Charged Particle Beam Weapons: Should We? Could We?

 

T

. he national news media, from magazine and newspaper articles to a CBS 60 Minutes presentation, ^ave focused considerable attention on the possible ^evelopment of charged particle beam weapons, both ^ere and in the Soviet Union. Controversy has grown ^rapidly in the wake of articles in Aviation Week & Space Technology reporting retired Air Force Major General George Keegan’s estimate that the Soviets ^are very close to developing a charged particle beam ^weapon (CPBW). Some respected scientists doubt that ^Such a weapon could be built or would have any ad­^Vantage over existing systems, but they feel the pres­^ent research program should be continued in order to ^resolve unanswered questions. Others have convinced k ^ernselves that there is no possibility of a useful earn weapon and advocate curtailing current inves­tigations. Indeed, the step from the high energy

P ysicists’ particle accelerator to a weapon is a long °ne,

Respite possible obstacles, a modest national re- ^arch program is in progress to determine whether ^arged particle beam weapons have a place in our ^tUre arsenal. Scientists most familiar with the pro- £ram feel that the admittedly high-risk investigation ^rnust be conducted simply because the potential Payoff i_{s ver}y j_arg_e if _th_e remaining questions can ^ favorably resolved, a CPBW application could be ^e defense of ships from missile attack. Even up­graded versions of shipboard missile and gun systems probably prove inadequate for coping with the ^Uc^ more capable antiship missiles of the future. If CPBW can do the job, the value in terms of fleet ^s|rrvivability would be enormous. The capabilities of th ^S ^^0I:ent*^a^ ^weaP^on constitute a revolution greater an the leap from naval guns to guided missiles; really new tactical thinking will be required to ^{1 e} maximum advantage of these new systems. In ⁿ> *r is this tactical reformulation which must in- ^ev'tably direct technological research in order that

areas of maximum potential payoff can be fully in­vestigated and so that possible weak points can be identified early.

A charged particle beam weapon, as the name im­plies, projects a high current pulse of electrons or ions in a beam from a nozzle. These particles can carry a great deal of energy and travel at nearly the speed of light. On striking a target, such as an in­coming missile, the beam penetrates deeply and de­posits its energy in a long, narrow cone throughout the target. For high-energy electrons, this cone can penetrate a few feet of solid aluminum, making shielding very difficult. Damage can be immediate and severe and includes nearly instantaneous high explosive detonation, structural damage, and elec­tronics destruction or disruption by a variety of mechanisms.

Some conceivable missions for CPBWs are ballistic missile defense (both ground-based silo defense and space-based destruction of ICBMs in the boost phase), space-based antisatellite defense, and fleet defense against antiship missiles. There is great variation in the details of the CPBW systems required for each of these missions. We are concerned with the applica­tion which could have the most immediate interest to the Navy and which, since it is the least demand­ing, has the most promise of relatively near-term development—antiship missile defense (ASMD).

The ASMD mission, whether using guns, missiles, lasers, or CPBWs, encompasses two possible applica­tions, point defense and area defense. The latter would provide protection from nuclear-armed cruise missiles as well as conventional ones, and the point defense system would provide protection from “leak­ers” from massive conventional raids and from low- warning attacks at close range. For the present, we will confine ourselves to a hypothetical point defense CPBW.

The major component of the CPBW would be an accelerator which has a function similar to the elec­tron gun in a television picture tube, although hav­ing far greater size and power. In addition to a weapon system based on firing electron beams, we could just as well use beams of protons or other ions.

The Stanford Linear Accelerator at Stanford University accelerates electrons to high energies for, use in physics experiments. No one seriously consid­ers a device of this type to be usable as a weapon. For one thing, it is 2 miles long. However, the use of this specific example will make the following discus­sion somewhat more clear; an accelerator designed for a weapon would have very different characteristics, such as higher current and lower electron energy, but the principles would be the same.

