426

THE LIGHT ION FUSION EXPERIMENT (LIFE) ACCELERATOR SYSTEM FOR ICF*

Zaven G.T. GuiragossianTRW Defense and Space Systems Group, Redondo Beach, CA 90278

ABSTRACT

A Light Ion Fusion Experiment (LIFE) accelerator system is under studyas a driver for ICF. The system consists of separate functional ele~ents.Light ions are extracted from a pulsed cold plasma source and accelerated inmulti-grid and mUlti-aperture accelerator structures, with provision forstrong compression of beam pulses. 0 or He ion beams, 20 kA, 1-10 MeV, will bemade to ballistically propagate in 10-4 torr gas by externally generated co-moving electrons which provide space-charge and current neutralization. Thepropagation method is relevant to the Heavy Ion Fusion program and helps

to reduce the large number of heavy ion beams entering a reactor to a mana-geable few. The system is also a useful test bed to perform several propa-gation experiments in the near term. The reactor requirements in HeavyIon Fusion and LIFE are identical.

*Work performed under the auspices of U.S. DOE, contract No. OE-AC08-790P40109

427

INTRODUCTION

A novel light ion-accelerator system was conceived(l) at TRW to serve

as a driver in ICF, and a study is now in progress to validate some of the

key concepts in the system. The LIFE accelerator system is differentin many respects from the mainline light ion driver approach at SandiaLaboratories(2) which consists of magnetically insulated diodes to produceMA 1eve1 beam currents and propagate in preformed plasma channel s, typi cally

in a 50 torr ambient gas pressure. Because of the differences to bedescribed, the LIFE system can best be viewed as an alternate-back up tothe light ion mainline system and also as a useful test bed, to perform se-veral propagation experiments in the near term which are relevant to theHeavy Ion Fusion program. Such propagation experiments appear to be criti-cally needed to provi de the exper-j menta1 feedback on detai 1ed ")arti c1 e-in-cell" simulations such as the one presented by D.S. Lemons(3 for theLASL-TRW collaborative effort.

CONCEPT DESCRIPTION

The LIFE accelerator is based on the use of separate function hard-ware elements and the system is optimized to fulfill the requirements ofan IeF ion beam driver. Considerations of potential IeF reactor scenariosand the state-of-the-art in pulse power technologies have also been madein the conception of the LIFE system. Separate hardware provisions aremade to provide the following functional elements:

1. Generation of Intense Cold Ions: A pulsed intense cold plasmaion source is under development in which plasma induction occursby pulsed RF power coupling in a large area. Ion extraction istimed to occur in the near afterglow regime of the plasma, about2 uS after RF power is terminated. In this regime, ion densityis high but ion temperature has cooled by ion-neutral atomic colli-sions. Light ions, D or He at densities of ni = 5 x l012 cm-3,and temperatures of Ti ~ 0.1 eV can be obtaine~ over a 2 m diameterarea by 1 MW, 1 ms, RF power at 300 kHz.

428

2. Intense Beam Acceleration: The LIFE accelerator concept is basedon beam stacking by a multi-aperture multi-grid accelerator struc-

ture in which each beamlet is channel foclJsed by shaped electro-static fields. A different example of this principle is the recentMEQALAC accelerator system by A.W. Maschke.(4) Current densities

of Jb : 1 A/cm2 are extracted and accelerated in a multi-gridsector with constant applied voltage, Vl' over a pulse duration oft p ~ 1 ~s. Beamlets are further accelerated in a following sec-tor with time dependent applied voltage, V2 + V(t), such that

Va = Vl + V2 is the final energy of particles in the beam front.V(t) = Va [(to/to - t)2 - 1], for t < to where to is the flighttime of the slowest particles at the beam front, in post-acce-leration ballistic drift. Typically, a total current of lace ~ 20 kA,constant to within a few percent will be accelerated with Vo ~ 1 MVand V(t)max ~ 10 MV, for 0 < t < t p. Beam1et extraction and initialacceleration must be performed with constant applied voltages topreserv~ a well defined perveance condition and Child's law at the

level of the plasma sheath. In this sector with Vl ~ 0.3 MV, beamletoptics are designed to focus extracted beams of rb ~ 6 mm down toa few mm, so that paraxial optics will apply in the subsequ~nt timedependent acceleration. An example of the multi-aperture electro-

static focusing accelerator structure is shown in Figure 1. Theoptics of radial apertures as the one shown or hole geometries areunder investigation with use of the codes(5) EGUN and ESQ. Figure 2presents the manner whereby the accelator structure is energized.

