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Siam Physics Congress SPC2013 Thai Physics Society on the Road to ASEAN Community 21-23 March 2013

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Numerical Experiments on Ion Beams from Plasma Focus

S H Saw1,2 & S Lee1,2,3

1INTI International University, 71800 Nilai, Malaysia

2Institute for Plasma Focus Studies, Chadstone, VIC 3148, Australia

3University of Malaya, Kuala Lumpur, Malaysia

e-mail: [email protected]; [email protected]

Siam Physics Congress SPC2013Thai Physics Society on the Road to ASEAN Community 21-23 March 2013

Summary- Previous work

Much work using variety of diagnostics reported on plasma focus ion beams, mainly experimental

Confusing picture- even units are confusing un-correlated across devices and experiments

No benchmark or scaling patterns appears to have been reported until:

Our Previous work: We adapted beam- gas target neutron yield mechanism for D beams from plasma focus

Our Previous results: (first plasma focus results on ion beam scaling- D)

Ion number fluence: 2.4-5.7 x 1020 ions m-2; independent of E0

Ion Number: 1.2-2 x 1015 ions per kJ; dependent on E0

Summary- New work

Our New work: First principle derivation of ion number flux and fluence equations applicable to all gases.

New results:

Fluence, flux, ion number and ion current decrease from the lightest to the heaviest gas

Energy fluence, energy flux and damage factors are constant from H2 to Ne;

but increase for the 3 high-Z gases Ar, Kr and Xedue to toradiative collapse.

The FIB energy has a range of 4-9% E0.

- Measurements of ion beams from PFâ€™s have produced a wide variety of results using different units;
- Less correlated than expected
- No discernable pattern or benchmarks.
- In summarizing experimental results, Bernard el al1 in 1996, it was reported that total yields of ions reach 1010-1014sr-1depending on energetics and experimental conditions.
- In a single discharge fast ions are emitted from point-like (sub-mm) sources mostly as narrow micro-beams with duration times of 2-8 ns forming intense bunches having total powers reaching 1011 to 1012W.

- 19.4 kJ (43 mF, 30 kV 500 kA peak current) PF published nitrogen ion beam power brightness of 0.23 GW cm-2 sr (maximum ion energy at 0.5 MeV) using a solid anode;
- and 1.6 GW cm-2 sr (maximum ion energy of 1 MeV) using a hollow anode.
- Peak ion current densities of 1100-1300 A cm-2 (50-60 ns) were recorded. [which would give a beam ion current <1kA!!]

- reported beam ion densities of 9.7-15.5x1019m-3(ion energy 15-50 keV) at the aperture of Faraday Cups for 0-25 degree angular positions 6 cm from the anode top in a 40 kV 2.2 kJ neon PF.
- Track densities (CR-39 film) had a maximum value of 10.9x109 tracks m-2 at 30 degrees.
- In another experiment, Bhuyan6 operating a 1.8 kJ methane PF quoted a flux of 2x1022m-2s-1 multiple-charged carbon ions (50-120 keV).

- UBA PF II (4.75 kJ 30 kV) in nitrogen; used Faraday Cup and Thomson spectra to measure nitrogen ions (50-1000 keV) recording 3.2x1013ions/sterad with energy content of 0.74 J/sterad.
- In another experiment5 same machine in dueterium, Kelly surmised a total number of 1015deuterons at 20-50 keV.

- foil-covered CR-39 track detectors states that the PF-1000 generates 105 ions/mm2 (energy above several dozen keV) with neutron yields of 1010-1011 neutrons/shot.

- 5.4 kJ PF using TOF and ion filters recorded fluence of 1014(MeV.sr)-1for the energy spectrum of deuterons (0.3-0.5 MeV) with FWHM of 40-60 ns and 1012 (MeV.sr)-1 (at 1-9 MeV) with FWHM of 10-20 ns.

