Survey of Compact High Current Electron Accelerators

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SURVEY OF COMPACT HIGH CURRENT ELECTRON ACCELERATORS S.D. Putnam Pulse Sciences, San Leandro, CA

INTRODUCTION Development of compact, multi-kiloampere electron accelerators is currently underway in support These accelerators of several national programs. can be substantially less costly and, if properly designed, significantly lower in weight than the more developed linear induction accelerators (LIA) such as the Advanced Test Accelerator (50 MeV, 10 kA, 70 nsec) at the Lawrence Livermore National Laboratory (LLNL)(‘) or the RADLAC II accelerator (15 MeV, 40 kA 40 nsec) at t e Sandia National Laboratories, Accelerating these high Albuqierque (SNLA)(2? currents in compact geometry to kinetic energies Of - 100 MeV or higher is a new and challenging goal, requiring developments in both technology and the physics of beam transport. Generally, high currents have significant self field and image effects which affect orbital stability; collective instability and strong magnetic fields growth can be very rapid; are needed to confine the space charge at losi energy and to suppress instability growth, leading in particular to injection and extraction problems with closed orbit configurations. At this time compact accelerators are in an early experimental stage, but recent progress in conceptual accelerator designs, power technology, and in theory and numerical simulation capability all suggest the possibility of In this paper, we survey the achieving the goals. new developments in high current compact accelerators, with concentration on recirculating induction concepts which are designed to be lightweight; i.e., factors of at least several lighter than an LIA.

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20 passes with near-term induction technology. There are several possible recirculation configurations: (1) a folded LIA (not really a recirculator); (2) a spiral line wherein each recirculation has independent beam pipes and all pipes pass through common accelerating cores; (3) a re-entrant recirculator a microtron) in which beams transit (RER) (e.g., through the accelerating region on the same path, but return orbits vary with particle energy; and (4) a closed orbit device wherein all transits are around a common trajectory. The folded LIA and the spiral line are openended, thereby allowing beam pulse train widths which greatly exceed the accelerator transit time, and injection and high quality and efficient eXtraCtion are straightforward. These devices are probably practically limited to < 20 passes/unit, but can be staged for higher energies. The RER is also limited in number of passes to values similar to the spiral line, but is a complicated alternative for high currents with low energy injection. Then solenoidal focussing fields are required, and switching the beam into the common orbit of the accelerating region appears very difficult. A closed orbit accelerator allows a large number of passes, limited by the energy gain/pass and the desired interpulse time within the macropulse. Injection and extraction are now difficult problems with low energy injection and the necessary solenoidal fields and/or strong focussing windings. Historically, the configuration and the type of induction acceleration have been determined by available power technology. Among the first high current accelerator proposals were the betatron in which input power levels were in the 107 - 10 % W range, requiring tens of thousands of passes at energy gains of a few tens of keV/turn in a closed orbit device. Acceleration times were several milliseconds, corresponding to frequencies of KHz or less within the burst. The betatrons have two attractive features. First, the particle energy is automatically synchronized with the bending field and, secondly, the acceleration is continuous around the orbit. Neither of these features is shared by discrete gap induction drivers.

REVIEW OF ACCELERATOR TYPES A high effective gradient is, of course, required to render an accelerator compact, and we consider gradients > 10 MV/m. High gradients can be achieved either directly in a linear device or LIAs are probably through recirculation of the beam. limited to < 10 MV/m, depending on efficiency and ATA operates at - 0.6 MV/m and weight constraints. RADLAC II at - 1 MV/m; development is underway for accelerating units giving - 3 MV/m, and advance P3) . designs are being studied to achieve - 10 MV/m Higher gradients with induction technology require recirculation.