The energy contained in a pulse of electrons fired by an accelerator is equal to the product of the cur­rent, the accelerating voltage, and the duration of the pulse. The Stanford accelerator has 50 milliamps of electrons accelerated to a potential of 20 billion volts. The energy contained in the 1.6 microsecond pulse of electrons is therefore 1,600 joules, or 1.6 kilojoules. In each second, the accelerator fires 360 of these pulses for a total energy of about 0.6 mega­joules. A practical weapon might put about three times this energy into one large pulse or a few pulses fired close together. It would thus have a bolt energy of 1.8 megajoules.

What is the significance of this amount of energy when deposited in a target? The bolt energy of 1.8 megajoules is roughly the energy released by one pound of high explosive. Not all of the beam energy would be translated into destructive shock waves in the target, but the analogy of embedding a small stick of dynamite deep inside the target is an apt one. The high energy of the Stanford accelerator’s beam has, at times, made itself very apparent by lit­erally exploding small metal structures when inad­vertently directed at them by an equipment malfunc­tion. Stopping electrons at such high energy is a dif­ficult problem and is solved at Stanford by running the beam into the side of a hill as shown in the photo on page 31.

Not all of the energy in a pulse reaches the target, however, because energy is lost during propagation through the air. There are several complex mecha­nisms causing this energy loss, but the loss is roughly proportional to the density of the air. Under normal conditions the pulse will lose half its energy after traveling about 200 meters. This would seem to limit the range of any CPBW severely, but the answer lies in what happens to the energy lost from the pulse. Much of it goes into heating the air along the path of the beam. In a few microseconds this hot air will expand, leaving a tube or “channel” of much lower density behind for the next pulse to follow with minimal energy loss. Using bolts consisting of a string of pulses may allow propagation over long dis­tances .

Propagation through air also tends to scatter the electrons in the pulse so that the beam would gradu­ally spread out and lose its effectiveness. Inhibiting this, however, is an effect without which a CPBW would probably be useless. The large currents in the beam create a magnetic field wrapped around the beam; the field, in turn, pinches the electrons closer together and keeps the beam from expanding. This effect works only when the beam is traveling through air or some other gas and is one of the reasons large currents are required. Typically, this “self-focusing effect would keep the beam diameter down to a cen­timeter or so in air, so that when the beam hits the target, large quantities of energy are deposited in a very small volume. These high energy densities pro­vide the beam its great lethality.

One great advantage any CPBW might have is the simple way it can be aimed at the target. The beam of electrons will curve in a magnetic field. Running the electron beam through a powerful magnet changes the direction. Change the current in the magnet’s coils, and the beam direction can be altered at will. At least two types of nozzles could be builti one of which trains mechanically and uses the mag' net for elevation. The other has two-axis magnetic steering, like the deflection coils on a television tube, and uses no moving parts. Very rapid slewing would be possible with this nozzle.

Other aspects of CPBWs also have great tactical significance. Much of the energy lost by electrons moving through air consists of high energy photons (gamma rays) emitted in a very narrow cone about the beam. This radiation can damage or disrupt eleC' tronics in arming, fuzing, guidance, and control cir­cuits in the antiship missile, even though the beam may not score a direct hit. The short burst of high current in the beam also produces a very strong elec­tromagnetic pulse. These phenomena associated with the beam could have significant effect on an enemy missile and may be sources of interference or damag^e to friendly systems if not properly controlled.

The present national CPBW effort can be charac­terized as a high-risk, potentially high-payoff inves­tigation of the basic physics issues behind the genera­tion and propagation of a charged particle beam- These are very difficult problems requiring eith^er elaborate computer simulations taking years of deveh

°pment time or direct experimentation with large, expensive accelerators. The program combines the two approaches by developing theory, predicting be­havior of a small accelerator, building the accelerator to check the theory, improving the theory to devise a larger device, and so on. This is the only logical way to proceed with a high-risk (large uncertainty of suc­cess) technology program, because it avoids enor­mously expensive blind alleys and mistakes which ^might occur if intermediate steps were skipped.

Important to note is the fact that if the physics of propagation and control proves satisfactory, no great

eaP in technology will be required to implement a ^ in a ship. The technology of some present ac- ^erators, while yielding a CPBW less compact than ^ab is adequate to provide prototype weapons. Fu­. ^{e te}chnological improvements might significantly ⁰¹ Prove a CPBW, but they would not be necessary to . ^e it work. The system would probably be rela- ^e*y simple, reliable, easily maintainable, and have ^ne*ther exotic technology nor critical tolerances.

