A design for a split-coaxial, normally-conducting, 12 MHz RFQ structure is being developed to accelerate singly charged ions of mass 132 and heavier to a velocity v/c = 0.008, suitable for injection into the ATLAS superconducting heavy-ion linac. Numerical studies have shown that a transverse (normalized) acceptance of 0.25 {pi} mm-mrad can be achieved while maintaining a longitudinal emittance as small as a few keV-nsec. A novel feature is the use of drift-tubes at the entrance and exit of the RFQ which make use of the voltage offsets intrinsic to the split-coaxial structure to increase the voltage gain by about 30%. A half-scale model of the RFQ has been built and tested. The model, with no provision for cooling, was not operable cw but was pulsed to vane-vane voltages as high as 59 kV for periods of several milliseconds. The achieved level, limited by arcing in an rf feedthrough and so not a fundamental limit for the structure, corresponds to 1.2 times the (frequency and gap dependent) Kilpatrick limit. Assuming the model results scale, a 2 meter long 12 MHz RFQ, with 8 mm minimum aperture radius, will operate at 100 kV intervane voltage with an RF input of slightly less than 25 kW. Design and construction status of a full-scale prototype is discussed

Sellyey, W C

Shepard, K W

Shepard, K.W.

Sellyey, W.C.

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r .' -960 8ra 3- To be published in the Proceedings of the 1996 Linear Accelerator Conference A LOW-FREQUENCY RFQ FOR A LOW-CHARGE-STATE INJECTOR FOR ATLAS - A NL/fH y , ? -  - 9 lpq4 K. W. Shepard and W. C. Sellyey Physics Division, Argonne National Laboratory 9700 South Cass, Argonne, IL 60439, USA Abstract A design for a split-coaxial, normally-conducting, 12 MHz RFQ structure is being developed to accelerate singly charged ions of mass 132 and heavier to a velocity v/c = 0.008, suitable for injection into the ATLAS superconducting heavy-ion linac. Numerical studies have shown that a transverse (normalized) acceptance of 0.25 IC mm-mrad can be achieved while maintaining a longitudinal emittance as small as a few keV-nsec. A novel feature is the use of drift-tubes at the entrance and exit of the RFQ which make use of the voltage offsets intrinsic to the split-coaxial structure to increase the voltage gain by about 30%. A half-scale model of the RFQ has been built and tested. The model, with no provision for cooling, was not operable cw but was pulsed to vane-vane voltages as high as 59 kV for periods of several milliseconds. The achieved level, limited by arcing in an rf feedthrough and so not a fundamental limit for the structure, corresponds to 1.2 times the (frequency and gap dependent) Kilpatrick limit. Assuming the model results scale, a 2 meter long 12 MHz RFQ, with 8 mm minimum aperture radius, will operate at 100 kV intervane voltage with an RF input of slightly less than 25 kW. Design and construction status of a full-scale prototype is discussed. Introduction This paper discusses the ongoing development of a low- frequency RFQ as the initial element of a secondary beam linac for an ISOL-type radioactive beam facility. The proposed facility would upgrade the existing ATLAS superconducting heavy-ion linac for the acceleration of radioactive beams [1,2]. In its present form, ATLAS can accelerate any ion with a sufficiently high charge state (q/A > 0.1). For efficient production of most radioactive beams, much lower charge states must be accelerated, at least for the first few MV of the linac. Adapting ATLAS to radioactive beams, therefore, requires development of a low-charge-state injector which can maintain the good features of ATLAS, Le. large transverse acceptance, flexibility in configuration, and most particularly, excellent beam quality [2,3]. To meet these goals, it is necessary to maintain substantially smaller longitudinal emittance than is typical for RFQ implement- ations. Several features of the RFQ discussed here ensure this result. The RFQ should operate at as low a frequency as is practicable both to minimize longitudinal emittance growth and also to maximize the transverse acceptance. The split- coaxial RFQ geometry is appropriate for this frequency range: RFQ structures have already been developed &!t bear the frequency and fields requiredr41. Injecting the RFQ with a pre-bunched beam maintains the longitudinal emittance at a smaller value than is practicabie using adiabatic bunching within the RFQ structure [5]. Also, by removing the bunching function, the length of the RFQ is reduced and the efficiency enhanced. A suitable bunching system exists: the 12 M H z  gridded-gap, four-harmonic system which is presently in use on the ATLAS accelerator can bunch 70% of a dc beam into 1 nsec bunches[6]. The efficiency can be further improved by development of a finer grid