Several betatron-type accelerators for high currents have been proposed: (1) a conventiona betatron with high energy injection (- 50 MeV) &J) ; (2) modified betatrons, which add a toroidal field ~~o”,““,‘n~“,~~e,““;wc~~~7~~f~~~s~~~g~~~a~r~ec-

New concepts in high power RF linacs project These gradients from tens to several hundred MV/m. accelerators operate at high frequency (- l-35 Fflf) to avoid breakdown and are of the two-beam type or direct drive with ultra-high power RF sources (5). For average currents in the macropulse of a few amperes or higher, RF powers/length of several to These requirements are tens of CW/m are required. several orders of magnitude higher than existing RF technology and are probably the major technical challenge of the high current RF linac approach. Induction driver technology, on the other hand, can generate such power/length with existing technology. However, if desired frequencies within the macropulse exceed about 50 MHz, the RF driver is the only presently viable technique.

which also adds strong focussing windings (a. = 2 stellar tot-) to provide greater tolerance to field errors(‘) ; and 4) the MYTRON, a version of the stellatron by Kapetanakos using torsatron instead of stellarator windings (See Figure 1). We do not further discuss these accelerators since they will be covered in the next paper of this session(g). In the last several years, advances in power technology and the interest in shorter acceleration times have resulted in accelerator designs which have input powers in the 10 GW range and accelerate over - 1 O-50 usec timescales. These accelerators typically involve hundreds of passes and energy gains of a few MeV/pass. All these designs are

Recirculating accelerators effectively increase the gradient by the number of passes and in principle can achieve gradients of - 100 MV/m over as few as 887

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closed orbit, discrete gap induction accelerators. Those using more conventional power technology apply a continuous accelerating voltage over the acceleration time. The beam thus necessarily fills the entire accelerator, and the geometry of the accelerator is determined by the pulsewidth. The first scch accelerator prop se tion Stellatron (RIS)?l of ~,“,‘,,““pn~~,“,“,“~~~~dI~~~stellatron windings for energy bandwidth desirable Other variations on(,t,q for the high gain/pass. theme are the PEBATRON (torsatron windings) n (SLB) which usis a ‘1d the Solenoidal Lens Betatr Figure 1 indialternating solenoidal lenses. 02) cates the field configurations of these acceleratars. A design of tne driving p~;“$f ;;,st,em2,“,asMbe;n derived for the RIS by Schlitt 112 kA, 50 nsec beam, accelerated oVer 5-10 psec.‘He devised a passive synchronization scheme for the This technique is in principle bending fields. determined by the enerpossible up to some energy, gy zandwidth frcm orbital dynamics and the detaiis of the vertical field coil and accelerating module The REBATRON is a faster accelerator circuitry. (5 1 Geii over 10 psec) than the RIS design of Schlitt, and most likely requires active synchronization of the bending field with particle energy. A long accelerating pulse, discrete gap recirculation accelerator in a spiral line configurat’on t141 has been studied by Wilson and Leiss at the NBS. This accelerator concept relied upon gas fOCUSSing for beam transport and encountered difficulties from beam-plasma instabilities due to the gas focussing An outgrowth of their studies was a Vacuum/ tecnnique. transport design for energies up to several magnetic hundred MeV. A characteristic of the long accelerating pulse, discrete gap class of induction accelerators above is the substantial weight of the isolation cores in the designs using ferromagnetic materials. These core weights are comparable to or exceed those of the silicon-iron cores of the For example, an LIA. 200 MeV, 50 nsec RIS design of Schlitt weigh about 100 tons. We now consider new technical developments which can significantly reduce the accelerator weight. LIGHTWEIGHT INDUCTION ACCELERATORS