- Many different experiments
- Many different machines
- Different gases
- Many types of diagnostics
- Many sets of data
- Data- some total (FC), some sampling (track detectors) ie not all ions recorded
- Different perspectives,
- different units:
number sr-1; bunch power in W;

beam power brightness in GW cm-2sr;

ion current densities in A cm-2 ; beam ion densities inm-3;

tracks m-2; ions/sterad; J/sterad ;

total ion numbers; flux in m-2s-1 ;ion fluence in (MeV.sr)-1

- Correlation among experiments? Benchmarking? Scaling? Obvious errors of orders of magnitude!!

- The ion beam numerical experiments adds on as a branch to the integrated view which our numerical experiments strive to present of the plasma focus

Modelling:

Ion beam fluence

Post focus axial shock waves

Plasma streams

Anode sputtered material

Summary of basic physical picture

Plasma Focus Pinch with plasma stream (Paul Lee- INTI PF)

Emissions from the PF Pinch region

Mach500 Plasma stream

Mach20 anode material jet

Sequence of shadowgraphs of PF Pinch- (M ShahidRafique PhD Thesis NTU/NIE Singapore 2000)

Highest post-pinch axial shock waves speed ~50cm/us M500

Highest pre-pinch radial speed>25cm/us M250

Comparing large and small PFâ€™s- Dimensions and lifetimes- putting shadowgraphs side-by-side, same scale

Anode radius 1 cm 11.6 cm

Pinch Radius: 1mm 12mm

Pinch length: 8mm 90mm

Lifetime ~10ns order of ~100 ns

Flux out of Plasma Focus

Charged particle beams

Neutron emission when operating with D

Radiation including Bremsstrahlung, line radiation, SXR and HXR

Plasma stream

Anode sputtered material

- Beam number fluence Fib(ions m-2)
- Beam energy fluence(J m-2)
Flux =fluence x pulse duration

- Beam number flux Fibt(ions m-2s-1)
- Beam energy flux (W m-2)

- Beam of fast deuteron ions produced by diode action in thin layer close to the anode; plasma disruptions generating the high voltages. Beam interacts with the hot dense plasma of focus pinch column to produce the fusion neutrons.
- In our modeling of the neutron yields, each factor contributing to the yield is estimated as a proportional quantity. The yield is obtained as an expression with proportionality constant. The neutorn yield is then calibrated against a known experimental point.
- We start with the neutron yield (rather than the ion beam number) because our method requires a calibration point; and for neutron yields by placing published yields on a chart we can obtain a good fitted calibration point. Had we started with the D ion beam number, we would not have been able to get a reliable calibration point

Ion beam flux and fluence equations

Ion beam flux Jb =nbvbwhere

nb= number of beam ions Nb divided by volume of plasma traversed

vb = effective speed of the beam ions.

All quantities in SI units, except where otherwise stated.

Note that nbvb has units of ions per m-2 s-1.

We derive nb from pinch inductive energy considerations.

Total number of beam ions Nb (each ion mass Mmp, speed vb) has

KE= (1/2) Nb Mmpvb2

where mp =1.673x10-27 kg is proton mass; M=mass number of ion e.g. neon ion has mass number M=20.

Assume this KE is imparted by a fraction fe of the inductive pinch energy (1/2) Lp Ipinch2 where Lp =(m/2p) (ln[b/rp])zp; where m=4p x10-7 Hm-1, b=outer electrode of PF carrying the return current,

rp= pinch radius and zp= length of the pinch.

The pinch current Ipinch is the value taken at start of pinch.

Thus:

(1/2) Nb Mmp vb2 = (1/2) fe (m/2p) (ln[b/rp]) zp Ipinch2 ; nb= Nb/(prp2zp)

nb = (m/[2p2 mp]) (fe /M) {(ln[b/rp])/(rp2)} (Ipinch2 / vb2) â€“ (1)

We derive vb from the accelerating voltage taken as the diode voltage U

Each ionmass Mmp, speed vb ,effective charge Zeff is given KE (1/2) Mmpvb2 by diode voltage U. Therefore:

