Hewett, D W

Rutkowski, H L

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Particle Accelerators, 1992, Vols. 37-38, pp. 55-59Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomCHARACTERIZATION OF THE PLASMA-SWITCHINTERACTION IN THE LBL HIF ION SOURCED. W. HEWETTUniversity of California, Lawrence Livermore National Laboratory,Livermore, California 94550andH. L. RUTKOWSKIUniversity of California, Lawrence Berkeley Laboratory, Berkeley, California 94720(Received 3 December J990)A new way to characterize the performance of the LBL HIF ion source has been found. In the LBLsource, ions are drawn from an arc-generated plasma reservoir in which the electrons are confined by anegative-biased "switch" mesh. Stagnation of the plasma is prevented by absorption of the excess ionflow on this mesh. The ion beam is generated by an external negative voltage that provides Child-Langmuirextraction of the ions through the switch mesh. We elucidate the physics requirements of the source anddeduce switch mesh parameters needed for successful operation.1 INTRODUCTIONProposed accelerators for Heavy Ion inertial confinement Fusion (HIF) have string-ent requirements on ion beam quality so that the necessary energy can be focusedon the target with the needed precision. A crucial beam parameter, the normalizedemittance en, can only increase in the HIF scheme using induction accelerators andthus can be no smaller than the level delivered by the ion source. The ion sourceemerges as a critical part of the entire system; it must meet several demands. It mustprovide a source of low temperature ions of the desired mass with the proper chargestate. The source must prevent these ions from entering the accelerator until thedesired beginning of the beam pulse and, thereafter, supply enough ions to fill aconstant-flux beam pulse until the end of that pulse. Finally, en should be as small aspossible; the goal is en ~ 5 X 10- 7 n m rad.Though these requirements can be met using metals with a + 1 valence as acoating on a hot plate, sources based on this technology do not offer much flexibilityin mass or charge state and they tend to "plate out" on surfaces in the low-energyend of the accelerator. An arc-source alternative has been pursued at LBL 1 in aneffort to provide more flexibility in ion mass by eliminating the + 1 valence constraint.The arc-generated plasma easily satisfies the ion flux requirements but encounters5556 D. W. HEWETT AND H. L. RUTKOWSKIdifficulties in achieving the required low level of emittance. Normalized emittance encan be expressed asoren = nf3yr-,ozwhere f3 and yare the usual relativistic factors and rand or/oz are the maximumvalues of the beam radius and the derivative of that radius with respect to the axialcoordinate z, respectively. This expression can be rewritten asv.1en = nyr-,cwhere V.1/c is the perpendicular velocity divided by the speed of light, y is taken tobe unity in that the beam energy leaving the source will be of order 1-2 MeV. Basicallythere are two ways to control or reduce en: (1) reduce r and still provide the necessarycurrent, or (2) reduce V.1. Option (1) will cause the space-charge density to increase,and that would cause increased focusing difficulties. Thus we have reconsidered theorigin of V.1 and are discovering ways to reduce it.We have found the predominant source of ion V.1 is not the intrinsic temperatureof the arc, but rather the ion interaction with the "switch" mesh. To understand thisinteraction we must first consider what function the switch mesh is expected toperform. The electrons must be confined by the switch mesh; without electrons, theions, no longer shielded from external potentials, remain localized about the switchmesh that has been charged to a small negative voltage. Thus, the switch meshconfines electrons and localizes/absorbs ions near this mesh until the extractionvoltage is turned on. If the electrons remain confined near the switch mesh even afterthe extraction voltage is turned on, then an ion-space-charge-limited flow from theswitch mesh to the extractor electrode will provide constant ion current and furtherisolate the beam from the fluctuations inherent in the arc source. In practice, thesefunctions place the switch mesh in a harsh environment. To insure technicalfeasibility, we look at how closely spaced and thin the mesh must actually be forrealistic ion source parameters.We rely on sheaths around the switch mesh to provide electron confinement. Debyesheaths form around objects in plasmas due to the greater electron mobility. Excesselectrons charge these objects toIf the object is held at an even lower voltage, electrons are forced farther back andmore ions are absorbed. To solve for the ¢ profile, a nonlinear Poisson's equationmust be solved. The electron density in this equation is represented well by aBoltzmann factorPLASMA-SWITCH INTERACTION 57but thermal ion distribution requires a more elaborate treatment to get ni • Analysisby Forrester2 leads to a quantitative understanding of the electron standoff distance.Using a numerical solution of the nonlinear Poisson equation with thermal ions, wefind the number of Debye lengths AD that electrons are repelled by a specified voltagecan be determined from Figure 1.For typical parameters <Psw = - 50 V and Te= 7 eV, electrons are repelled ~ 8AD ,where AD = 743jT;frz, and n is the plasma feed density. For minimal confinement,we must make the interwire spacing Siw less than twice the distance the electrons arerepelled. To insure electron confinement even during the extraction phase we preferSiw ;5 8AD· For the parameters given above and Siw = 0.0035 inches (0.005 centerto center), the limit on density n is n ;5 3.13 x 1012 em - 3. This is the first of severalconstraints on the arc-generated plasma that must be satisfied for successful ionsource performance.We now assemble some additional constraints that must be satisfied for properoperation of the plasma switch mesh. Looking at the parameter space defined by thearc plasma feed density n and the thermal velocity Vth in Figure 2, we define anobvious constraint: the diagonal line represents the product nvth, and if this productis below the line, then the arc is not providing sufficient plasma flux to satisfy theion beam current requirement during the extraction phase. The electron confinementconstraint discussed above is depicted by the horizontal line. A given Siw along witha given Te impose a maximum arc feed density n that can be confined by the switchmesh. Closer mesh spacing or lower <Psw allows confinement of denser plasma.:.