Even though the most difficult of the outstanding questions are ones of physics, and even though the whole issue of whether or not a CPBW can be made to work will not be resolved until the physics questions can be answered, serious investigation of the tactical characteristics and potential employment of an elec­tron beam antiship missile defense weapon needs to be conducted now. Decisions based on the tactical employment of a potential CPBW will define specific characteristics to be achieved and requirements for key support systems so that any needed long-lead de­velopment can be started. In order for us to examine

a potential CPBW system critically from an opera­tional viewpoint, we need first to understand the tac­tical, as opposed to technical, dimensions of the CPBW and the trade-offs between these characteris­tics. We should begin with some of the problems with current systems.

Why is there a need to explore an admittedly high-risk rechnology? The answer lies in the inability of present systems to protect ships from future gener­ations of antiship missiles. As new ASMs become fast­er, more maneuverable, and fly at lower altitudes, the limitations of current types of ASMD weapons will become more pronounced. One major limitation is the velocity of the ASMD weapon compared with that of the ASM. This affects the engagement in sev­eral ways. First, for unguided projectiles the weapon system must predict where the target will be at the time of intercept. Low weapon velocity lengthens time to intercept and makes the prediction more and more uncertain, particularly if the incoming missile is very fast and makes high-g maneuvers between the time of firing and intercept. Closed-loop aiming sys­tems do not greatly reduce the problem, because the time constant of the loop is governed not by how many rounds are put through the muzzle per second, but by how long it takes one round to get to the vicinity of the target. Until the first round reaches the target area, no aiming correction can be gener­ated and subsequent rounds will all have the same aiming error. Clearly, if the ASM significantly alters its velocity on a time scale shorter than projectile time of flight, even the best fire control system can be defeated. Many of the ASMs threatening the Navy today are, of course, large, relatively slow, and non-maneuverable, but it is not difficult to imagine sea-level, hypersonic (Mach 4), highly maneuverable (lateral accelerations greater than 50 g) ASMs in the 1980s. And where does that leave even the best un­guided system?

Of course, as the antiship missile approaches the ship, time of flight decreases and the ASM has less time to maneuver so that at some range the unguided projectiles will catch it. However, for a rapid firing machine gun which must actually hit the target with a small projectile, this range for assured stopping of >' Mach 4, 50g ASM is less than 60 meters. This range cannot be improved no matter how smart the fire control system is; it is purely a function of the speed of the projectile and the geometry of the problem.

Avoidance maneuvers by the ASM can be, °t course, countered by maneuvers of guided munitions (missiles, laser-guided projectiles) so that initial intercept predictions are nowhere nearly so critical- Even for such ASMD weapons, relative velocity effects constitute serious limitations. The ASMD missile must have significantly higher speed and greater ma­neuverability than the ASM to make intercept possi­ble. For equal capabilities, the attacking missile al­ways has an advantage over the defending missile- This drives up the cost and size of the missile and limits rounds carried. There is some point of di­minishing returns in improving missile performance- If rhe ASMD missile should miss the ASM, the time available for firing additional rounds is still a func­tion of the speed ratio. At hypersonic velocities, launcher loading and slewing times become highly significant and may easily prevent a second shot for a point defense system.

The relative velocity problem is a potentially fat^;d flaw of our present systems when they are examined in the light of expected threats of the future. Exist­ing systems are, of course, constantly being evaluated and upgraded to meet more and more potent threats- To guard against reaching a dead end where no pos­- sible improvements can make one or more of out present weapons effective against a new threat, "’^e must examine new technologies. We cannot assume 3 “pie-in-the-sky” attitude in which we sacrifice pres­ent security for something we may achieve in the future, but we also cannot ignore the possibility that we may someday desperately require new concepts- Investigating them ahead of time is only prudent.

