Dolling, R

Gross, P

Klein, H

Pozimski, J

Riehl, G

Weis, T

Wiegand, J

CERN Document Server

Particle Accelerators, 1992, Vols. 37-38, pp. 215-221Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomTECHNIQUES FOR HIGH CURRENT IONBEAM DIAGNOSTICStT. WEIS, R. DOLLING, P. GROSS, J. POZIMSKI, G. RIEHL, J. WIEGANDand H. KLEINInstitut fur Angewandte Physik der Johann Wolfgang Goethe Universitiit,Postfach 111932, D-6000 Frankfurt am Main, Germany.( Received 3 December 1990)Sophisticated systems for the transport and the acceleration of low energy, high current ion beams arenecessary for a driver for heavy ion inertial confinement fusion. The components of the accelerator systemas well as the space charge of the ion beam itself may exhibit field nonlinearities, which give rise toemittance degradation. These losses of beam quality should of course be reduced as far as possible. Whilethe nonlinearities of external fields can be investigated mainly by numerical methods, the properties ofthe internal space charge fields of compensated and uncompensated beams can only be examined bymeans of experimental diagnostic techniques.During the last three years several techniques for destructive and non-destructive beam diagnosticshave been further developed and tested at the Institut fur Angewandte Physik, Frankfurt. Retarding fieldspectrometers and 1270 electrostatic energy analyzers are the nondestructive tools used to measure theintegral or differential energy spectra of residual gas ions created and expelled from the beam; they allowthe evaluation of potentials of the transverse space charge of uncompensated and space-charge-compen-sated beams. By means of an external transverse electron beam (typically 1 keV, 1 /lA) even non-destructivedetermination of the distribution of the radial charge density of high intensity ion beams is possible. Withthe aid of a multifunctional profile and emittance measurement system, which, in addition to our standardslit-to-grid mode, can be operated now in a point-to-grid mode, a better resolution of the transverseemittances and the transverse density in real space can be achieved.INTRODUCTIONDegradation of beam quality measured by the increase of rms-emittance is causedby unlinearities of fields acting on the ion beam. Especially at low energies, the maincontribution to possible emittance increase is believed to originate from the re-arrangement of the transverse distribution of the ion density, thus minimizing thenonlinear field energy of the ions in their own space charge field 1,2,3. Therefore it isstrongly recommended to focus attention especially on the low energy part of anaccelerator system (where the field energy is high) to keep the growth of overallemittance as low as possible.The precise measurement of transverse emittances, along with the evaluation ofradial distributions of ions and compensating electrons as well as the overall degreeof compensation, is necessary to gain information on field nonlinearities acting onthe system, whether caused by external or internal fields.t Work supported by BMFT under contract no. 06 OF 1861.215T. WEIS et ale70in tegra 1 (1)10compensated3.3 V~q:;beamfb = 84 %earn\ ftotal = 92 %(2)216~CI} 1.0......,a• ..-4~::s~ 0.8CI} .0......,a ;..c• ..-4 ct$~ '--" 0.6::3 ~~ ......,a;..c ~ 0.4ct$ (1)'--";..c$-4::J ::s~ 0 0.2";..c~ 0~ ......,a0 0.0(1)~..-40u-0·20Retarding VoltageFIGURE 1 Integral (1) and differential (2) energy spectra obtained from a residual gas ion spectrometerfor a compensated and decompensated 10 keY, 1.5 rnA He+ beam. The evaluated degrees of compensationf vary according to the different definitions of f (for explanation see text).2 ELECTROSTATIC ENERGY SPECTROMETERS FOR RESIDUALGAS IONSThe collisions of beam ions and residual gas atoms give rise to plasma build-up. Inthe absence of external electric fields, the low energy electrons are trapped by thepositive beam potential (in the case of a positive beam) thus lowering the acting spacecharge forces significantly. The residual gas ions created via ionization or chargeexchange are expelled from the beam, the final energy (measured by the spectrometer)depending on the location of creation and the acting space charge potentiaI4 ,5.Typical integral output signals versus retarding voltage are given in Figure 1together with the calculated differential energy distributions. The system allows fora quick evaluation of the degrees of compensation by comparing correspondingmeasurements on the same beam in the uncompensated and compensated case. Thehighest energy present in the distribution has been gained by an ion created on thebeam axis and corresponds to the overall beam potential, cPtotal (beam axis-groundpotential at outer wall), whereas the lowest energy is linked directly to the beampotential at the edge of the beam, cPedge. The