J. Plasma Fusion Res. SERIES, Vol.2 (1999) 320-323

X-Ray Emission from Dense Plasma Sequential Focus Device
MOHANTY Smruti R.r, RAWAT Rajdeep S.2 and SRMSTAVA Mahesh p.*
Department of Physics and Astrophysics, University of Delhi, Delhi-ll0 007, INDIA
lCenter of Plasma Physics, Saptawahid Path, Dispur, Guwahati, Assam, INDIA
2

Department of Physic s/Electronic s, SGTB Khatsa Colle ge,
University of Delhi, Delhi-ll0 007, INDIA(Received: 19 January 1999 | Accepted: 8 March 1999)

Abstrast
The Sequential Dense Plasma Focus (DPF) under investigation is a 3.3 kJ device having a coaxial
electrode assembly with a modified central electrode surrounded by six outer electrodes in squirrel cage

fashion. In conventional DPF device, one or two compression phases are generally observed. In
sequential dense plasma focusing device more than two compression phases (i.e. multiple focusing) havebeen achieved. Sequential focusing of current sheath was achieved on DPF device using a modified formof central electrode. The sequential focusing was optimized by taking different electrode designs fordifferent filling gas pressures of argon. The observation of multiple spikes in the voltage probe signal
confirmed the sequential focusing. We have observed the X-ray emission characteristics from the bestoptimized Sequential DPF device. The time resolved X-ray signals from Sequential DPF device havebeen taken using one of the channels of five channel Diode X-ray Spectrometer. Multiple X-ray spikes
corresponding to different voltage spikes at different instants have been observed indicating thereby thepossible use of Sequential DPF device for fast X-ray cinematography.

Keywords:
Plasma focus, Sequential focusing, X-rays, Diode X-ray spectrometer, voltage spike

1. Introduction

second focus at the target disc placed above the central

device with most of its studies being done in hydrogenand its isotopes. Since DPF is also a source of X-rays.
energetic ions and relativistic electrons it has founds its
applications in other areas too. Energetic ions from DPF
have recently been used for material processing [4-7]electrode of DPF device and thus predict the possibility

and deposition of thin films [8,9]. The relativistic

Sequential dense plasma focus has its importance in

X-ray cinematography and neutron radiography. Lee etal llf were the first one to observe the formation ofof development of Sequential DPF device. Sub-electrons of this device have been used forsequently, Lee 12) proposed a theoretical model for

microlithography t101. The DPF device also emits soft
X-rays during pinch and subsequent phases. Presently,
the problem is no longer that of reducing the radiation
Iosses from plasma focus but is that of increasing thetotal X-ray yield in certain region of spectrum. This isdue to increasing potential application of plasma X-ray

sequential focusing. Nisar er al. [3] reported theexperimental evidence of Sequential DPF by having
target disc hanged by two off centered insulating
support instead of single axially centered support usedby Lee et al. [11.
Earlier, DPF device has been developed as a fusion@1999 by The Japan Society of Plasma
Science and Nuclear Fusion Research*Corresponding author's e-mail: mps @ himalaya.du.ac.in320

Mohanty S. et aI., X-Ray Emission from Dense Plasma Sequential Focus Device

sources in X-ray spectroscopy, microfabrication,

2. Experimental Setup

microscopy, laser pumping and high'density
microelectronlc lithography tl 1- l4l.
Recently, we have attempted to optimise thesequential DPF by modifying the central electrodedesign with variable radius copper cylinders attached

Mather type device whose schematic is shown in figurel. The modified central electrode is used to obtain thesequential focusing. The modified central electrode is

The DPF device used in this study is a 3.3kJ

the one in which two cylinders of decreasing radii are
carved on the top of it. The outer electrode consists ofwith the central electrode by single or double support
tl5l. It has been found that a decreasing radii type ofcylindrical attachment at the top of the central electrodeby single support gives strong sequential focusingaction. Nisar er al. [3] have claimed the sequential
focusing in their focus by observing two spikes in their
voltage probe signal as well as in neutron pulses. A
conventional DPF device many times shows two orthree spikes in voltage probe signal and therefore thesix copper rods whose design is same as that ofconventional focus device. The device is operated withargon as the filling gas in the pressure range of 50 tol50Pa. The device is powered by a low inductance 30 pf
capacitor bank charged up to 14kV. The voltage probe

and X-ray diode spectrometer signals were recorded
using Tektronix TDS 784 digital storage oscilloscope.
The best sequential focusing is indicated by strong
multispike voltage probe signal for the central electrodewith cylindrical attachment of decreasing radii withsingle support as shown in inset of figure l.

result reported by Nisar et al does not confirm
sequential focusing. In present paper, we report the time

resolved X-ray emission characteristics of sequential
focusing DPF device with central electrode design
optimized earlier [5] for strong sequential focusing.
This is to explore the possibility of developing plasma

The X-ray signals are detected by a multichannel
diode X-ray spectrometer (DXS) tl6l. The diodes usedin the DXS are planar silicon PIN photodiodes (BPX65) without glass windows, housed in a modified TO-18

sequential pulsed X-raY source.

Pressure
Gause

AnodeFig. 1 Schematic of experimental set up with the inset showing modified central electrode design for sequentialfocusing.

