Direct Extraction of Ions from Expanding Laser Ablation Plasma

JP0655057

Mitsuo Nakajima, N. Kobayashi, and K. Horioka Tokyo Institute of Technology, Energy Sciencies

J. Hasegawa and M. Ogawa Tokyo Institute of Technology Research Laboratory for Nuclear Reactors Ions are extracted directly from the expanding plasma ablated on small spots and linear without a drifting space. We investigate behaviors of the emitting surface by measuring ion beam current and its image. In the experiment of the small laser spot, a matching-like operation was founded in which the emitting boundary didn't move so much and the current had a fiat-topped waveform under suitable conditions. We obtained copper ion current of 80 mA with en = 0.25?rmm • mrad, fast rising of 40 ns and flat-top waveform. In the long linear spot, the divergent angle was decreased. Based on the experimental results, some indication of the possibility of large area laser ion source with direct extraction is obtained.

I. INTRODUCTION temperature even though low irradiation[6, 7]. Because the ion drift velocity is considerably faster than the ion 1/2 acoustic velocity(va = (kTe/M) ), the pre-sheath ac- Heavy ion fusion(HIF) requires high-current( A) and celeration is not needed. In the case of drifting plasma high-brightness(en < lmum • mrad) heavy ion beams. source, a supply current density is represented as ~ nqvj. However, current density is severely restricted by break- not the Bohm current density(~ nqva) and it has suffi- down and beam radius(a) by the emittance scaling(gn oc cient supply capacity. a • (Ti/M)1/2) . Beams are dominated by space charge force, so the charge state must be low(l+ — 3+) and rect- Conventionally, powerful lasers have been used for gen- angular current pulses are demanded.[l] erating of highly charged ions with a long drifting space. If low charge states of ion, especially a pure single charge In the case of high current ion sources, the efficiencies state considered to be easy to make, are desirable the of the source gas consumption and its ionization must laser fluence must be just above the threshold of plasma be low, so gaseous discharge type ion sources may be ablation and naturally a plasma expansion length must difficult to use. Thermonic sources offer high brightness be short. beams with a charges state limited to 1+, but flux levels are relatively low[2]. We propose an ion extraction scheme, where ions are directly extracted from an ablating plasma made by large A pulsed solid vapor type such as a vacuum arc source area, low-fluence laser irradiation. This scheme is ex- or laser ion source is one of possible candidates. The pected to have the following characteristics. It facilitates matching problem which the plasma meniscus changes the generation of a pulsed beam with a fast current rise- temporally with a balance of the extraction current and time due to a rapid plasma generation without adding the supply current (as shown in fig.l) is severe for pulsed another electrode. Secondary, as schematically shown in ion sources due to a fluctuation of plasma supply. The vacuum arc ion source has been overcoming these two problems namely beam noise and poor reproducibility. 1) Plasma meniscus in a conventional tonextracto r Recently, up to 17 mA/cm2 of Gd ions ware produced with about 85% of the ions in 3+ state, a pulse rise-time close to 1/xs and beam fluctuation level of less than 3% rms[3]. To avoid pre-fill of plasma in the extraction gap and to solve the matching problem, a grid control method is effective[4, 5]. Since the extractor operates in a space- a) Supply current msfehsd Ct#j current b) Escsss pfasma ton flua charge-limited mode, a plasma fluctuation doesn't affect 2) Direct Extraction from an sfoSsJsd pJasms by farga area las«r S the beam current if the ion flux from the plasma ex- ceeds the space-charge-limited flow. Plat-topped current beam waveforms with fast current risetimes are obtained. However, the confinement grid increase the risk of break- pmoving down. Laser irradiation is well-suited to make a dense and low temperature plasma effectively. And there is no random plasma fluctuation like the vacuum arc discharge FIG. 1: A conventional extractor and a direct extractor from plasma. It is well known that ablation plasma has high an ablation plasma drift velocity(vd) up to 100 eV comparing with plasma

