Investigation of characteristics of hard x-rays produced during implosions of wire array loads on 1.6 MA Zebra generator

High Energy Density Physics 6 (2010) 113–120

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Investigation of characteristics of hard x-rays produced during implosions of wire array loads on 1.6 MA Zebra generator I. Shrestha*, V.L. Kantsyrev, A.S. Safronova, A.A. Esaulov, K.M. Williamson, N.D. Ouart, G.C. Osborne, M.E. Weller, M.F. Yilmaz Physics Department, University of Nevada, Reno, NV 89557, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 May 2009 Received in revised form 2 September 2009 Accepted 5 September 2009 Available online 12 September 2009

Experimental study of the characteristics of hard x-ray (HXR) emission from multi-planar wire arrays and compact-cylindrical wire arrays (CCWA) plasmas on the 1.6 MA Zebra generator at UNR has been carried out. The characteristics of HXR produced by multi-planar wire arrays such as single, double, and triple planar as well as compact-cylindrical wire arrays made from Al, Cu, brass, Mo, and W were analyzed. Data from spatially resolved time-integrated and spatially integrated time-gated x-ray spectra recorded by LiF spectrometers, x-ray pinhole images, and signals from fast x-ray detectors have been used to study spatial distribution and time history of HXR emission with different loads. The dependence of the HXR yield and power on the wire material, geometry of the load and load mass is observed. Both HXR yield and power are minimum for Al and maximum for W loads. The HXR yield increases with the rise of the atomic number of the material for all loads. The presence of aluminum wires in the load with the main material such as Cu, Mo, or W in combined wire arrays decreases HXR yield. For W plasma, the intensity of cold L-shell spectral lines (1–1.5 Å) correlates with corresponding amplitude of HXR signals which may suggest the evidence of generation of electron beams in plasma. It is found that HXRs are generated from different plasma regions by the interaction of electron-beam with the plasma trailing mass, with the material of anode and due to thermal radiation from plasma bright spots. The theoretical assumption of thermal mechanism of HXR emission predicts the different trends for dependency of HXR power on atomic number and load mass. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Hard x-ray Wire array Z-pinch

1. Introduction The emission of hard x-rays (HXR) with photon energies greater than 5 keV is a common feature in Z-pinch discharges such as vacuum sparks [1–3], plasma focus [4,5], gas-puff [6,7], X-pinch [8– 11] and wire array [12–16]. The previous studies showed that X-pinch had emitted HXR continuum and characteristic line radiations due to interaction of energetic electron beams with matter [11]. The energy of HXRs produced by X-pinch or single wire Z-pinch (C fiber, Al and Mo wires) discharges were experimentally measured at the different generators (current of 100–450 kA with rise time of 100–150 ns) [11–14]. It was found that the HXR photons in wire array experiment can reach energies of up to 4 MeV that is much higher than the applied voltage [12]. In earlier studies, the time evolution and spatial localization of the HXR in the spectral range >50 keV were studied on the 4 MA Angara generator [16].

* Corresponding author. Tel.: þ1 775 247 2763; fax: þ1 775 784 1398. E-mail address: [email protected] (I. Shrestha). 1574-1818/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hedp.2009.09.001

The total HXR energy was estimated in the range 0.02–2 J depending on the material of wires (Al and W) in the cylindrical wire array load. The time evolution of HXR emission from wire array Z-pinch plasma was investigated in Ref. [16]. The result showed that the HXR pulse was delayed 8–15 ns with respect to that of the soft x-ray signal depending on the type of loads. HXR may arise due to thermal radiation of hot plasma or the interaction of the energetic electron beam with the pinch plasma and electrode material. For example, three different types of such electron beams were observed in X-pinches [10]. The proposed mechanism of x-ray multiburst regime in 1 MA X-pinches was described in Ref. [15]. The HXR emission takes place when plasma undergoes disruption [12,13]. This should be associated with the break up of the plasma column into density islands. It can be assumed that this necking off of the plasma provides the accelerating mechanism for the electron beam and hits the electrodes that ultimately produce the HXR through bremsstrahlung mechanism. This model of generation of HXRs is described in Ref. [15]. In this paper, a systematic study of the characteristics of HXR emission from various plasmas such as Al (low atomic number Za),

