The US Inertial Fusion Energy (IFE) Program is developing induction accelerator technology toward the goal of electric power production using Heavy-Ion beam-driven inertial Fusion (HIF). The recirculating induction accelerator promises driver cost reduction by repeatedly passing the beam through the same set of accelerating and focusing elements. The authors present plans for and progress, toward a small (4.5-m diameter) prototype recirculator which will accelerate K{sup +} ions through 15 laps, from 80 to 320 keV and from 2 to 8 mA. Beam confinement is effected via permanent-magnet quadrupoles; bending is via electric dipoles. Scaling laws, and extensive particle and fluid simulations of the space-charge dominated beam behavior, have been used to arrive at the design. An injector and matching section are operational. Initial experiments are investigating intense-beam transport in a linear magnetic channel; near-term plans include studies of transport around a bend. Later experiments will study, insertion/extraction and acceleration with centroid control

Barnard, J.J.

Cable, M.D.

Friedman, A.

UNT Digital Library

RECEIVED \. OCT 0 6 1995 UCRL-JC-119538 PREPRINT O S T I  &F- 950SJt2 PROGRESS TOWARD A PROTOTYPE RECIRCULATING INDUCTION ACCELERATOR FOR HEAVY-ION FUSION A. Friedman, J. J. Bamard, M. D. Cable, D. A. Callahan, E J. Deadrick, S. Eylon,* T. J. Fessenden; D. I?. Grote, D. L. Judd; H. C. Kirbie, D. B. Longinofti? S. M. Lund, L. A. Naffrass, M. B. Nelson, M. A. Newton, T. C. Sangster, W. M. Sharp, and S. S. Yut Lawrence Livennore National Laboratory, University of California P.O. Box 5508, L-440, Livermore, Califoniia 94550, USA tLawrence Berkeley Laborato y, Berkeley, Califonria 94720, USA *Titan Beta, 6780-R Sierra Court, Dublin, California 94568, USA §EG&G Energy Measurements, Inc., Ainador Valley Operations, 5667 Gibralter Drive, P.O. Box 8051, Pleasaizton, California 94588, USA This paper was prepared for submittal to the Proceedings of €he IEEE 1995 Pnrficle Accelerafor Conference Dallas, Tans  .. May 14,1995 April 28,1995 'I'his is :I preprint of 8 paper intcndcd for publication in ajournal or proceedings. Since ch;ingcs iiiiiy be made bcfore publication, this preprint Is made available with the uiitlcrsl:iiitling that it will not be cited or reproduced without the permialon of the :i i i t l iw.  DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED DISCLAIMER 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, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to  any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 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. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. _. - . . . . 4 -  3 -  2 -  1. 0 .  -1 -2 PROGRESS TOWARD A PROTOTYPE RECIRCULATING A. Friedman, J. J. Barnard, M. D. Cable, D. A. Callahan, F. J. Deadrick, S. Eylon,* T. J. Fessenden,? D. P. Grote, D. L. Judd,t H. C. Kirbie, D. B. Longinotti,§ S. M. Lund, L. A. Nattrass, M. B. Nelson, M. A. Newton, T. C. Sangster, W. M. Sharp, and S. S. Yut INDUCTION ACCELERATOR FOR HEAVY-ION FUSION* Lawrence Livermore National Laboratory, L440, Livermore CA 94550 USA bzwrence Berkeley .Laboratoly; $Titan Beta; &G&G L. -3 -2 -1 0 1 2 3 . - The U.S. Inertial Fusion Energy (IF%) Program is developing induction accelerator technology toward the goal of electric power production using Heavy-Ion beam- driven inertial Fusion (HIF). The recirculating induction accelerator promises driver cost reduction by repeatedly passing the beam through the same set of accelerating and focFsing elements.[ I] We present plans for and progress toward a small (4.5-m diameter) prototype recirculator,[2] which will accelerate K+ ions through 15 laps, from 80 to 320keV and from 2 to 8mA. Beam confinement is effected via permanent-magnet quadrupol&s: bending is via electric dipoles. Scaling laws, and extensive particle and fluid simulations of the space-charge dominated beam behavior, have been used to amve at the design. An injector and matching section are operational. Initial experiments are investigating intense-beam transport in a linear magnetic channel; near-tern plans include studies of transport around a bend. Later experiments will study Present technology has achieved 200 kHz bursts at 5 kV and 800 A with pulse widths of 0.5-2 ms, but with a non- variable format.