difference of the potential inside thebeam (~<Pbeam = <Ptotal - <Pcedge), which is responsible for the acting space charge forceson the beam, can be compared for uncompensated and compensated transport andleads to the degree of compensation f beamfbeam = 1 - ~<Pbeam, compensated/~<Pbeam,uncompensated·HIGH CURRENT ION BEAM DIAGNOSTICSThe total degree of compensation hotal can be defined analogously byhotal = 1 - ~<Ptotal, compensated/~<Ptotal,uncompensated·217Both values of f are identical only if no electrons are located outside the beam area.Determining the radial distributions of the potential and of the space chargedensity, respectively, is in principle possible, but suffers from uncertainties concerningthe angle of observation (aperture diameter 8 mm) and secondary processes such assecondary emission of electrons and ions in the device. Therefore the operation ofthe spectrometers is applicable for ion energies ranging from as low as approximately2 eV up to a few hundreds of eV with an uncertainty of +/-1 eV.Our retarding field spectrometers are very compact (60 mm diameter, 80 mmlength) and can be used even at locations with little available mounting space.A compact 1270 deflection electrostatic spectrometer has also been built and tested.The device directly gives the differential energy spectra of the ions with enhancedenergy resolution. First attempts have been made to correct the measured energyspectra with respect to energy resolution and spectrometer properties, and anevaluation of the transverse space charge distribution shows promising results.Using Helmholtz-like coils in order to make sure that very low energy electronsare not deflected by stray fields, even electrons with energies < 10 eV escaping fromthe beam could be detected.Both types of spectrometers are used routinely as non-destructive diagnosticdevices in our high current beam transport experiments 3 ,6.3 TRANSVERSE ELECTRON BEAM PROBEAn electron beam probe (typical values: 1 keY, 1 f.1A) has been developed for themeasurement of the space charge potential and charge density as functions of radiusfor intense ion beams under uncompensated and compensated conditions. Theelectron beam probe (EPB) uses an external transverse electron beam, which isdeflected by the electric field of the ion beam 7,8,9. The EPB consists of a Pierce-likeelectron gun and an electrostatic deflection system to obtain a variation of thetransverse coordinate with respect to the ion beam. The probing beam is detectedwith a resistor coated plate of ceramics by analyzing two partial detector currents(position resolution: 0.2 mm, time resolution: 5 f.1sec). The device is computer con-trolled and allows for automatic scanning of the ion beam. A typical deflectioncharacteristic is shown in Figure 2. For a given beam the degree of compensationcan easily be evaluated by the comparison of the deflections for large transversecoordinates of the electron beam with respect to the ion beam in the compensatedand uncompensated cases.For small deflection angles we were able to perform an Abel inversion of themeasured deflection characteristics8 ,1 0 giving the space charge potential and the netcharge density (ions and compensating electrons e.g.) as a function of radius. Figure3 shows the radial distribution of the net charge density of a compensated beamcalculated from the deflection characteristic of Figure 2. The beam is hollow with a218 T. WEIS et al.10.0 t---+----~7.55.02.50.0-2.5-5.0-7.5-10.0transverse c'oordinateDeflection characteristic for a compensated and decompensated Ar + beam (0.8 rnA, 10 keV).FIGURE 2IY)E"uOJVl 8o<!.~'0 6~~......-Vi•CQJ-02CDenc....d a..c:.u10•20 19 a 10 20 38r[mm]FIGURE 3 Radial distribution of the net charge density calculated from the deflection characteristicfor the compensated beam shown in Figure 2 (points). The curve is a fit to calculated values obtainedfrom a symmetric deflection characteristic (average deflection at specific radius f). The values near thecenter of the peak as well as the central dip of the curve are uncertain due to the used Abel transformation.HIGH CURRENT ION BEAM DIAGNOSTICS 219sharp center peak due to aberrations of an Einzel lens system. The distribution ofthe compensating electrons causes regions of net negative charge density between themaxima and outside the beam area. Due to the typical properties of an Abel inversionthe obtained values near the beam center are uncertain. The comparison of measure-ments under compensated and decompensated conditions even allows for the separateevaluation of ion and electron distribution respectively.The device is capable of resolving time dependent processes such as the evolutionof increasing degrees of compensation at the front end of a beam macropulse. Therise time of compensation has been measured giving ionization cross