and the driving power system. Two types of isclation cavity designs are used in induction acceleraferromagnetic (ATA-like) and dielectric tors: The ferromagnetic technology and (RADLAC II-like). associated power conditioning systems are the most developed, but dielectric cavities and their drivers are currently under active study at SNLA. The properties of these cavities a d their relative merits are discussed by Humphries. ?15) . For brevity we discuss the ferromagnetic technology developed at LLNL. To reduce weight, the volt-second requirement of the cores must be minimized by using the cores over and over through common recirculation. The cores are reset during the beam transit outside the accelerating gaps. The multiple pass use of cores in turn requires development of a high frequency driving circuit for the cells. The driver provides both the accelerating voltages and the reset pulse. The invention and small scale t sting of the branched magnetic driver (BMD) by Birx (1%) provides a driver which can perform these functions in the L: 35 MHz range over a limited number of pulses. This frequency nicely fits the compact accelerator transit time requirements. Tne circuit can in principle be recycled for a larger number of pulses at a rate determined by the recovery time of the first intermediate store switch (- 30 usec with present thyratron technology). Faster recovery switches are under development; e.g., the crossatron (- 1 usec) and photoconductive switches. The BMD utilizes magnetic cores (saturable inductors) for pulse Compression and these com?onents dominate the weight of the driver. The weight is roughly proportional to the energy supplied to the beam within the switch recovery time. It is noteworthy that use of a BMD in a recirculating device reduces driver weight by factors of 2-4 below the weight of magnetic pulse compressor designs of a linear system; the later stages of compression are used several times ir. the BMD. In 1984, two new accelerator driver designs were proposed by Smith to reduce weight. Birx designed one version, the Cyclic Induction Accelerator, using BMD and ferrite cores, and Smith de igned Cl38 a similar device using dielectric cavities. Both accelerators were closed orbit configurations with toroidal and stellarator fields, as in the Racetrack Induction Stellatron. Weights were reduced by a factor of five or so below the longaccelerating-pulse design of Schlitt referred to above. A novel accelerator concept also using bunched beams and multiple pulse cavities h s been recently proposed by Hasti et al. at SNLA (177 (see Figure 2). The accelerator uses mismatched dielectric cavities which ring q-8 times to provide the high frequency switching for multiple pulses. This accelerator is the highest gain/pass (- 100 MeV) concept yet considered. Both spiral line and closed orbit configurations have been proposed, but the closed orbit device is the preferred embodiment. The novel feature of this accelerator is the use of ion focussed tr nsCl.27 port for beam focussing and energy bandwidth. Finally, we mention a new lightweight embodiment of the spiral line concept proposed by us which uses the BMD/ferrite core technology with gain/pass ;y,“,“,“,‘,

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PHYSICS ISSUES It is not possible in this survey to discuss the physics issues of the compact accelerators in Orbital dynamics and the negative mass any detail. and other collective instabilities, in particular, g and strong will be covered for the various gui “tbl focussing schemes in the next paper . Perhaps the main concern of high current, finite gap induction accelerators is the beam breakup (BBU) instability, a problem arising because interest has been directed towards faster acceleration times than practically achievable in conventional air or iron core betaFinite gaps introduce discontinuities in the trons. beam pipe and have an associated transverse shunt [Generally the accelerating cavities are impedance. very low Q (< 10) compared to RF cavities.] Offaxis beams excite transverse magnetic deflecting modes which grow during the beam pulse and from cavity to cavity. The BBU limited the current transport in the early ATA experiments and was finally suppressed by the use of the ion focussing technique which provided an equivalent strong focussing field (lo-20 kG) and, because of the non-linear radial forces about the ion channel, phase mix damping.(‘) Without phase mix damping, the BBU growth is controlled by increasing the beam pipe radius, the focussing field strength, and the electric field in the gap. These parameters, of course, have practical limits. The ATA experience at LLNL has motivated development of lower shunt impedance and higher gap ;;?;;a~~~/~~ designs. Recent theoretical work by has shown that the instability can be stabilized for arbitrarily high energies by introducing a spread of betatron wavelengths such that the damping rate exceeds the instability growth rate. Energy spread and the ion focussed channel and/or sextupole or octupole magnets can provide the betatron detuning. Possible attendant emittance growth from nonlinear radial forces can be controlled by damping at low excitation levels, not surprisingly. All in all, there is reason to hope that the BBU growth can be adequately suppressed by designing cavities and power feeds which avoid reflections of the excited TM modes, by increasing the electric field in the gaps, and by introducing betatron freThis topic clearly requires more quency spread. theoretical and experimental study. A summary of current recirculating ;+;u;;;,~;+s#erical