32r

Mohanty S. et al., X-Ray Emission from Dense plasma Sequential Focus Device

case. Each diode of DXS is reverse biased to 45 V andcorresponds to the first compression at the top of themain part of the anode. This is followed by one or two

is connected to the oscilloscope through a resistor
capacitor combination.

small peaks, separated by 100-150ns, which is seen
even in conventional plasma focus. The next peak is

3. Results and Discussion

stronger and occurs about 600-650ns after the first peak

The typical voltage probe and DXS signals at 60,

which corresponds to second focus at the first

100 and 120 Pa filling gas pressure of argon are givenby (a), (b) and (c) of figure 2, respectively. The X-ray
signals shown in figure2 are taken using the channel ofcylindrical attachment. The third peak which occurs at
the second cylindrical attachment is about 300-350ns

DXS in which the PIN diode window is covered by 149
pm aluminized mylar. Each of the voltage probe signals

and third focus (300-350ns) is less than time gap

after the second focus. The time gap between second
between the first and second focus (600-650ns) because

posseses multispike structure which clearly indicatesof the shortened support as well as the smaller width of
the second cylindrical attachment. As can be seen fromsequential focusing in device. Majority of voltage probe
signals contain four to five voltage spikes. The first peakfigure 2, no X-ray emission corresponding to first
voltage spike is observed. This is possiblly due to thefact that the first focus is occuring little too earlybecause of the shortening of the main part of the anode.

Therefore not nearly the peak discharge current isflowing through the first focused plasma column andthus the plasma is not hot enough to radiate in X-ray
regime. The first X-ray spike starts approximately at thepeak in the second spike of the voltage probe signal. It

is also apparent from figure 2 that the second spike inthe voltage probe is always most intense and has
smallest FWHM of about 50-60ns. The third and fourthr L-*-"Jvoltage spike are relatively less intense with more half

width. The second and third X-ray peaks in DXS
matches well, in a similar fashion. to third and fourth
voltage peaks, respectively. The second X-ray spike wasalways more intense in comparison to the fisrt one and,'h^the third peak was the weakest in all DXS signals atdifferent filling gas pressures. In some of the DXS
signals we have noticed more than three peaks with thefirst the first three similar to normal one and the laterones being even lesser in magnitude with more half
width as seen in figure 2(b).

4. Conclusion
This study clearly establishes Sequential DPF as asource of sequential X-ray pulses as well. This device
thus can find suitable application in the field of X-ray
cinematography.

Acknowledgment
One of us (SRM) is grateful to Delhi University foroffering TPSC grant to enable him to be associated withthis work at Delhi Universitv.

Fig. 2 Typical voltage probe (upper trace) and DXS(lower trace) signals at different filling gas

pressures of (a) 60 Pa, (b) 100 Pa and (c) 120 Pawith time base of 1 ps/div and voltage base of lV/div.

322

Mohanty S. et al., X-Ray Emission from Dense Plasma Sequential Focus Device[9] Chayya R. Kant, M.P. Srivastava and R.S. Rawat,

References[]

Phys. Lett. A239,109 (1998).

S. Lee, M.A. Alabraba, A.V. Gholap, S. Kumar,[10] P. Lee, X. Feng, G.X.Zhang, M.H. Liu and S. Lee,
Plasma Source Science and Technology 6, 343

K.H. Kwek, R.S. Rawat and J. Singh, IEEE Trans.
on Plasma Sci. 18, 1028 (1990).

[2] S. Lee, IEEE Trans. of Plasma Sci. 19, 912 (1991).

(1997).

I l] J.S. Pearlman and J.C. Riordan, J. of Vac. Sci. andl3l M.Nisar, F.Y. Khattak, G. Murtaza, M. Zakaullah

Tech. 19, I 190 (1981).

and N. Rashid, Physica Scripta 47,814 (1993).

[12] R. Feder, J.S. Pearlman, J.C. Riordan and J.L.
Costa, Journal of Microscopy 135, 347 (1984).
[3] J.L. Porter, R.B. Spielman, M.K. Matzen, E.J.
Mcguire, L.E. Ruggles and M.F. Mvargas, Phys.

t4l R.S. Rawat, M.P. Srivastava, S. Tandon and A.
Mansingh, Phys. Rev. B 47,4858 (1993).

[5] Ratna Sagar and M.P. Srivastava, Phys. Lett. A
r83, 209 (1993).
M.P.
Srivastava, S.R. Mohanty, S. Annapoorni and[6]R.S. Rawat, Phys. Letr. a 215,63 (1996).
[7] Priti Agarwala, S. Annapoorni, M.P. Srivastava,R.S. Rawat and Pratima Chauhan. Phvs. Lett. A
231,434 (1997).
[8] Chayya R. Kant, M.P. Srivastava and R.S. Rawat,Phys. Lett. A226,212 (1997).

Rev. 68, 796 (1992).

ll4l Y. Kato and S.H. Be, Appl. Phys. Lett. 48, 686(r986).
ll5l S.R. Mohanty, Ph.D. thesis, University of Delhi,Delhi, India (1997).
[16] S.R. Mohanty, M.P. Srivastava and R.S. Rawat,
Phys. Lett. L 234, 472 (1998).

323