3. Strong Time Compression: Provisions are made to program strong timecompression of beam pulses at the level of 100:1 or 50:1, to deliver1-2 MA beam currents at a target as 1 ~s, 20 kA beam pulses fromthe accelerator are compressed to 10 ns in a ballistic transportdistance of 10-15 m. Strong pulse compression is not feasible insingle gap diode accelerators without introducing also strong beamdefocusing effects; for these, pulse compression is confined to thelevel of 3:1 to 5:1 and MA currents need to be accelerated in orderto deliver MA beams at targets. Accordingly, one of the main ad-vantage in the LIFE system will be the use of relatively low power

technology to energize the multi-grid accelerator structures. A

flexible, programmable and highly efficient pulse power method(6)

429

is conceived by I. Smith for this project whereby the desired V(t)waveform can be tuned with a few tenth of a percent regularity.

4. Focusing: In addition to beamlet channel electrostatic focusing,the accelerator grid planes are spherically shaped with a radiusof 10-15 m to provide the radial focusing of the entire beam.Provisions are made independently to apply systematic correctionsof trajectories by shaped electrodes, both as a function of beam

pulse time and overall beam radius.

5. Current Neutralization: The provision of current neutralization ofintense light ion beamlets will be made at the exit end of theaccelerator structure. It is required to produce co-moving andco-located electron-ion beams in order to have space-charge and

current neutralized ballistic propagation over distances of 10-15 min an ambient gas pressure of 10-4 torr. A detailed calculation(?)

of the single and multiple scattering of ions off the nuclear cou-lomb field and ion energy straggling due to atomic ionization of amedium produced the 10-4 torr requirement, so that 1-10 MeV 0

or He ions wi "11 not spread more than ~ 2 mm in 10m of transport in

N2 gas. Therefore, beam neutralization by the ionization of ambientgas at 0.1 - 1 torr pressures is not acceptable and externally

prepared co-moving electrons must be provided to the ion beam.The alternate propagation scheme is through pre-formed high returncurrent plasma channels which are not considered here. A method ofgenerating co-moving electrons is sketched in Figure 3. Theseelectrons must become co-located with the ions within 20-50 cm oftravel, otherwise the electrostatic field among electron and ionbeamlets will raise the temperature of electrons causing a pressurewhich limits the amount of ballistic focusing of ions. Co-locationis made possible by cusp magnetic fields which fan out co-movingelectrons to quickly mingle with the ion beamlets and become trappedin their potential wells. A special test bed is being prepared atTRW to address experimentally the issues of current neutralizationof intense and energetic ion beams.

6. Beam Power Profile Shaping: The flexibility in the LIFE acceleratorconcept in principle makes it possible to deliver a desired beampulse profile from each beamline while providing also a strong beam

430

pulse compression at the target focus. Ordinarily, in a multi-beam target compression arrangement each beam would be programmed to

fill a specified portion of the power profile, so that the totalityof beams but no each beamline provide the power shape desired by

target designers. Starting with one of the target design casespresented by R.O. Bangerter~8) a smooth pmlJer raise from 'V 10% to100% in 14 ns and flat top for 6 ns (see Figure 4), the timevarying acceleration voltage waveform in the LIFE system can beprogrammed(9) according to the shape displayed in Figure 5 to de-liver also a current amplified beam of 66:1 at the target focus,as shown in Figure 6. The accelerator will produce a nearlyconstant current source of 20 kA and in the above case, each beam-line will impart 60 kJ energy at the target. A configuration of34 beamlines is envisaged for a 150 TW, 2 MJ, 300 TW/cm2 LIFEdri ver.