149 a) Set up for emlttancs measurement with a psppsfpot piste

iPhospher lOOjuti Tantalum ptet® ^ — Aaoum hdss in ths plate

PIG. 2: A Schematic view of the induction accelerator and the laser ion source fig.l, the shape of the emitting surface does not vibrate with plasma fluctuation like a conventional ion extractor but the boundary can move. Since laser plasmas have FIG. 3: A schematic apparatus of measuring beam emittance no random fluctuation as seen in a vacuum arc discharge plasma, we can expect to make the state where plasma ion flux almost matches, so the emitting boundary does III. RESULTS AND DISCUSSION not move so much with time. However, dynamics of the emitting surface may exert a bad influence upon beam optics. A. Experiment in small spot irrdiation Figure 4a) shows typical beam waveforms and pepper- pot images in the case of a direct extraction as a function II. EXPERIMENTAL APPARATUS of the time delay of voltage application from a laser pulse td. The gate of MCP was opened between 100 to 400 ns A schematic diagram of experimental apparatus is after gap voltage application. In the case of 0-200 ns shown in fig.2. Four induction cavities made of ferrite delay, the current waveforms increases and beam images cores, connected to a pulse forming line, can generate expands. Obviously, the ion supply current is in excess voltages up to 50kV during 400 ns. Four cavities were and the emitting surface moves forward. stacked in a voltage adder configuration. In our experi- At 400 ns delay, there is a small spike in the current ments, extraction voltage was about 120 kV with 400 ns waveform. This is caused by plasma pre-fill in the gap duration. since the front of ablation plasma, although it is low den- We used a frequency doubled Nd:YAG laser with pulse sity by the diffusion, can approach the cathode. The duration of 5-7 nsec for copper rods and an excimer laser current waveform is similar to that of a plasma opening for zinc rods. The small spot size of laser evaporation switch. 2 2 was 1.6 x 0.5mm and the linear size was 0.5 x 10mm . Between 200 to 400 ns delay, current waveforms are Electrodes were made of SUS plates and the extraction flat-top and beam patterns also become a dot-like shape. hole was 10 mm in diameter. The gap length was 20 mm. Because the effective gap length becomes short and the This configuration of extraction was not optimized for the space charge limit current increases with time, the ion beam optics to see the emitting surface behaviors more supply current becomes to match the extracted current. clearly. As shown in fig.4b), when the laser output energy We performed the experiments under a background downs to 28 mJ, it becomes easy to obtain a flat cur- pressure of the order of 10~6 torr. To avoid contamina- rent waveform, and the beam pattern is also improved. tions, we shot the laser four times per one measurement It means the emitting surface doesn't move so much. with a few Hz and the last one was used for the source In order to estimate rough position and shape of the plasma. emitting surface, we drew straight lines that connect the As shown in fig. 3, we observed beam images through dots of screen image and pepper-pot holes as shown in a pepper-pot plate of 100 jj,m thickness made of tanta- fig.6. Figure 5 shows that the extension of each lines lum which has 90 /xm holes with 1mm intervals. Im- intersected in the small point in the all conditions. As ages were visualized by a multi-channel-plate(MCP) + shown in fig.6, we can understand that dot patterns de- phosphor screen opened during 50-300 nsec with a cable form in elliptical shape at the outer side. While in the gate pulser. The pepper-pot plate was located behind case of excess supply, the intersection moves with the the cathode electrode and the MCP was placedl5.2 mm plasma drift velocity. It means that we can consider the from the pepper-pot plate. emitting surface as a point ion source for small spot ir- Beam current was measured with a Faraday cup which radiation in our extraction configuration. It may be sup- has an aperture of 17 mm and also located behind the ported by electric field distortion with plasma expanding. cathode. This means all of extracted ions were captured. Figure 7 shows the position of the intersection that

150 a) 33 mJ b) 28 mJ td = Ons td ~ Ons 200ns/div 200ns/div lion Volfage

i ! i ^

! |

100ns td = 100ns

= 200ns td = 200ns trf 1 j . j" j V I . — 4-.., i |

_L_

i = 300ns = 300ns m f , i j ---•[- l- 4- iik

400ns 400ns r 1, _—j. — - (; - mJ_mJra_s

FIG. 4: Current waveforms and beam images with direct extraction from ablating plasma of Cu target at laser energies of a) 33 mJ and b) 28 mJ. td represents time delay of voltage application after laser irradiation. The MCP gate was opened between 100 to 400 ns after gap voltage application. means the position of emitting surface. As one expected, B. Experiment in line irradiation the emitting surface goes forward when excess supply and come back when insufficient supply. However, in the case of 26.1mJ , a matching condition which the emitting sur- A point source leads to high brightness but divergent face doesn't move so much is observed. beam. We made line plasma in a vertical direction to improve the divergence. In a horizontal direction, the