(1/2) Mmpvb2 = Zeff eU where e is the electronic (or unit) charge 1.6x10-19 C; Hence

vb= (2e/mp)1/2 (Zeff /M)1/2 U1/2 â€“ (2)

From (1) multiplying both sides of equation by vb, we have

Algebraic manipulations:

nb vb = (m/[2p2 mp]) (fe /M) {(ln[b/rp])/(rp2)} (Ipinch2 / vb)

Eliminate vb on RHS of this equation by using Eqn (2) gives

Jb =nb vb = (m/[2p2 mp])(fe /M){(ln[b/rp])/(rp2)}(Ipinch2)(mp/2e)1/2(M/Zeff)1/2/U1/2

= (m/[2.83p2 (emp)1/2])(fe/[M Zeff]1/2){(ln[b/rp])/(rp2)}(Ipinch2)/U1/2

Noting that: (m/[2.83p2 (emp)1/2]) = 2.74x1015. We have:

Result:

Flux = Jb = 2.75x1015 (fe/[M Zeff]1/2){(ln[b/rp])/(rp2)}(Ipinch2)/U1/2 ions m-2s-1 (3)

The fluence is the flux multiplied by pulse duration t; Thus:

Fluence:

Jbt = 2.75x1015 t(fe/[M Zeff]1/2){(ln[b/rp])/(rp2)}(Ipinch2)/U1/2ions m-2 (4)

Value of fe

The parameter fe is the fraction of energy converted into beam energy from the inductive energy of the pinch.

By analyzing neutron yield data1,3,4 and pinch dimensional and temporal relationships15 we estimate a value of fe =0.14.

This condition fe =0.14 is equivalent to ion beam energy of 3%-6% E0 in the case when the pinch inductive energy holds 20% -40% of E0. Our extensive study of high performance low inductance plasma focus classified16 as Type 1 shows that this estimate of fe is consistent with data.

We summarise the assumptions:

Ion beam flux Jb is nbvb with units of ions m-2 s-1.

Ion beam is produced by diode mechanism (ref).

The beam is produced uniformly across the whole cross-section of the pinch

The beam speed is characterized by an average value vb.

The beam energy is a fraction fe of the pinch inductive energy, taken as 0.14 in the first instance; to be adjusted as numerical experiments indicate.

The beam ion energy is derived from the diode voltage U

The diode voltage U is proportional to the maximum induced voltage Vmax; with U=3Vmax (ref) taken from data fitting in extensive earlier numerical experiments.

Procedure

The value of the ion flux is deduced in each situation (specific machine using specific gas)

by computing the values of Zeff, rp, Ipinch and U by configuring the Lee Model code with the parameters of the specific machine and specific gas.

Example: Numerical Experiment for NX2 based on following fitted parameters:

L0=20 nH, C0=28 uF, r0=2.3 mWb=4.1cm, a= 1.9 cm, z0=5 cmfm=0.08, fc=0.7, fmr=0.2, fcr=0.7V0=14 kV, P0= within appropriate P range for each gas

Range of Pressures

PF axial run-down time covers a range which encompasses at least from 0.5 to to 1.3 of the short-circuit rise time 1.57*(L0/C0)0.5.

The matched condition with the strongest energy transfer into the plasma focus pinch is well covered within the range;

also the range covers conditions of high enough pressures that the focus pinch is almost not ocurring as defined by the condition that the reflected shock is barely able to reach the rapidly decelerating magnetic piston.

Collection of data

For each shot the dynamics is computed and displayed by the code; which also calculates and displays the ion beam properties.

For H2, D2, He, N2 and Ne the procedure is relatively simple even though Ne already exhibits enhanced compression due to radiative cooling.

RESULTSFig 2(a) shows a typical PF discharge current computed for NX2 and fitted to the measured discharge current in order to obtain the model parameters fm, fc, fmr and fcr32,33,41. Fig 2(b) shows the computed radial trajectories of the radially inward shock wave, the reflected radially outward shock wave, the piston trajectory and the pinch length elongation trajectory. Range of pressures: widest for lightest gas H2 (1 Torr -70 Torr ). For D2 and He 1- 40 Torr; for Ne we successfully ran numerical experiments 0.1-10 Torr; N2 from 0.1 -6Torr; Xe 0.05- to 1.8 Torr.