STRONG BEAM PULSE COMPRESSION TEST

Since strong beam pulse compression is one of the key features of

the LIFE system, a small scale experiment was performed at TRW using most ofthe ingredients -in the concept, to explore some of the issues of technical

feasibility and hardware requirements. The experimental setup is givenin Figure 7 and details are reported elsewhere. (10) The time varying

acceleration voltage waveform was shaped by a 20 channel circuit, toaccelerate and pulse compress He+ ions up to 7 keV. A beam current of 0.6 rnAwas extracted at constant voltage and subsequently, a 1 ~s duration pulsecompressing voltage was applied in the following accelerator grids. Thepulse compression time focus was located 66 cm downstream from the endof the accelerator. After careful shaping of the time varying voltagewaveform, the 1 ~s beam pulse was compressed to 8 ns at the time focus.Figures 8 and 9 show a collection of measurements in the temporal behaviorof the detected beam current as a function of axial position from theaccelerator. Voltage waveform tuning was accomplished by a variety ofdiagnostic techniques including the behavior of the beam itself. Oncethe desired waveform was tuned,highly reproducible results were obtainedover long periods in time. No attempts were made to provide externally

431

current neutralization electrons or to design a good spatially focusingaccelerator structure. The achieved beam pulse time compression is125:1 in 66 cm of travel.

A test facility is being upgraded at TRW to scale up these measure-ments to the 500 keV level with 500 A, 1 ~s, He+ beams produced in a pre-prototype LIFE accelerator system.

432

REFERENCES

1. Z.G.T. Guiragossian, "Light Ion Fusion Experiment (LIFE), Key ConceptVa 1i dati on Study," TRW Document No. 33876.000, September, 1978;IINovel Light Ion Inertial Confinement Fusion Accelerator,1I PatentDisclosure TRW No. 11-170, June, 1979; and Bull. Amer. Phys. Soc.24,1033 (1979).

2. P.A. Miller et.a1. Comments on Plasma Physics and Controlled Fusion,5 (1979); D.J. Johnson et.a1. Phys. Rev. Lett. 42,610 (1979); G. Yonas,TEEE Trans. Nucl. Sci. NS-26, 4160 (1979).

3. D.S. Lemons, ilIon Beam Propagation Simulations," -in this proceedings.

4. A. W. Maschke; R. Adams, et.al. IIDescription of the Ml MEQALAC andOperati ng Resu1 ts ," in th is proceedi ngs .

5. W.B. Herrmannsfe1dt, EGUN, SLAC-166 (1973); A.C. Paul, EBQ, UCID-8005(1978).

6. I. Smith (private communication).

7. J.L. Orthel and Z.G.T. Guiraqossian, "Scatterinq and Straggling of the LIFEBeam," LIFE-TN-001.

8. R.O. Bangerter, in this proceedings (and private communication).

9. J.L. Orthel, IILIFE Beam Pulse Profile," LIFE-TN-003.

10. B.H. Quon, W.F. DiVergilio, W. Gekelman, Z.G.T. Guiragossian andR.L. Stenzel, IIGeneration of Strong Time Compressed Light Ion Beams,1I(s~bmitted to Jour. Appl. Phys.); and Bull. Amer. Phys. Soc. 24,1033 (1979). --

W0::::JI-U:::>~

l-V")

0::::

oI-

«0::::w...JW

Uw u.«

u..w_ 0::::.:J

...J I-0::::W0..

«I-

I-...J

::>~

...Q

0:::

l?I

I-...J

:J~

433

...W0:::::Jl?u.

L.I.F.E.MULTIGRID ACCELERATOR VOLTAGE DISTRIBUTION CHAIN

AC COUPLEDRF POWER

1\It) It'!ELECTRON I o ...: 0

FILAMENTS . . . I··· I~ I I~ I I Iu Z (J)C)w 9 C)~z

I~ I I I I I I I I I I I I~ I I~ ~~I I I D+C) u w «z BEAMI~ I I I I I I I I I

~ .~I II I I~ I~I 8~1P :::l ~ ii: • ~ ..L a.