151 Cu Target Pepper- pot MCP 10

- - ^~- E --—' r—- E o si 1 . . " _ • % ——, P -. - . 100 200 300 Tmm May from Acutoraliofi Voltage [ns] -5 •\

\ FIG. 7: In the case of excess plasma supply, the emitting surface moved forward with plasma drift velocity. In insufficient -10 supply, it moved back. In matching, it didn't move. -20 -10 0 10 20 z(mm) Zn Target Pepper-pot MCP 10 FIG. 5: This is an example that each lines intersects in a small point.

Anode Cathode Io MCP '1 0-100ris — 100-200ns — 200-300ns -10 300-400ns ;— 10 20 30 40 z(mm)

FIG. 8: Current waveforms and beam images with direct extraction from ablating plasma of Cu target at laser energies FIG. 6: We can explain the beam pattern with a moving point of a) 33 mJ and b) 28 mJ. t

straight lines connecting images and pinholes intersected IV. SUMMARY in a small point again like a small spot irradiation. So we can estimate the position of the emitting surface. We have proposed a new scheme for high-flux ion sources, which utilize direct injection of laser ablation Figure 8 shows envelopes at the positions of the emit- plasma into the acceleration gap. From the proof-of- ting surface estimated from horizontal direction. The principle experiments, we could point out that main length of envelope was a few mm since the uniformity cause of emittance degradation is the space charge po- of laser irradiation was a few mm. It decreased as ex- tential in front of the point-like, moving emission surface panding. It seems the emitting surface becomes to curve. of the drifting plasma, and when the ion supply of the Uniformity of laser irrdiation is important. source plasma is close to the space charge limited current of the effective acceleration gap, the drift of injected As shown fig.9, as for the horizontal direction, diver- plasma is counterbalanced. gent angles are still large. However, in the vertical direction, especially in near the center, the divergent angles When we operate it at proper condition with low- become small for 0-lOOnsec. fluence laser small spot irradiation on Cu target , high flux ions are extracted with current density of 100 If laser beam irradiates uniformly, the emittance will mA/cm2, flat-top waveform, fast rising time of less improve to 0.15 Trmm • mrad which is estimated from the than 40 nsec, and with emittance of less than 0.35 center. Trmm • mrad.

152 a) Horizontal b) Vertical Based on the obtained results, we can conclude that if we increase the ablation area using a high energy laser, the direct injection scheme will be able to additionally improve not only ion current level but also the beam quality as well.

Pepper-pot Hole Position [mm] Pepper-pot Hole Position [mm]

FIG. 9: Divergent angle of a) horizontal and b) vertical direction. In horizontal direction, it was the same as in the case of small spot irradiation.

[1] E.P. Lee, J. Hovingh, AIP Conf. Proc. No.249 PT.2 Nakajima, K. Horioka, M. Ogawa Jpn. J. Appl. Phys. (1992)1713-24 42(2003)5367-71 [2] F.M. Bieniosek, J.W. Kwan, E. Henestroza, C. Kim, Nucl. [6] G.C. Tyrrell, L.G. Coccia, T.H. York, I.W. Boyd, Appl. Instrum. Methods Phys. Res. A464(2001)592-8 Surf. Sci. 96-98(1996)227-32 [3] N. Qi, J. Schein, R.R. Prasad, M. Krishnan, A. Anders, J. [7] J.G. Lunney, Appl. Surf. Sci. 86(1995)79-85 Kwan, I.Brown, Nucl. Instr. and Meth. A464(2001)576-81 [8] G.P. Bazhenov, O.B. Ladyzhensky, E.A. Litvinov, S.M. [4] L.K. Len, S. Humphries Jr. C. Burkhart, AIP Conf. Proc. Chesnokov, Zh. Tekh. Fiz. 47(1977)2086-91 152(1986)314-22 [9] R.W. Dreyfus, J. Appl. Phys. 69(3) (1991)1721-9 [5] M. Yoshida, J. Hasegawa, J.W. Kwan, Y. Oguri, M.

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