[4] Because of its long (~200 km) beam path length and the,fact that the beam repeatedly visits each section of the beam line, a recirculator driver will require a vacuum of-10-10 to 10-11 torr. II. DESIGN OF SMALL RFCIRCULATOR LLNL, in collaboration with LBL, Titan-Beta, and EG&G, is currently developing a small prototype ion recirculator. This “small recirculator” is intended to explore, in a scaled manner, the beamdynamics issues in a recirculating driver for inertial fusion energy. The small recirculator will be assembled and operated as a series of experiments over several years’.time. Fig. 1. illustrates the overall physics design of the final small recirculator, and lists some of the elements which must all work together, . I. RECIRCULATOR CONCEPT A recirculating induction accelerator‘gotentially offers cost reduction relative to a “conventional” ion induction linac. The overall accelerator length is reduced (by a factor of -2-3, to about 3.6 km in the “Cdesign” recirculator of Ref. [l] and possibly shorter), and the accelerating cores are smaller because acceIeration can be slower. Research on recirculator drivers has centered omfour-beam multi- ring designs, with each ring augmenting the beam’s energy by an order of magnitude over 50 to 100 laps. In contrast with most HIF induction linac concepts,[3] the recirculator designs considered to date do not employ beam merging. Hybrid designs (with a recirculator at the low-energy end) are also possible and may prove attractive. The beam-dynamics issues which must be resolved before a recirculating driver can be built include centroid control, longitudinal control, .avoidance of phase-space dilution in bends, and insertiodextraction of the beam into/out of the rings. As described ,below, these can be addressed at reduced scale in a small prototype recirculator. The waveform generators in a driver must supply variable accelerating pulses at repetition frequencies -50 kHz, and accurate time-varying dipole fields with good energy recovery. These requirements are challenging, but advances in solid-state power electronics should make it possible to meet them through a technology development program. *This work was performed under the auspices of the U.S. Department of Energy by LLNL and LBL under contracts W-7405-ENG-48 and DE-AC03-76SF00098. The small recirculator will have a .circumference of 14.4 meters, a 3.5 cm aperture radius (pipe radius) for the beam focusing and bending elements, and a half-lattice period.of 36 cm. The beam wilI be transverseIy focused . with alternating-gradient permanent magnet quadrupoles with a field of -0.294 T at the pipe wall, and will be bent . .  #bc olSmIBUTION OF THIS DOCUMENT IS UNLlMlTED with electric dipole deflector plates. These quadrupoles and dipoles will each physically occupy about 30% of the axial lattice length, and the full recirculator ring will consist of 40 half-lattice periods, including two using special large- aperture quadrupole magnets through which the beam will be inserted and/or extracted. The beam ion is K+ (mass 39 MU), and the beam will be accelerated from an initial particle kinetic energy of 80 keV to 320 keV over 15 laps by 34 induction cores. The initial current of the beam will be 2 mA, corresponding to a line-charge density of 0.0036 pC/m and characteristic beam radius of 1.1 cm, and the initial pulse duration will be 4 pec. Also, the initial phase advance per lattice period will be 00 =78", depressed to c = 16" by space .charge: After 15 laps of acceleration, the beam current will have incrwed to 8 mA, with a corresponding linecharge density of 0.00721 pC/m, an average beam radius of 1.3cm, and a final pulse duration of 1 psec. Also, the phase advances will decrease to 00 = 45" and o = 12'. Because the heavy-ion beam in the small recirculator is nonrelativistic and accelerating, the variable-format waveforms required to accelerate and bend the beam will require repetition rates ranging from approximately 40 to 90 kHz at the initial and final beam energies.