The tactical characteristics of-a conceptual CPB^ are listed in Table 1 and a subjective comparison with other systems, including additional potential!) fatal flaws, is shown in Table 2. Some capabilities o‘ the CPBW' are radically new and will have a profound effect on our tactical thinking:

Y Essentially unlimited ammunition suppl)'' Maximum firing rate could be used against hundreds of ASM attacks.

► Very high rate of fire. An energy storage schem^e

^ All-weather operation.

Clearly, a weapon with these characteristics de­mands a careful examination of the most effective ^t;lctics for employment and of the concepts to be ^Used in the controlling weapon system. Typical of ^rfi^e kind of new thinking required is the following j'rgutnent. Reliable, rapid kill assessment has always ^een important in weapon system design, yet it is often very difficult to achieve. Surprisingly enough, O'vever, kill assessment may be completely irrele- ^v‘>nt for the point defense CPBW. In any engagement rfiere are three actions that might be taken after mak- 'ⁿS a kill determination: (1) if the target has been

Range Firing rate

Lethality (hits to kill)

Slew time Velocity of pulse Effective bolt velocity

Fuel consumption (“ammunition”)

(assuming 10% efficiency)

"Ammunition" capacity

^c°uk| either boost peak firing rates for short periods ^Hr reduce the size requirements on the primary power source (for example, gas turbines).

^ Near speed-of-light velocities. Even if the effec­tive bolt velocity required for channel formation is considered, target motion during propagation time is nearly zero and target maneuvering is irrelevant.

^ Extreme lethality. One hit can detonate the target 'varhead.

^ Very high slew rate obtainable since there are no moving parts. This enables nearly simultaneous en­gagement of multiple targets with only one CPBW. Slewing to a new target can be done without decreas- ^lng firing rate.

\ Unsaturable. A combination of lethality, Empower, and slew rate can be achieved to defeat any conceivable coordinated attack.

^ Relatively difficult to defeat with countermea­sures.

killed, shift fire to another threat; (2) if the target has been killed, cease firing to conserve ammunition; and (3) if the target has not yet been destroyed, bring additional weapons to bear, possibly releasing them from less threatening targets. Since our elec­tron beam has an essentially unlimited ammunition supply, knowing exactly when to cease fire is not as important as in an ammunition-limited system. If there is only one threatening target within range, the tactics are obvious: fire at the target at full firing rate until the missile splashes, passes out of range, or hits the ship. In this case, a kill assessment is not impor­tant.

A characteristic of kill assessment is that it gener­ally takes a significant amount of time. In the event

Table 1

Conceptual Tactical Characteristics of a Point Defense CPBW'

1-3 kilometers 100 bolts/second 1 (1.8 megajoules/bolt)

10 milliseconds

300.0             kilometers/second

1.0                   kilometers/second 450 kilograms per second of sustained firing

Table 2

Subjective Comparison of Various Point Defense ASMD Concepts

Velocity	+	—	—	—	—	+
Firepower	+	+	___	___	___	+
F-ethality	+	—	+	+	+	—
^stained fire	+	+	+	—	—	+
^*11 radius	- (+?)	-	+	+	+	-
Countermeasures	+	—	+	-	-	—
Range		—	+	+	+	-
father ^°te< The implicit assumption research program. Certainly ~~ an advantage ~~ a disadvantage	+ is that the CPBW no fatal flaws are	+ will work and that no fatal fl: obvious at the present.	+ aws will be found by	+
~~ a fatal flaw which cannot	be overcome by merely improving performance