The ion channel is generated in the curved geometry by a low energy (< 1 keV), low current (1. 1 Amp), long pulse electron beam which is guided by a weak bending field, a technique proposed by B. Godfrey. The vertical field in the bends is rapidly ramped in time to provide the guiding of the accelerated beam. The ion focussed channel is projected to provide as much as + 100 MeV bandwidth at 500 MeV in the bends to accommodate the significant head-to-tail variation in the bending field during transit of the bends and the energy spread inherent in ringing dielectric cavities. Beam injection and extraction are not yet defined in detail, but wire guiding, gas and laser channel techniques are being ConSiThe experiment at SNLA uses a 1.5 MeV, 15 kA, dered. 50 nsec injector and is studying the transport around a full turn, including passage through an acceleraPrelimiting cell giving a 1 MeV energy gain/pass. nary results show that about 60% of a 9 kA injected beam is transported around 270°. Maintenance of the integrity of the ion channel during multi-pass and multiple pulse acceleration cycles is a key issue in the development of this accelerator. THE SPIRAL LINE INDUCTION ACCELERATOR (SLIA) The new SLIA embodiment proposed by the author and his colleagues builds upon the BMD and ferrite cavity technology developed at LLNL (see Figure 3). ,,C,W:,,lO”

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A toroidal field threads the transport tubes, and vertical and stellarator fields are incorporated in the bends. Two of the main attractive features of the device are the open-endedness, as mentioned and the static fields in the bends over the earlier, macropulse which allow very low field error designs. A single toroidal coil is being considered around all the beam pipes in the accelerating regions for mechanical simplicity, and all field coils are enclosed in conductors in the bends to avoid “cross The shields are designed to prevent field talk”. diffusion over the time of the macropulse. The beam pipes are necessarily off-axis in the accelerating regions and shielded gaps (Figure 4) are used to pre-

many of the physics issues of high accelerators, including many simulations, is given by Godfrey

THE ION FOCUSSED REGIME RECIRCULATOR (IFRR) SNLA is currently experimentally investigating the IFRR concept which uses ion channels for beam energy bandwidth, and BBU suppression.(’ focussing, A schematic of the accelerator is shown in Figure

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vent low frequency beam deflection due to asymmetries in the return currents. A similar gap design was used by Wilson at NBS in his recirculation experiments. The high frequency modes of these currently being studied by Miller at SNLA.Y%3 are Growth of the EBU is probably the main concern of this accelerator. A beam transport experiment for the SLIA withis in progress at Pulse Sciences, c eleration :$%2cr A nominal 1 MeV, 1 kA beam is injected into a transport line including a 180°, 0.5 m radius bend to study the energy bandwidth of the stellara;;?2;fndings, the Hughes-Godfrey 3-wave instabili, and control of emittance growth.

Sciences, Inc., communication.

San Leandro,

I. Smith, CA 94577,

4)

See, e.g., Sessler, A.M., and Yu, S., Relativistic Klystron Two Beam Accelerator, UCRL Report 96083, LLNL, February 1987.

5)

M. Friedman and V. Serlin, Phys. Rev. Lett., 55, 2860 (1985), and App. Phys. Lett., 2, 596 (lm6); also S. Humphries, Jr. et al., Bull. of APS, Vol. 31, No. 9, 1506 (Oct. 1986).

6)

of Accel. J.M. Peterson, 7th Conf. on Applications North Texas State Univ., in Res. and Industry, Denton, TX Nov. 8-10, 1982; See also D. Kerst, 5th International Conf. on High Power Particle Beams, CONF-830911, LLNL, 433 (Sept. 1985).

