There are presented basic results of experiments on the LIA-10M accelerator since its putting into operation (1994) till today. There were investigated various modes of accelerator operation and its output characteristics depending on the parameters of injected electron beam, number of connected accelerator modules, time program of inductors switch-in etc. There was obtained a large scope of experimental data that are of interest for LIA-10M accelerator practical use and for the development of new facilities of this type. The investigations that have been performed recently make it possible to considerably (half as much again) increase the output dose parameters of the accelerator as compared to the level achieved before: maximal dose (Si) and dose rate on the output flange constitute 400 Gy and 2.5.1010 Gy/s, while at a 1 meter distance from the target they are equal to 7.5 Gy and 5.108 Gy/s, respectively

Filippov, V.O.

Gordeev, V.S.

Myskov, G.A.

Viktorov, I.V.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

RESULTS OF LIA-10M ACCELERATOR INVESTIGATIONSV.S. Gordeev, G.A. Myskov, V.O. Filippov, I.V. ViktorovRussian Federal Nuclear Center – All-Russia Scientific ResearchInstitute of Experimental Physics (RFNC-VNIIEF)607188, Sarov, Nizhni Novgorod region, Russia E-mail: gordeev@expd.vniief.ru There are presented basic results of experiments on the LIA-10M accelerator since its putting into operation (1994) till today. There were investigated various modes of accelerator operation and its output characteristics depending on the parameters of injected electron beam, number of connected accelerator modules, time program of inductors switch-in etc. There was obtained a large scope of experimental data that are of interest for LIA-10M accelerator practical use and for the development of new facilities of this type. The investigations that have been performed re-cently make it possible to considerably (half as much again) increase the output dose parameters of the accelerator as compared to the level achieved before: maximal dose (Si) and dose rate on the output flange constitute 400 Gy and 2.5⋅1010 Gy/s, while at a 1 meter distance from the target they are equal to 7.5 Gy and 5⋅108 Gy/s, respectively.PACS numbers: 29.17.+wDuring the period beginning from the time of LIA-10M [1-3]  putting  into  operation  till  now there  have been made about 1800 work pulses. In the course of the accelerator  experimental  development  there  was  per-formed a large scope of experimental investigations pur-posed at studying the peculiarities of its operation and aimed at achieving maximal dose parameters at the fa-cility output. In the experiments the conditions of opti-mal  injection,  acceleration  and  transport  of  electron beam  were  investigated  depending  on  its  parameters (amplitude and current pulse duration of the beam, its diameter and injection energy),  delay time of injected beam pulse as related to the accelerating voltage pulses of inductors and time program of inductors turn-on as related to each other.One of  the  tasks  at  the  initial  stage of  researches consisted  in  defining  the  optimal  correlation  between the pulse durations of injected beam current and the ac-celerating  voltage  formed by  the  inductors.  In  Fig.  1 presented  is  an  oscillogram  illustrating  the  shape  of voltage pulses at the output of LIA-10M inductor in the idle mode. The inductor forms the group of pulses of al-ternate polarity with different amplitude and duration. To accelerate the beam there was used the second pulse (of negative polarity) with the amplitude of 2 MV and 22 ns duration at a half-maximum. The maximal dura-tion (FWHM) of the beam current pulse formed by the LIA-10M injector [4] is approximately twice as large. However,  there  exists  the  possibility  of  its  regulation within the limits from 20 to 40 ns through the change of the turn-on time of the controlled pre-pulse switch in-stalled at the output of the injector forming system.Fig. 1. Shape of voltage pulses at LIA-10M inductor  output.                         a                                          b                         c                                          dFig. 2. Oscillograms of injected beam current pulses  (duration at half-maximum: a – 40 ns, b – 35 ns,  c -  30 ns, d – 25 ns).In the experiments performed the duration of the in-jection current pulse varied from 40 to 25 ns with a step of  ~5  ns.  