CPBW Small caliber guns Large caliber guns Missiles Guided projectiles Lasers

12.0             bolts in 20,000 gallon tank

of warhead detonation, of course, topside observers or tracking radar operators could detect destruction very quickly and relay the information to the man or ma­chine in tactical control of the beam. Even this best case would take a few seconds, which is much too long for any meaningful engagement. A “soft-kill” might not be determined until the missile has splashed or flown past the ship. A Mach 1 missile will reach a ship from 1 kilometer in less than three seconds. The degree of sophistication in timing an attack in which more than one missile arrives within a three-second period surpasses current or planned capabilities of the United States and presumably those of the Soviet Union. If missiles arrive more than three seconds apart, they may be engaged as individual targets by firing at them continuously as long as they are in range and closing. If a multiple, simultaneous attack A mounted, a CPBW has the abil­ity to engage several targets sequentially in only a few milliseconds with the extremely fast slew of an all-magnetic nozzle. Division of fire has been a tacti­cally unsound idea in the past since the firing rate was low. A CPBW with an appropriate power source could fire perhaps 100 bolts per second, any one of which, if it were to hit the missile warhead, would destroy the missile. Thus, a logical tactic for engag­ing multiple targets would be to engage the first one as it crosses the range envelope, alternate between the first and second as soon as the second is in range, and add others to the rotation as they enter the envelope. Those which have been killed or pass by the ship would be dropped. A modification might be to increase the number of bolts fired at a given target as it neared the ship. Clearly, this calls for an ex­tremely fast and sophisticated weapon control system in which men could play only a minimal role.

In the face of the capabilities of a properly inte­grated CPBW system, secondary batteries for point defense make little sense (of course, redundancy is wise to provide ability to withstand damage or in event of failure). Thus, the third reason for needing a kill assessment, allocation of additional assets, is also unimportant, because saturation of a CPBW is virtu­ally impossible.

This line of thought establishes a trade-off be­tween kill assessment and system characteristics. If the CPBW concept works, we can make a system that will work well without kill assessment; but if we can be assured of quick, accurate kill determinations, we might feel safe in reducing bolt rate and overall sys­tem size, weight, and cost. The possibility of trade­off suggests several lines of investigation for the physics and technology program.

Other questions become apparent on reflection.

How do we tactically employ to its greatest advan­tage a weapon capable of engaging several targets on a millisecond time scale? With decisions required on such a time scale, people will certainly be out of the immediate problem, which means tactical decisions must be made well before the engagement and pro­grammed into the CPBW system. There are all sorts of trade-offs between bolt energy, range, average power, peak power, size and weight of the ac­celerator, and size and weight of power sources. Keeping in mind these trade-offs, what is the best set of weapon characteristics for a given ship and a given mission? This question can be answered only by a detailed study of tactical employment.

Effectiveness of a weapon is strongly dependent on the countermeasures employed by the enemy. It has been suggested that a particle beam weapon could easily be defeated by supposedly simple expedients such as smoke screens, chaff, screening explosions which may deflect the beam, and decoys. In fact, a CPBW is probably the least susceptible of the weapon systems shown in Table 2. All these weapons have some sort of search and fire control system which would share nearly the same vulnerabilities; but at the close ranges permitted by the great destructive­ness of the CPBW, countermeasures become much less effective. The beam itself penetrates deeply into the target so that shielding would be relatively ineffec­tive. The beam would not have the weaknesses of ^a high energy laser which could be partially defeated by an ablative coating on the ASM, or of a small- caliber projectile which may be armored against. The multiple target situation of decoys could probably h^e fought more successfully by a rapid-firing CPBW than by any other weapon.

Deploying this host of countermeasures is no eas)' task for the enemy. These would force trade-off^s upon him in terms of the size, speed, and maneu­verability of the ASM, making it more susceptible to other systems. Recognizing that any given threat can be countered and that any given defense can be pene­trated if one has the time and pays the price, we se^e that what a CPBW really buys for us is a much greater assurance of survivability in the 1990s. As a part 0 the “defense in depth” doctrine, a CPBW could mak^e the enemy’s job very difficult—and very costly.

Weapon control system concepts also need reex­amination. We must ensure that the accompanying fire control system (radar, electro-optical) is design^e to match CPBW capabilities. For example, since v'^e are trying to achieve a direct hit on the target with ^a small-diameter beam, it might be thought that would need a closed-loop aiming system, analogou⁵ to the Vulcan/Phalanx system, which measures m<^sS

and

covert force operations?

distances and automatically corrects the aiming. In fact, for the application and characteristics cited, a closed-loop fire control system may not be necessary ^ar>d might simply open an area for the enemy to try ^t0 defeat with countermeasures. A cruise missile with ^a diameter of 0.5 meters subtends 0.5 milliradians at ¹000 meters. Assuming beam propagation can be Properly managed, a system accuracy of 0.3 milli­radians or less should be readily achievable. There­fore, with good battery alignment techniques, it should be possible to hit targets without adaptive ^a>rning. The high firing rate also permits firing a Psrtern to allow for uncertainties and systematic errors or countermeasures.