structure, which should be straightforward for the very low beam currents anticipated. Placing the RFQ on a variable voltage platform (350 kV) allows operation at a constant velocity profile for a wide range of ion masses and also increases the velocity at the entrance of the RFQ, thus increasing the transverse acceptance. RFQ Design and Numerical Modeling We have analyzed an RFQ structure configured for beams of charge to mass ratio of 1/132 [2]. Ions of higher charge state are accommodated by scaling the platform voltage to maintain a constant injection velocity and the RFQ voltage as the ratio of mass to charge. The RFQ parameters are as follows: Operating frequency Resonant Structure Vane-vane voltage Maximum Electric field Minimum Aperture Modulation factor Entrance Velocity Exit Velocity Number of cells Length Synchronous phase 12.125 MHz sptit-coaxial 100 kV 12.8 MV/m (1.2 KP) 8.00 mm radius 1.5 0.00247 c 0.00493 c 44 222 cm -30 degrees The split-coaxial structure is characterized by a voltage offset of 95 the vane-vane voltage (VJ at both entrance and exit. We make use of this offset to provide accelerating potential by attaching a drift tube to the high-voltage vane pair at both the entrance and exit of the RFQ. The energy gain provided by the two drift tubes is somewhat greater than twice Vv and, as shown below, increases the voltage gain through the RFQ by more than 20%. The electric fields near the vane tips have been numerically modeled using TOSCA and also RELAX3D. 1 The submitted manuscript has been authored I by a contractor of the U. S. Government under contract No. W-31-109ENG-38. Accordingly, the U. S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U. S. Government purposes. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usc- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not nectssarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. c v To be published in the Proceedings of the l 996 Linear Accelerator Conference I I I I I I I Hardware Modeling and Tests I .An roughly half-scale model (19.6 MHz) of the RFQ, [ T T I  with unmodulated vanes, has been constructed of copper and aluminum. Measurements of the electromagnetic fields in the model yielded the following results (where we have scaled both the structure size and the observed RF surface resistance to 12.125 MHz) for Vv = 100 kV: I 4 t  4 ..a*.. E :' '. ... RF Input Power = 23.6 kW Peak surface electric field = 12.3 MV/m RF Energy = 2.3 joules Even though no efforts at vibration isolation were made, the effects of ambient mechanical vibrations on the RF eigen- frequency were much less than the intrinsic resonator band- width and should pose no operational problem. 0.05 0.10 0.15 0.20 0.25 The model was so constructed that it was possible to evacuate the resonator and operate it at high fields. No provision for water cooling was made, however, so that high- field operation was limited to periods of a few milliseconds at a repetition rate Of a few pu'ses per second* An rf pickup loop to monitor the vane voltage was calibrated by three different methods. The first method, using with direct measurements Of the energy 'Ontent Of the resonator, gave values for Vv in good agreement with a direct I * '*.'*.*** 1 1 , ;; , L-:-;-;/;.{ .* '* ................* : $ 0  .. \ E f : w - 2 ~ ~  -2 : *  . *  .=*..= . .  . .  . .  \ 4 -6 .. . .  . *  .do Z-POSmON (X=Y=O.UOSm) Fig. l.Electric fields at the enma of the RFQ, born mSCA. A drift-tube 20 mm long. centered at z = .06 meters, is tied to the high-voltage vane pair. The vane geometry used has a Constant radius of curvature of the Of perturbation measurements Of the together 10 mm, 1.25 times the minimum aperture radius. At entrance and exit the vanes are tapered over a 4 cm interval. Figure 2 shows the transverse and longitudinal RFQ and includes matching electrostatic triplets at the entrance and exit. Some parameters for the beam shown are: I N  P 21 411 dimensions of a beam bunch numerically traced through the 20 I I I I I I Species Sn"' Charge state I +  Entrance Velocity .00247 (378 keV) Exit velocity Emittance (normalized) .00493 (1 508 keV) Input: Transverse 0.27 IC mmmrad Longitudinal 4.7 K keV.nsec Transverse 0.27 n mm-mrad Longitudinal 6.1 IC keV-nsec Output: The small increase in longitudinal emittance through the RFQ indicates that beam quality in this system will be determined primarily by the bunching system. It should be noted that the transverse acceptance projected above is sub- stantially greater than required for the ISOL - type ion sources contemplated for this machine. In fact, for the vane voltage 16- h Z - POSITION (m) assumed above, it is possible to accelerate singly charged uranium ions and achieve similarly small longitudinal emittance values by reducing the RFQ modulation factor from 1.5 to 