The  obtained  oscillograms of  beam current pulses injected to the accelerating channel are given in fig. 2. At reducing the duration of pulse its shape was changed as well as the amplitude (towards the increase by 10-15% at pulse shortening). For each version of in-jection there was selected the optimal time of beam cur-rent pulse delay as related to the accelerating voltage of inductors. The criterion of optimality consisted in get-ting the maximal dose and dose rate of bremsstrahlung registered at the facility output. The unique conclusion on the fact that the best conditions for the beam acceler-ation are achieved at approximately equal  duration of injected current  and accelerating voltage pulses is  the result of the investigations performed. Thus, in case of a shortened to 25 ns pulse of injection current the dose (Si) of bremsstrahlung at 1 meter distance from the tar-get was 20-30% higher than in case of a “long” (40 ns) pulse. The advantage by dose rate was even more con-siderable and constituted 50-70% and in separate pulses it  was close to  100%. This  is  conditioned by the de-crease of bremsstrahlung pulse duration at the accelera-tor output (Fig. 3) in case of short-pulse injection (13-15 ns instead of 15-18 ns) at simultaneous growth of dose. The pulse of injected current can be even shorter by du-ration than the pulse of accelerating voltage, however, in this case the efficiency of accelerating system use de-creases.ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3. Серия: Ядерно-физические исследования (38), с. 38-40.38abFig. 3. Oscillograms of bremsstrahlung pulses corre-sponding to 25-ns (a) and 40-ns (b) duration of injec-tion current pulses.Fig. 4. Dependence of bremsstrahlung dose on the delay time of the injected current pulse as related to  the accelerating voltage of the first inductor.In  Fig.  4  presented  is  the  dependence  of bremsstrahlung dose on the delay time of injected cur-rent pulse as related to the accelerating voltage of the first inductor of the accelerating system. The solid line corresponds to the dose at 1 meter distance from the tar-get, the dashed one – to the dose at the output flange of the accelerator (~50 mm from the target). From the fig-ure  it  is  evident  that  maximal  dose  parameters  are achieved at 3-4 ns delay time of pulse injection relative to the accelerating voltage pulse. The delay of the in-jected current is required to compensate inductive decel-eration of electrons on the beam front by the accelerat-ing voltage of inductors. It should be mentioned that the required delay time depends not only on the properties of the accelerating structure and characteristics of accel-erating voltage pulse but on the injection current pulse parameters  –  its  amplitude,  duration,  rise-  and  decay time. In each specific case it is selected individually.The other task solved in the course of experimental accelerator development was to optimize time program of accelerator modules switching as related to each oth-er. At the velocity of electrons close to the velocity of light in vacuum the time of their flight through a sepa-rate inductor (axial length – 0.58 m) of the accelerating track is equal to 1.9 ns. In the initial series of experi-ments the time shift of turn-on for each consequent in-ductor  as related to the previous one corresponded to this value. However, at the accelerating structure input the velocities of electrons near the current pulse front and slope are considerably lower than the velocity of light while at short-pulse injection this group of elec-trons constitutes the noticeable part of the beam. There-fore later the program of accelerating modules turn-on was corrected in terms of this factor.Fig. 5. Time diagram of LIA-10M inductors turn-on.The time diagram of LIA-10M inductors operation is presented  in  Fig.  5  (solid  line).  The  first  4  inductors turned on in accord with the velocity of light in vacuum (dashed line), the second group of inductors is addition-ally shifted in time by +2.5 ns as related to the first one. The last group of 8 inductors was delayed in run-up by 5 ns as related to the time schedule corresponding to the velocity of light. The time shift between the two neigh-boring  inductors  in  each  group  is  retained  equal  to 1.9 ns. Such program of inductors switching was select-ed basing on the results of experiments in a set of ~100 work pulses of LIA-10M accelerator what provided for getting  sufficiently  reliable  statistics.  Its  realization made it possible to additionally increase by 15-20% the dose characteristics of the generated by the accelerator bremsstrahlung and finally  achieve the  dose  levels of ~300 Gy (Si) at the output flange and ~ 5 Gy - at 1 me-ter distance from the target.In one series of experiments there was investigated the dependence of the dose at the accelerator output on the  number  of  the  active  accelerating  modules.  Here only n=0, 4, 8, 11 and 16 of the first accelerating mod-ules were charged and they formed the pulses of accel-erating voltage. The case of n=0 corresponds to the op-eration of a single injector (with no inductors turned on) while the case of n=16 – to the operation of the acceler-ator in a body. The chart of this dependence is given in Fig. 6. The dose curve can be approximated by the de-gree function 3.2≅NnDD Nn , where Dn – dose at the facility axis at  n number of inductors turned on, while DN – dose value at the same point at all N=16 accelerat-ing modules turned on. The obtained data make it possi-ble to predict the bremsstrahlung dose at a definite point near the accelerator axis for the accelerator operation in any configuration. It is of particular convenience for the work with large-scale objects when the change of dose load to the object through its approach or moving away from the target