[5] The voltage pulses for the electric dipoles must be.correctly ramped in concert with the changing beam energy. Detailed "ear" pulses and lap-to-lap variation of the pulse duration and shape must be added to the accelerating waveforms to maintain or decrease the beam length. In order to switch the beam into or out of the ring, it is necessary to apply dipole fields that vary in time. Transverse confinement of the beam must be carried out during the insertion or extraction process. The design .we are developing uses a permanent-magnet quadrupole with an expanded aperture. III. EXPERIMENT PLANS Linear experiments now getting underway will measure space-charge-dominated beam quality after transport through a permanent-magnet quadrupole lattice, will characterize the beam prior to injkpion into the recirculator, provide a &est-bed for diagnostic development and afford a preliminary assessment of the role of electrons in magnetic beam transport (see Fig. 2). The next experiments wiIl study beam transport around I 1'1 1 ".;r b r,'- ,.,, ...-.., 1'1 \/Diagnostic  ports Electric Quadrupoles I/ a -90" bend (without acceleration, at first). Emittance growth can result from the non-uniform distribution of beam space-charge resulting from the action of centrifugal forces. As revealed in particle simulations using WARP3d [6] and interpreted theoretically [7], it occurs at changes in the accelerator's curvature where the distribution of beam particles relaxes toward a new equilibrium. Also, the electric dipoles introduce field aberrations. Detailed 3-D simulations show that proper shaping of the dipole plates should render the beam distortion minimal. In the small recirculator, a measurable amount of emittance growth is expected to take place over the fifteen laps, mostly occurring in the first two laps.[8] Later experiments will study insertion and extraction, acceleration (at first in a partial ring '.to facilitate measurement of the beam using intercepting diagnostics), beam steering, bunch compression, and full integrated operation of the recirculator.[2] Preservation of a ,small emittance will again be the central beam physics issue to be addressed. The principal non-intercepting diagnostic is a segmented capacitive pickup located. inside the quadrupoles.[9] Until the ring is complete it will be possible to employ intercepting 'diagnostics. As currently planned, the ring will incorporate two extraction sections 180" apart, and the extracted beam can be diagnosed with detailed intercepting diagnostics twice each lap. As with earlier linac expedments at LBL, excellent shot-to-shot repeatability is anticipated and,so far, observed; The long beam residence in the machine, up to (and possibly exceeding) 300 full lattice periods, will provide a unique opportunity to observe and characterize the longitudinal propagation of space-charge waves along the beam. Such waves will be launched (deliberately or otherwise) by mismatching the applied ear fields. The small recirculator will afford the longest beam path length of any near-term facility, and so will be able to explore issues such as slow thermalization which are important to both recirculating and linear drivers. IV. STATUS .AND INITIAL RESULTS The injector diode[lO], matching section, and straight experiment have been fabricated and are now operating; the layout is depicted schematically in Fig. 2. Fifteen Magnetic Quadrupoles Diagnostics and pumping Faraday cup Injector Matching section Drift section Fig, 2. Linear configuration (overall length 4.568 m from source plate to final Faraday cup). .- . i permanent-magnet quadrupoles have been procured; seven will be used in the straight experiment (a shorter line will serve as the link from matching section to ring). The mechanical design of the “half-lattice period” is nearly complete.