1.3, and the transverse acceptance from 0.27 to 0.17 IC Fig. 2. Transverse and longitudinal beam size through entrance and exit matching electrostatic quad triplets and through the 12 MHz, 2 meter long RFQ structure. mm.mrad. To be published in the Proceedings of the 1996 Linear Accelerator Conference 0.1 1 10 100 GAP (cm) Fig. 3. Electric field limits in the Kilpatrick model, showing both gap and frequency dependence. The model test results and extrapolation are discussed in the text. measurement using a high-impedance probe. The third method was to observe the high-energy cutoff energy for bremsnahlung produced during high-field tests: this indicated voltages some 20% higher than the voltage determined by the two previously mentioned methods. This may be due to bremstrahlung resulting from electron trajectories which traverse an appreciable portion of the length of the RFQ while traveling between vane pairs. In the split-coaxial structure, such trajectories can yield voltages greater than Vv. The calibration for the results discussed here was from the first two methods discussed. On operation following initial pumpdown, the structure exhibited low-level multipacting, which conditioned away in less than an hour. High power operation was with pulses of a few milliseconds duration, and with a duty factor less than 1%. At higher field levels, the structure arced between the RFQ vanes repeatedly but within a few hours had conditioned to Vr = 59 kV. At this level the input power was limited (at approximately 15 kW) by arcing in a vacuum feedthrough to the power coupling loop Figure 3 shows the results of this test, in the context of the Kilpatrick model for limiting electric fields in RF snuctures[71. It should be noted that for most RFQ implementations, the rf frequency is sufficiently high that for practical vane-vane spacing. the Kilpatrick limit is in the asymptotic limit of large gap, and the gap dependence is generally ignored. For the present case of a very low frequency RFQ, however. we are not in the asymptotic region for the Kilpatrick model, and the gap dependence is quite imponant. This is born out by the experimental result for the 19 MHz model shown in Figure 3, which is nearly three times the asymptotic Kilpatrick limit for this frequency, but only 1.23 times the Kilpatrick limit when the gap dependence is included. Using the complete Kilpatrick model to scale the present result to a 12 MHz structure, it appears that operation at an intervane voltage of 100 kV is feasible. Conclusions By operating at low frequency, and by pre-bunching the beam, it seems feasible to provide good transverse acceptance and excellent longitudinal beam quality in an RFQ structure for singly-charged ions heavier than mass 132. It should be noted that two two-meter sections of 12 MHz RFQ will be required for an injector for ATLAS. This paper has focused on the critical entrance section, as subsequent sections are similar, but technically less demanding. A two-meter section of 12 MHz RFQ is presently under construction. Tests are planned initially to be with unmod- ulated vanes, for the purpose of demonstrating cw operation and determining voltage limits. Subsequently, the vanes will be modulated and tests with beam performed. Acknowledgements The authors gratefully acknowledge numerous helpful conversations with Rolf Muller, Jerry Nolen, and John Staples. John Vincent, John Brandon, and other staff members of the National Cyclotron Laboratory made possible the high power RF tests, which were performed at Michigan State University. We also thank Michael Bruns for perform- ing the RELAX 3D calculations. References [ I ]  J. A. Nolen, in the Proc. 1995 IEEE Particle Accelerator Conference, May 1-5, Dallas, Texas, 95CH35843, p354 (1996). [2] K. W. Shepad and J. W. Kim, in the Pmc. 1995 IEEE Particle Accelerator Conference, May 1-5, Dallas, Texas, 95CH35843, pl128 (1 996). [3] R. C. Pardo, et a].,, in the Proc. 1992 Linear Accelerator Conference, August 24-28, Ottawa, Ontan’o, AECL-I 0728. p70 (1992). [4] R. W. Muller, GSI Report GSI-90-25-ISSN 01714546, (1990). [5] John Staples, in the Proc. of the Workshop on Post-Accelerator Issues at the Isospin Laboratory, Berkeley, CA, October 22-29, 1993, edited by S. Chattopadhyay and J. M. Nitschke, PlOl (1994). [6] F. J. Lynch, R. N. Lewis, L. M. Bollinger, W. Henning. and 0. D. Despe, Nucl. Instr. and Meth. 159, p245 (1979). [7] W. D. Kilpatrick, Rev. Sci. Instr. 28, p824 (1957). 

A low-frequency RFQ for a low-charge-state injector for ATLAS

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A Low-Frequency RFQ for a Low-Charge-State Injector for ATLAS

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A Low-Frequency RFQ for a Low-Charge-State Injector for ATLAS

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