Even more important to the question of accuracy is ^che fact that a direct hit may not be necessary at all, •dthough a warhead kill would be desirable to guarantee that the ship wouldn’t be hit, not even by ^a disabled missile. Radiation cone effects may well ^neutralize targets at tens of meters from the beam, taking aiming requirements trivial.

Again, tactical considerations (What is the ^rnimum tolerable kill range? How long can the ^ystem afford to fire at one target before other ASMs ^ec°me a problem?) drive technological questions (Is ^a faosed-loop system needed? How much accuracy is deeded in the nozzle design? What are the trade-offs etween fire control system type and total system ^s,2e, cost and weight?). Without innovative tactics ^ernPl°ying the potential of the CPBW to the fullest, ^and without accurate, timely threat assessment a '-f’BW may be saddled with a weapon control system ^esigned to cope with a ten-year-old threat or a sys-

tern so overly capable as to be prohibitively expen­sive.

. Analysis of mission, threat, technological capabil- '7’ and tactical employment leads to system defini- ^t|Qn. The process is a closed loop and must be con- ^tlfiUous throughout the research and development of ^atly new weapon system. While detailed hardware ^and systems development is not appropriate until ^w physics issues are resolved, there is a definite ^need for pragmatic, tactical evaluation by profes- ^Sl°nal naval officers. Among the questions which ^'SEt profitably be considered are these:

^ What are the issues of task force employment ⁿ8e, electromagnetic interference, command and

J-ontrol) and what might a fleet tactical doctrine look like?

^ What is the ideal balance between bolt rate and ^stem size for threats of the 1990s?

^hat is the impact of CPBWs on emission control

^                                            * >■         *c/i V.S. wjfVv-i uuuuj.

What are fire control and sensor requirements as a function of threat and mission?

►   What tactics would be employed with existing weapons in attacking an enemy ship armed with a CPBW? What new offensive systems might be re­quired?

►   Will present computers be adequate to cope with weapons control and tactical computations for a high-firing-rate CPBW?

Answers to these questions depend on the antici­pated engagements and how we would like to fight them. Work by officers at the Naval Postgraduate School or the Naval War College might help decide which particular CPBW characteristics are tactically most important to maximize the effectiveness of this potentially revolutionary weapon. This can help guide research in the directions of greatest payoff and prevent unpleasant surprises in areas which might otherwise be overlooked. The ultimate payoff may well be a weapon system which will radically alter the capabilities of the fleet at the end of the 20th century.

PI Lieutenant Commander Wright was graduated V with high honor from Michigan State University J in 1967 with a B.S. in physics and mathematics. I He spent the years 1967-1969 as a National Sci- I ence Foundation fellow at the California Institute ^ of Technology, where he received an M.S. in ' physics. Following commissioning via Officer Candidate School in 1969, he served as main battery assistant in the USS Harwood (DD-861) through 1970. He then resumed his NSF fellowship at Cal Tech under the Navy scholarship program and received a Ph.D. in physics and astronomy in 1973. After that, he served tours as fire control officer in the USS William V. Pratt (DLG-13) and weapons officer in the USS Henry B. Wilson (DDG-7). Since 1978, he has been a military research associate at the Lawrence Livermore Laboratory, Livermore, California. He is doing theoretical physics work and systems analysis in support of the particle beam program.

----------------------------------------- Bibliography_________________________

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Garwin. "60 Minutes on Particle Beam Weapons." The Bulletin of the Atomic Scientists, February 1979, pp. 50-52.

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"The Next Generation of Weapons.” Nature, 18 January 1979, p. 161. Paramentola, John, and Kosta Tsipis. "Particle Beam Weapons." Scien­tific American. April 1979, pp. 54-65.

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