£i>kTe10oFIGURE 1 The number of Debye lengths )"D required for a plasma to shield an external negativepotential. A surface with a potential of e<l>= -20 kTe requires about 16)"D before e<l>= -kTe, orquasineutrality, can be established. Reading to the left, a surface with e<l> = -10 kTe is about 6 )"D closerto being a quasineutral plasma than the - 20 kTe surface. Thus, a-I 0 k~ surface forms a sheath thatextends only about 10 )"D'58 D. W. HEWETT AND H. L. RUTKOWSKI5 mil meshSource limited105VThermal (em/sec)1011 ~--"'-~--Iooo""'&-"""""''''''''''''''''''''--_J...---'----L--L-..L-IJ..-J..J...,l104FIGURE 2 Physics constraints of the switch-mesh sheath restrict the density n, thermal velocity Vth , andelectron temperature T e of the arc-generated plasma to the triangular region depicted above. For anassumed Te = 7 eV, a 5-mil switch mesh with interwire spacing of 3.5 mil can only confine electrons inthe shutoff phase if the plasma density is less than 3.13 x 1012 em - 3. This forms the constraint on thetop. On the right, if the ion flow needed to feed the sheath current requirements exceeds the Bohm velocity,the concept of switch-mesh confinement breaks down. The diagonal line, forming the constraint in thelower and left part of the region, reflects the requirement that the product of ion density and Vth from theplasma arc source must be sufficient to supply the needs of the needed ion pulse where we usenivth > 1.8 x 1011) when the switch is opened.The constraint that bounds the operation region on the right has its origin in theBohm sheath criterion2 • In essence, this criterion reflects the fact that ions begin toleave electrons behind, thus starting the sheath, when the ion flow velocity exceedsthe sound velocityWe require the arc-generated plasma feed to the switch mesh to be large enough toisolate the switch mesh operation from fluctuations in the arc source. Thus we musthave the plasma density from the arc source large enough so that the needed currentcan be delivered with the ion Vth less than the ion sound speed Cs up to edge of thesheath. If the ion flow velocity in the plasma emerging from the arc source is fasterthan cs ' the concept of switch mesh confinement breaks down.We have displayed these constraints in Figure 2. It is easy to see that if the switchmesh Siw is too coarse or if the electrons are too cold, there is no window for ionsource operation. To allow for some arc fluctuation, we need an operating windowrather than a single operating point. We must balance window size against the needfor reliable, repetitive operation that argues for the coarser, more robust mesh.We now manage this tradeoff successfully in the desired parameter range, but wehave encountered another physics constraint. It is impossible for the ions to passthrough this switch mesh without some enhanced v1. depending on the proximity oftheir trajectory to a switch mesh wire. This velocity enhancement is unavoidable andPLASMA-SWITCH INTERACTION 59acts to increase en' The increase is not large but is just large enough to produceexperimental en around 2n x 10- 6 m-rad, a factor of four above the goal. Techniquesto minimize this emittance growth are not obvious, though some trends are nowemerging from particle-in-cell (PIC) simulation. One possibility is now being testedexperimentally. The idea is to couple the extraction voltage to the switch bias voltageso that when the extraction voltage is turned on, the switch voltage is reduced inmagnitude to the plasma potential. The electrons now move within a AD of the switchmesh, and shield more of the ions from the transverse kick where they wouldotherwise get from the switch mesh. The switch mesh, however, is held at the plasmapotential so that the extraction potential cannot push the electrons farther backtowards the arc, thereby changing the extraction-switch mesh distance (d) so crucialfor constant space-charge-limited ion flow.A recent suggestion has been made that rf cusp-field sources could provide iontemperatures that are only a fraction of an eV, as compared to the present arc-sourceion temperature, suspected to be ~ 3 eV. Low ion temperature appears to provideno advantage because the electrons must still be confined by the switch mesh, andpresent designs require electron temperatures ~ 7 eVe With constant switch voltagethe transverse ion temperature resulting from the transverse kick of the switch meshseems to make further reduction of the ion temperature of the source plasmairrelevant. Present configurations show enhancements of the effective transverse iontemperature from the switch mesh to be on the order of 25 eV, an effect that woulddominate the relatively small reduction in initial ion temperature that could beachieved by using the rf cusp-field source.In conclusion, we are beginning to understand the sheath physics of the plasma-mesh interaction of the LBL HIF curse. We can now infer arc-source plasmaproperties based on the behavior of the plasma switch mesh. Further we have nowprocedures for estimating the properties of the switch (mesh spacing, wire size,voltage, etc.) that are needed for it to function in the HIF application. Thoughprogress to date has given emittance that is within a factor of 3-4 of the HIFrequirement, a new procedure that involves reducing the voltage magnitude on theswitch mesh during the ion pulse extraction phase is now being investigated. If thenew procedure works as anticipated from simulation, the present plasma arc sourcewill meet HIF design goals, with the promise that the rf cusp source will work evenbetter.ACKNOWLEDGEMENTSWork performed under the auspices of the United States Department of Energy bythe Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.REFERENCES1. H. L. Rutkowski, R. M. Johnson, W. G. Greenway, M. Gross, D. W. Hewett, and S. Humphries, Jr.,"Multiple arc ion sources for heavy ion fusion," Rev. Sci. Instrum. 61, 1, (1990) 553.2. A. T. Forrester, Large Ion Beams (J. Wiley 1988).

Characterization of the plasma-switch interaction in the LBL HIF ion source

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