I I fa: 1C)lo~~1A I~ f I I I I I I I I I I CO-MOVINGS w ~ C)=> ELECTRONM a:l W I- BEAMA I I I I I I I I I I I I I I IU:•••

RFINDUCTIONLOOP

VG IJ L10 kV,

104 A

~ Veextract Velt}2 kV, 104 A

.poW.po

I • (rvl • ("V)' • I

V, V2 + V (t/

I QUIESCENT PLASMA-1 CONSTANT VOLTAGE r-T1ME DEPENDENT VOLTAGE -+GENERATION OF~CURRENTAND ~• GENERATION • ACCELERATION • ACCELERATION tA BUNCHING CO-MOVING SPACE CHARGE

ELECTRON BEAM NEUTRALIZATION

FIGURE 2

ELECTROLYTICALLY ETCHEDTUNG~TEN NEEDLES ORRAZOR EDGES

435

J --·lOAlcm2eEXTRACT

CURRENTINEU TRAlIZATlONCHARGE

EeMAX -" 1.32 (keV,

GENERATION OF CO MOVING FlECTRON BEAMFOR CURRENT & CHARGE NEUTRAlIZ.4.TlONOF ACCELERA.TED DEUTFRON.'.

FIGURE 3

F.IGURE 4

0.0

436

SINGLE BEAM POWER PROFILE AT TARGETfP(t)dt = 60 kJ

POWER RISE TO 4.4 TW IN 14 ns (5~, FLAT FOR 6 ns (~. POWER PROFILE ACCORDINGTO R.O. BANGERTER (LLL) TARGET CALCULATIONS.

~

olf)- ."...

!J')I

-=lI

o

oci -!-------r------"T""------r-..---......"

0.5 I.u 1.5 2.01

roeARRI'/:: TI'~::, T iSLe) ~.~

437

FIGURE 5

oco

otel

0 I

Lri1I

> !U !z:: j

C I. ...,,,- -:r::..sLL..;Z

:-.~

r'".

I

L.I.F.E

He'" BEAM VOLTAGE PROFILE AT ACCELERATOR EXIT

E1= 0.65 MeV at t = 0

E2 = 8.0 MeV at t = 1.277 J,ls.

//

.. j-----,.I----r-I--------.-1- -'T - -- ----'-----T---,- -- 1

0.2 O.i 0.6 0.8 1.0 1.2 ;.~LEAVE TI["IE.. TO ('3[C j

438

o(0

oLf)

L.1.F.EHo+ BEAM VOLTAGE PROFILE AT TARGET

I I = 20 kA, t I = 1.28~, CA· 65.6ace. occeIto t· 1.3 MA, Eta t = 60 kJ, T • 3 MeVrg. r9. ave

2R I = I m, A = 10 m, RL = 4 mm, A~ = 0.5 cmacce JQcus rOCUSocD

. .FIGURE 6

/

~j

~~/

~ J-----,-I----------.---_..-.--- --_..-,--------0.0 0.5 1.0 1.5

ARRIVE TiME, T (SELJ2.0

_ -0'\

-""' 1'j

LIGHT ION BEAM COMPRESSION EXPERIMENT--------------------------------------------

~W\D

LAMPS, VACUUM VESSEL40 IN. DIA., 138 IN. LONG

--------_-1

----

ION BEAM

r\. BEAM TIME COMPRESSION +-~ MEASUREMENT PROBES

--

l = 4..-H

Rplasrna = 30 OHM

EXTRACTOR!ACCELERATOR

ION SOURCE vet) + V0

1- 18" "1//Vlt~O----.-- --------- -12"

P=5kWn = 2 x 1011 em-3 He+,Te == 2 eV

---------- ~------------

FIGURE 7

440

76,..-----------------------------

2.72.62.5

~ Vend -= 4.1 x 107 cm/s

2.3 2.4

TIME t (ps)

222.1

56 L-...I-_---J.:..-_....L- ..L...- ..L...-----..L.-----oL...-------

72

60

58

74

E 70.:=.N'

~'

XUJ 68a:o~

FlGURE 8

FIGURE 9 I I90 I

'n=125mVI' ----_..

t = 8 ns..80

30~ /V front = 2.7 x 107 cm/s~

E~ 70

N

I It~~••jjllil.~ (REGION OF --X EXPANDED VIEW)u.J

ex:0~ 60