design against dynamic Lorentz force detuning of superconducting radiofrequency cavities of any shape.

A study of dynamic Lorentz force detuning of 650 MHz βg=0.9 superconducting radiofrequency cavity

In contrast to a normal conducting RF cavity, a superconducting RF cavity is very susceptible to shifts in its resonance frequency. The main sources of the shift are Lorentz force detuning and microphonics. In a spallation neutron source, to compensate for the frequency shift, a feedforward control is to be applied. In this paper, as an initiative step, a frequency shift observer is proposed which is simple enough to be implemented with a digital signal processor in real time. Simulation results of the proposed frequency shift observer show reliable performance and acceptable computation time for the real time implementation.

/pdf/frequency-shift-observer-for-an-sns-superconducting-rf-4pug6tq04i.pdf

Frequency shift observer for an SNS superconducting RF cavity

In contrast to a normal conducting RF cavity, a superconducting RF cavity is very susceptible to shifts in its resonance frequency. The main sources of the shift are Lorentz force detuning and microphonics. In SNS, to compensate for the frequency shift, a feedforward control is to be applied. In this paper, as an initiative step, a frequency shift observer is proposed which is simple enough to be implemented with a digital signal processor in real time. Simulation results of the proposed frequency shift observer show reliable performance and acceptable computation time for the real time implementation.

/pdf/frequency-shift-observer-for-sns-superconducting-rf-cavity-394na7cjuu.pdf

Frequency shift observer for SNS superconducting RF cavity.

The Spallation Neutron Source (SNS) project incorporates a superconducting radio-frequency (SRF) accelerator for the final section of the pulsed mode linac Cavities with geometrical β values of β =0.61 and β=0.81 are utilized in the SRF section, and are constructed out of thin-walled niobium with stiffener rings welded between the cells near the iris. The welded titanium helium vessel and tuner assembly restrains the cavity beam tubes Cavities with β values less than one have relatively steep and flat side-walls making the cavities susceptible to lsed RF induces cyclic Lorentz pressures that mechanically excite the cavities, producing a dynamic Lorentz force detuning different from a continuous RF system. The amplitude of the dynamic detuning for a given cavity design is a function of the mechanical damping, stiffness of the tuner/helium vessel assembly, RF pulse profile, and the RF pulse rate. This paper presents analysis and testing results to date, and indicates areas where more investigation is required. The medium β =.61 cavity is a thin wall (3.8mm) niobium structure that utilizes electron beam welding at the iris and equator (see Fig. 2). A stiffening ring is welded near the iris at an 80mm radius. The cavity is welded into a pure titanium helium vessel. One end of the cavity is welded directly to the helium vessel while the opposing end is attached to the helium vessel by the stainless steel tuner. The medium β cavities are susceptible to Lorentz force detuning because they have relatively large, flat sides that are flexible compared to very high β cavities.

/pdf/lorentz-force-detuning-analysis-of-the-spallation-neutron-9zf8mm7axo.pdf

Lorentz force detuning analysis of the spallation neutron source (sns) accelerating cavities

The Spallation Neutron Source (SNS) project incorporates a superconducting radio-frequency (SRF) accelerator for the final section of the pulsed mode linac Cavities with geometrical {beta} values of {beta} = 0.61 and {beta} = 0.81 are utilized in the SRF section, and are constructed out of thin-walled niobium with stiffener rings welded between the cells near the iris. The welded titanium helium vessel and tuner assembly restrains the cavity beam tubes Cavities with {beta} values less than one have relatively steep and flat side-walls making the cavities susceptible to Ised RF induces cyclic Lorentz pressures that mechanically excite the cavities, producing a dynamic Lorentz force detuning different from a continuous RF system. The amplitude of the dynamic detuning for a given cavity design is a function of the mechanical damping, stiffness of the tuner/helium vessel assembly, RF pulse profile, and the RF pulse rate. This paper presents analysis and testing results to date, and indicates areas where more investigation is required.

Lorentz Force Detuning Analysis of the SNS Accelerating Cavities

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Lorentz force detuning analysis of the spallation neutron source (sns) accelerating cavities

Lorentz Force Detuning Analysis of the SNS Accelerating Cavities