sections thatare in excellent agreement with data from the literature7,8,11,12.Up to now the limited resolution of our detector system allows only for low electronenergies (1 keV) and the application of the EPB is not possible for the analysis ofalmost fully compensated beams. Moreover, the EPB can be used at the momentonly at particularly prepared test stands where all possible precautions have beentaken to reduce magnetic stray fields. An improvement of the detector resolution willeither allow for a better analysis of compensated beams or higher electron energiesin order to reduce the existing problems with magnetic stray fields.4 MULTIFUNCTIONAL PROFILE AND EMITTANCEMEASURING DEVICEStandard slit-grid emittance and profile measurement devices give only basic informa-tion on the transverse ion beam current distributions such as I(x), I(y), I(x, x') andI(y, y') due to the integration over the y and x direction respectively. Our existingdevice has been extended to allow for slit-to-grid as well as point-to-grid measure-ments, which, due to a better local resolution, give an enhanced physical insight intothe 3-dimensional phase spaces I(x, x', y) and I(y, y', x). From the obtained distribu-tions I(x, x', yJ and I(y, y', xJ, at different locations Xi and Yi respectively, all 2- andI-dimensional subspaces (standard emittance I(x, x') and ion density I(x, y), forexample) can be deduced, as indicated in Figure 4. From the distributions I(x, x')and I(y, y') obtained by slit-to-grid methods however a reconstruction of the densityin real space I(x, y)is not possible.Figure 5 shows a typical example of a 2-dimensional ion current distribution inreal space. It is calculated from measurements of I(x, x', yJ at different locations Yi'Up to 4 slit/profile harp combinations may be installed to 6 stepping motor drivenUHV feedthroughs. The profile currents of one harp are switched by a multiplexerto the 60-channel current amplifier. Motor and electronics are controlled by an 8 bitmicroprocessor system with digital I/O boards. A 68000 PC handles the interactivedialogs, the measurement procedure and data storage as well as comfortable datadisplay and evaluation 13 . The slits are made of tantalum, molybdenum or stainlesssteel, and cooled versions for beam powers up to 6 kW are available. An optionalkicker in front of the system may increase the measurable beam power up toapproximately 1 MWatt.The device is easy to operate and, especially when operated in the point-to-grid220 T. WEIS et al.r------------------------------------------,: I (x"x· "y"y.) :L--------------------T---------------------Jr---------~----l----~--------,I (x) I (y)FIGURE 4 Survey on phase space distributions, which may be measured (fat) or calculated frommeasurements.-11 -6 o , 11FIGURE 5 Radial density distribution of an intense He+ beam behind a magnetic lens with strongaberrative effects. The distribution is calculated from measurements of I(x, x', Yi) at different locations YioHIGH CURRENT ION BEAM DIAGNOSTICS 221mode, allows a detailed evaluation of emittance growth, for example, correspondingchanges of ion density distribution, and occurring field unlinearities.5 CONCLUDING REMARKSThe methods mentioned above offer an attractive supplementation to commonly useddiagnostic techniques for high current ion beam behavior. The development ofmeasuring devices for beam potentials and corresponding electron and ion densitydistributions in strong magnetic fields (e.g., those of solenoids and quadrupoles) is atask for the future. Beam analysis carried out with electric plasma probes in amagnetic solenoid have shown poor results thus far.REFERENCES1. T. Wangler et al., IEEE Trans. Nucl. Sci. NS-32 (1985) 2196.2. I. Hofmann and J. Struckmeier, Part. Accel. 21 (1987) 69.3. T. Weis, these Proceedings.4. A. Schonlein, thesis, Universitat Frankfurt, GSI-Report 87-4 (1987), GSI, Darmstadt, Germany.5. P. Kreisler, thesis, Universitat Frankfurt, GSI-Report 84-10 (1984), GSI, Darmstadt, Germany.6. T. Weis et al., in Proceedings of the 1990 Linear Accelerator Conference, LANL-Report LA-12004-C(1991), 358, Los Alamos, USA.7. P. GroB et al., in Proceedings of the 1990 European Particle Accelerator Conference, EditionsFrontieres, Gif-sur-Yvette (Cedex, France, 1990), 806.8. P. GroB, diploma thesis, Institut fur Angewandte Physik, Universitat Frankfurt, Germany.9. V. Shestak et al., Triumf Design Note TRI-DN-87-36 (1987).10. C. H. Stallings, J. Appl. Phys. (1971) 2831.11. T. Sluyterset al., Physika 25 (1959) 1376.12. H. Gilbody, J. Hasted, Proc. Roy. Soc. A240 (1957) 382.13. G. Riehl et aI., Proceedings of the 1990 European Particle Accelerator Conference, Editions Frontieres,Gif-sur-Yvette (Cedex, France, 1990) 756.

Techniques for high current ion beam diagnostics

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