does not always represent a simple task.A considerable scope of investigations was connect-ed with the study of LIA-10M accelerator at different-diameter electron beam injection and acceleration. With the increase of the number of its modules at a stage of tuning activities it was required - for the purpose of en-ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3. Серия: Ядерно-физические исследования (38), с. 39-40.39suring stable mode of electron beam transport to the tar-get - to gradually decrease the cathode diameter from 120 to 80 mm. For the accelerator in a body we had to decrease  as  well  the  energy  of  injection from 3  to  2 MeV. It has been established later that the problem was associated  with  parasitic  electron  emission  from  the conical segment of the injector cathode what led to the fact that the beam injected to the accelerating structure was surrounded by a circular  “aureole” of these elec-trons. Its diameter was sufficiently large, ~160 mm, that is  comparable  with  the  aperture  of  the  accelerating channel (∅200 mm). In the course of acceleration there was observed the increase of cross dimensions of the beam and, correspondingly, of the “aureole” surround-ing it. Finally, it led to the electrons hit on the accelerat-ing tubes of several last inductors what initiated electric breakdowns on the surfaces of their insulators.0.010.050.200.450.761.000.00.20.40.60.81.00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16Number of turned-on inductorsDose, rel. unitsFig.  6.  Relative  bremsstrahlung dose  change on LIA-10M accelerator axis at different number of turned-on accelerating units.Table 1. Dose parameters of LIA-10M accelerator de-pending on the cathode diameterParameter Cathode diameter, mm40 60 80Bremsstrahlung  dose  at the output flange, Gy250÷300 350÷400 300÷350Bremsstrahlung dose at 1 meter  distance  from  the target , Gy4÷5 6÷7 5÷6Dose  rate  at  the  output flange, Gy/s2⋅1010 2.7⋅1010 2.3⋅1010Dose rate at 1 meter dis-tance  from  the  target  , Gy/s3.3⋅108 4.7⋅108 4⋅108Diameter  of  irradiation spot with Dmax/Dmin=2:     at a flange, cm 5.5 9 12     at 1-meter distance      from the target, cm50÷55Bremsstrahlung pulse du-ration, ns14÷17For the purpose of this phenomenon elimination the configuration of  the  injector  cathode was changed by decreasing the axial length of its conical segment at the corresponding increase of the cylindrical one. This had a positive effect and made it possible to carry out the in-vestigations of the beam injection and acceleration with the use of cathodes of 40-80 mm diameter. We had not managed to do it before as the diameter decrease of the cylindrical cathode segment did not lead to the essential change of injection parameters: cross dimensions of the beam,  its  current  and  energy  remained  approximately the  same  as  at  80-mm  diameter.  The  parameters  of bremsstrahlung at the output of the facility fixed in the experiments are presented in Table 1. The best results by dose are achieved at 60-mm cathode diameter. The injection current and energy in this case constitute 20 kA and 2.3 MeV, respectively. In this mode there are fixed record dose characteristics for LIA-10M. In a set of pulses the dose at 1 meter distance from the target was as high as 7.5 Gy (Si), while at the output flange it achieved 400 Gy at 5⋅108 Gy/s and 2.7⋅1010 Gy/s, respec-tively.  The results  obtained by a dose  are  ~1.5 times higher than the levels that had been achieved before.In the course of experimental investigations at LIA-10M accelerator there were made noticeable strides in increasing its output dose parameters. However, the re-serves built in its design are not by far exhausted and one can expect to get the dose levels ~10 Gy and higher at 1 meter distance from the target. Both theoretical and experimental works are being performed along this line.REFERENCES1. V.S.Bossamykin,  V.S.Gordeev,  A.I.Pavlovskii  et al.  Pulsed  Power  Electron  Accelerators  with  the Forming Systems Based on Stepped Transmission Lines //  Proc. 9th Int. Conf. on High Power Parti-cle  Beams  (BEAMS-92),  Washington,  DC.  1992. v. 1. p. 505-510.2. V.S.Bossamykin,  V.S.Gordeev,  A.I.Pavlovskii  et al. Linear Induction Accelerator LIA-10M //  Proc.  9th  IEEE Int.  Pulsed  Power  Conf.,  Albuquerque, NM. 1993. v. 2. p. 905-907.3. V.S.Bossamykin,  V.S.Gordeev,  V.F.Basmanov  et al. Linear Induction Electron Accelerator LIA-10M with  Inductors  on  Stepped  Lines  //  Problems  of  Atomic  Science  and  Technology.  Issue:  Nuclear-Physics Research (31-32). 1997. # 4-5, p. 117-119 (in Russian).4. V.S.Bossamykin,  V.S.Gordeev,  V.F.Basmanov et. al.  Injector  of  the  Accelerator  LIA-10M //  Prob-lems of Atomic Science and Technology. Issue: Nu-clear-Physics  Research (31-32).  1997.  #  4-5.  p. 120-122 (in Rusian).ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3. Серия: Ядерно-физические исследования (38), с. 40-40.40

Results of LIA-10M accelerator investigations

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Results of LIA-10M accelerator investigations

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)