[ 1 11 The electrostaticquadrupole matching section gives the circular beam from the diode an elliptical cross-section, suitable for alternating-gradient transport in the transfer line and the recirculator. A section of the SBTE apparatus from LBL was adapted by EG&G to serve this function. The voltages applied to the various quadrupole elements in order to obtain a matched beam were derived using an envelope calculation and range from f 1.8 to f 4.0 kV. The fifth and seventh elements are intended for minor beam steering rather than for focusing. Insertable Faraday cups are located after the third and ninth elements. I I Time-resolved measurements of beam. properties have ! . and magnetic transport sections. The current has been measured using Faraday cups at positions .67 and 1.9 m downstream of the diode source in the matching section, as well as at a position 3.16 m downstream in the magnetic ’ transport section. An energy analyzer developed at LBL (consisting of curved electrostatic plates, across which various potential differences are placed) was located 1.75 m downstream of the source. A two-slit scanner was placed at positions,2 and 1.6 m downstream of the source, providing measurements of emittance, beam radius and I been obtained at various locations throughout the matching ‘beam ce;tr&d location. C P) “ 4  0 I ’  2 .  4 6 ,I, 10 12 Time (microseconds) Fig. 3. Experimental (black) and simulated (gray) current. Figure 3 shows an example of the current vs. time at the second Faraday cup location. Also plotted are results from the 1-D Code HINJ[12] showing close agreement betwkn simulation and experiment. The plot shows a large current spike at the head of the pulse. The spike arises because the rise time of the diode voltage (about 1 ps) is longer than the ideal rise time of 0.48ps.[13] With the slower rise time, particles emitted at the beginning of the pulse have significantly.lower energy than particles emitted subsequently and so particle overtaking occurs. A modification of the pulser circuitry to halve the rise time is planned. The code results are slightly noisier than the experiment; this results from a numerical deconvolution of the voltage waveform (to account for time lags in the voltage monitor), which introduced noise into the voltage waveform used by the code. Shown in Fig. 4 is a 4 I T 7 Normalized Emittance 0.12 0.10 0.08 0.06 0.04 0.02 0.00 I l l I l I l I I I  5 6 7 8 9 10 Time @) Fig. 4. Normalized emittance at end of matching section measurement of the horizontal normalized edge emittance (4x rms) at the end of the matching section. The high initial . value appears to be due to the instantaneously high line- charge density. REFERENCES [l] J. J Barnard, F. Deadrick, A. Friedman, D. P. Grote, L. V. Griffith, H. C. Kirbie, V. K. Neil, M. A. Newton, A. C. Paul, W. M. Sharp, H. D. Shay, R. 0. Bangerter, A. Faltens, C. G. Fong, D. L. Judd, E. P. Lee, L. L. Reginato, S. S. Yu, and T. F. Godlove, “Recirculating Induction Accelerators as Drivers for Heavy-Ion Fusion,” Phys. Fluids B: Plasma Physics 5, 2698 (1993). [2] A. Friedman, R. 0. Bangerter. and W. B. Herrmanns- feldt, “Progress in Heavy-Ion Drivers for Inertial Fusion,” Proc. IAEA Tech. Comm Mtg. on Drivers for Inertial Confinement Fusion, Paris, Nov. 1418,1994. [3] R. 0. Bangerter, ‘The Induction Approach to Heavy- Ion Inertial Fusion: Accelerator and Target Considerations,” ZI Nuovo Cimento 106 A, N. 11,1445- 1457 (1994). [4] H. C. Kirbie, W. R. Cravey, S. A. Hawkins, M. A. Newton and C. W. Ollis, “A FET-Switched Induction Accelerator Cell,” Proc. Ninth Int. Pulsed Power Con$, IEEE Cat. No. 93CH3350-6 p. 415 (1993) [5] T. J. Fessenden et. al., these Proceedings. [6] A. Friedman, D. P. Grote, and I. Haber, “Three- Dimensional Particle Simu1ation.of Heavy-Ion Fusion Beams,” Phys. Fluids B: Plasma Physics 4,2203 (1992) . [7] J. J. Barnard, H. D. Shay, S. S. Yu, A. Friedman, and D. P. Grote, “Emittance Growth in Heavy Ion Recirculators,” Proc. I992 Linear Acel. Conf.. Ottawa, Ontario, Canada, AECL-10728,229(1992). ’ [8] D. P. Grote et. al., these Proceedings. [9] F. J. Deadrick et. al., these Proceedings. [ 101 S .  Eylon et. al., these Proceedings. [ 1 13 V. Karpenko et. al., these Proceedings. [12] J. J. Barnard, G. C. Caporaso, S. S. Yu, “l-D Simula- tions of Heavy-Ion Injectors,” Proc. I993 Particle Accel.. Con$, Washington, DC.,,May 1993,1, 712. [ 131 M. Lampel and M. Tiefenback, “An Applied Voltage to Eliminate Current Transients in a One-Dimensional Diode,’’ App. Phys. Lett. 43,57 (1983). 

Progress toward a prototype recirculating induction accelerator for heavy-ion fusion

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Progress toward a prototype recirculating induction accelerator for heavy-ion fusion

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