7)

P. Sprangle, C. Kapetanakos, J. App. Phys. q4 Comments on PlasmaP. 1, 1978; N. Rostoker, Physics, 6, 91 (1980).

8)

C. Roberson, (1983); also,

9)

C. Kapetanakos, Toroidal, Induction Accelerators, conference.

THE INDUCTION SYNCHROTRON ACCELERATOR (ISA) Very recently the author has proposed a new hybrid accelerator concept which in principle is the lightest weight induction accelerator suitable for high kinetic energies. The ISA uses a SLIA as a high energy injector (- 100 MeVi) to a closed orbit induction Unit which accelerates the beam to high energies. (A high gradient LIA or a folded LIA inje-tor can also be used with weight penalty.) Injection and extraction of the beam with the SLIA and the beam is injected into are straightforward, A second the closed orbit unit via a kicker magnet. Because the kicker magnet is used for extraction. beam is bunched (- l/5 the transit time) and the closed orbit Iunit uses more conventional synchrotron transport magnet design without toroidal fields, the The beam injection and extraction appear feasible. circulates hundreds of times at a few MeV/pass in the main accelerating unit, depending upon the desired interpulse separation and constraints due to transport ir. the ramped vertical field. Tens of usec appear feasible for the acceleration time. The voltseconds and weight of the cores and the BM driver ireight are now reduced significantly, and we have avoided the injection and extraction problems associated with the low energy injection of other closed High energy induction accelerator designs. orbit injection also allows control of the tune shift due Careful design to self-field and image effects. studies are needed to establish the feasibility of the concept.

Pulse private

3)

et al., Part.

Phys. Accel.,

Rev. 2,

Lett. z, 79 (1985).

High Current Proceedings of

10) A. Mondelli and C. Roberson, Vol. 15, 221 (1984).

Part.

11) C. Kapetanakos (1985).

Accel.

et al.,

Part.

507

Electron this

Accel., fi,

73

12) S. Humphries, Jr. and D. Woodall, Bull. of APS, 28, 1054 (1983); also, S. Humphries, Jr., Vacuum Transport Experiments in the UNM High Current Betatron, Proceedings of this conference. 13) L. Schlitt et al., tor, PSI-FR-21-167, Leandro, CA 94577

Racetrack Induction AcceleraPulse Sciences, Inc. San (May 1984).

14) M.A. Wilson, IEEE Trans. NS-28, No. 3, 3375 (June 15) S. Humphries, Jr., ticle Acceleration, 1986, Ch. 10, pp.

SUMMARY We have discussed the evolution of high current, compact and lightweight accelerators over the last few years or so and shown that several key developments have occurred. Among the many contributions, we suggest the following iteas as potentially of high impact: (1) the ATA and RADLAC II accelerators and their associated power technology (2) the incorporation of strong focussed windings; (3) the invention of the branched magnetic driver; (4) the new ideas for high power/length RF drivers; (5) the use of ion focussed transport; and (6) the theory of and experiments on BBU suppression. As experiments it is very iikely that new problems will progress, be discovered and new ideas generated.

16) D. Birx, IEEE Conf. Swp . , 4 (1982).

on Nut. 19811.

Principles New York; 283-325. Record,

Sci.,

Vol.

of Charged John Wiley

15th

Power

Par& Sons, Modulator

17) See proceedings of this conference: S. Shope, et. al., Acceleration and Bending of a Relativistic Electron Beam on the Sandia Recirculation Linac; T. Hughes and B. Newberger, Numerical Studies of Ion-Focussed Transport in a Recirculating Accelerator; and W. Tucker, et. al., Recirculating Electron Beam Linac. 18) A. Mondelli et al., A Strong Focussed Spiral Line Recirculating Induction Linac, to be published in Conf. Proc. of “Beams ‘86”.

REFERENCES 1)

D.S. Prono vol.

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et al., NO. 5,

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19) C. Caporaso, SLAC Report

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20) 8. Godfrey and T. Hughes, High Current Electron Beam Transport in Recirculating Accelerators, Report AMRC-R-865, Mission Research Corp.; Albuquerque, NM 87106 (Nov. 1986).

IEEE Trans. on Nut. SCi., Vol. R.B. Miller, ~~-32, NOV. 5, 3149 (Oct. 1985); M. Mazarakis, RADLAC II Accelerator Beam Experiments, Proceedings of this Conference.

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V. Bailey, et. al., the Strong Focussed, Induction Accelerator, conference.

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