Fig 3 illustrates the different compression of the PF pinch. In H2, D2 & He radius ratio ~0.15 up to 10 Torr then rises towards 0.2. For N2 the radius ratio drops from 0.15 to about 0.13 over range of operation. Ne shows signs of enhanced compressions 3- 5 Torr; smaller radius ratio to 0.08 at 4 Torr. Ar shows strong radiative collapse with radius ratio of 0.04 (cut-off value) around 2.0 Torr. Kr strong radiative collapse from 0.5-2 Torr; Xe from 0.3 to 1.5 Torr.

Fig. 4a shows the flux in ions m-2 s-1.

H2 : 6x1027 at 1 Torr , rises to a peak 1.9x1028 at 25 Torr; pressure of best energy transfer for NX2 in H2.

The D2 and He curve show same trend but lower peak flux values at 15 Torr.

N2 shows same trend peaking at 3.6x1027 at 3 Torr.

Ne shows an accentuated peak of 6.6x1027at 4 Torr due to radiative enhanced compression.

Ar flux is even more accentuated with 8x1027 at 2 Torr.

For Kr although the radiative collapse is more severe than Ar, flux is flat at 1.4x1027 at 1 Torr.; this is due to the much greater energy per ion. Xe shows the same flat flux curve as Kr with a flat central value around 6x1026.

Conclusion: Beam ion flux drops as the mass number increases, with accentuating factors provided by radiatively enhanced compression.

Fig 5a shows the fluence in ions m-2.

The shape of the curves and the trend with gases are very similar to the flux

The peak values of the fluence (ions m-2) range from 8x1020 for H2 decreasing to 6x1018 for Xe; with clearly radiation enhanced values of 2x1020 and 1.7x1020 for Ar and Ne respectively..

Figure 6 a-c show that the beam ion number per kJ range from 1016 for the lightest gases decreasing to 1.5x1012 for Xe in the radiative enhanced regime.

Although the beam ion number is the lowest (see Fig 6) for the heaviest gases Ar Kr and Xe, yet these beams also carry the largest amounts of energy at 8-9% E0 compared to around 5-8% for the other gases.

This is because the energy per ion more than compensate for the low numbers.

The damage factor defined as power flow density multiplied by (pulse duration)0.5. This quantity is considered to be important for assessing the utility of a beam for damage simulation of plasma-facing wall materials in fusion test reactors.

The results show that the heaviest ions produce the biggest damage factors.

IV ConclusionIn this paper we deduce the flux equation of ion beams in plasma focus for any gas using experimental data from the case of deuterons to obtain a calibration constant for energy fraction. We configure the Lee Model code as the NX2 using best estimated average model mass and current factors obtained from fitting the computed current traces of several gases with experimentally measured current traces. The flux equation is incorporated into the code and the number and energy flux and fluence from different gases are computed together with other relevant properties. The results portray the properties of the ion beam at the pinch exit.

Results: The ion fluence range from 7x1020 for the lightest gas H2 decreasing through the heavier gases until a value of 1.7x1020 for Ar and decreases further dramatically to 0.03 x1020 for Xe. The very small fluence value of Xe is due to the very large energy of the Xe ion, estimated to have average charge state Zeff of 28 and accelerated by exceedingly large electric fields induced in the radiative collapse

- ion number fluenceions m-2
- The number of beam ions: beam ions per kJ for PFâ€™s with typical static inductances L0 of 33-55 nH.
- Total beam energy: for NX2
- Beam current: of Ipeak.
- Beam energy fluence: J m-2
- Beam Energy Flux: W m-2
The independence from E0 of the ion beam fluence is likely related to the constancy of energy density (energy per unit mass) that is one of the key scaling parameters of the PF throughout its E0 range of sub kJ to MJ14,28.