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Cu and brass (mid atomic number Za) and Mo and W (high atomic number Za) from wire arrays were performed. In particular, these plasmas were generated in multi-planar wire arrays such as single planar wire array (SPWA), double planar wire arrays (DPWA), triple planar wire arrays (TPWA), and also compact-cylindrical wire arrays (CCWA) at the 1.6 MA Zebra generator. The presented results are strongly relevant to ICF-related wire array experiments at Sandia National Laboratories (SNL). In Zebra experiments, one of the most important parameters, the inter-wire gap, varies from 0.4 to 0.7 mm for compact-cylindrical wire arrays which is close to the inter-wire gap for cylindrical wire arrays from 0.2 to 0.5 mm in SNL Z-experiments [17–19]. In most of planar wire arrays experiments at Zebra generator, the inter-wire gap was 0.5–0.7 mm [20] which is also close to the inter-wire gap of 0.5 mm in the single planar wire arrays on SNL Saturn generator [19]. This paper mostly presents the comparative study of intensity, spatial distribution and time history of HXRs generated from different wire arrays. Section 2 describes diagnostics and Z-pinch loads used in experiments. Section 3 includes discussion of the results and Section 4 gives the conclusion of this paper. 2. Expermental setup The experimental results were obtained from experiments at UNR Zebra generator with a peak current 1 MA in a standard mode, and up to 1.6 MA with Load Current Multiplier [21], current rise time of 100 ns, electrical power 1.5 TW, a maximum stored energy of 150 kJ in capacitors, and 1.9U pulse forming line impedance. The parameters of different loads are given in Table 1. The Al, Cu, brass, Mo, and W wires had diameters that vary from 5 mm to 30 mm. During the experiments; the wire anode–cathode gap was 20 mm for most of the tested loads. In some of the shots, the gap was 10 mm (for DPWA and SPWA loads). The SPWA is a Z-pinch load configuration, in which the wires are mounted in a row forming a plane between the anode and cathode at the center of the discharge chamber. The number of wires varies from 5 to 16 and an inter-wire gap (d) varies from 0.5 to 1 mm in SPWA experiments [22,23]. In DPWA, the wires are located in two parallel rows with an inter-planar gap (D) placed between two electrodes [24]. The D varies from 1.5 to 9 mm. The inter-wire gap varies from 0.7 to 1 mm. The number of wires varies from 8 to10 in each row. The diameter of wires varies from 6 mm to 17.8 mm. The TPWA consists of three parallel wire rows with a variable inter-planar gap, placed between two electrodes. The inter-row gap Table 1 Wire array configurations tested on Zebra generator. Configuration

Material

Total mass (mg/cm)

No. of wires

Width (D)/dia. (mm)

Compact cylindrical

W Mo Al (5056) W Mo Al (5056) W/Al Mo/Al W Mo Al (5056) W/Al Mo/Al W Mo Al (5056)

91–98 100–144 76–124 87 80–100 64 88 98 87–110 100–180 75–100 86–94 80–100 91 90 64

14–24 20–24 15–26 8/8 8/8–10/10 8/8 8/8 10/10 6–24 10–18 9 6/2–10/2 8/2–13/2 8/8/8 6/6/6 10/10/10

6 3,6 3,6 3 1.5,3,6 3 3 1.5,3,6,9 3.5–10.5 6.3–9.8 5.6 4.9–10.5 9.1–9.8 3 3 3

Double planar

Single planar

Triple planar

D was 3 mm. The inter-wire gap was 0.7 mm. The number of wires in each row varies from 6 to 10. The diameter of wires varies from 5 to 15 mm. Compact-cylindrical wire array is a cylindrical wire array that has a small diameter of 3 or 6 mm [23]. The inter-wire gap varies from 0.4 to 0.7 mm. The number of wires varies from 16 to 26. The plasma diagnostics consist of various x-rays and laser probing devices. X-ray/EUV diagnostic includes fast detectors, imaging systems and spectrometers. The fast x-ray detectors include x-ray diodes (XRD), photoconducting diodes (PCD) and hard x-ray Si-diodes. The calibrated XRD and PCD were used to measure the x-ray yields and power and x-ray radiation time history. The XRD was filtered by a 5 mm kimfoil and registered radiation in two regions of photon energy 0.18 keV < hn < 0.28 keV and hn > 0.7 keV (‘‘sub-keV’’ spectral range). The PCD with filter 8 mm Be registered radiation with hn > 0.75 keV, (‘‘keV’’ spectral range). To study the time dependence of HXR emission, the hard xray detectors with filtered fast Si-diodes (AXUV-HS5) were used. Three Si-diodes with filters 240 mm Al (E1/10 > 9 keV), 8 mm Be, and 140 mm Cu (E1/10 > 25 keV), and 120 mm Al and 200 mm Cu (E1/10 > 25 keV) registered hard x-rays with photon cut off energy E1/10 > 9 keV, E1/10 > 25 keV and E1/10 > 30 keV, respectively. The sensitivity factor of AXUV-HS5 Si-diode is 2.8 A/W [25]. An estimation of the total HXR intensity is based on integration of hard x-ray signal (cut off energy E1/10 > 9 keV) with taking into account sensitivity factors of Si-diodes. A standard nickel bolometer was used for measurements of total x-ray yields (0.01–5 keV) for various loads [15]. The x-ray/EUV spectroscopic diagnostics were employed to obtain more precise information about plasma electron temperature Te, density ne, and ion charge state Z. These devices include four different spectrometers: time-integrated spatially resolved and time-gated spatially integrated potassium hydrogen phthalate (KAP) convex crystal spectrometers; spatially resolved time-integrated and spatially integrated time-gated LiF crystal spectrometers were used in this experiment. In KAP crystal spectrometers, convex KAP crystals (radius of curvature 51 mm) and filters made of 8 mm kapton and 1 mm mylar (coated by 0.3 mm Al) were used. The spatial resolution was 1.5 mm. The frame duration in time-gated spectrometer was 4 or 5 ns. Inter-frame time was 5 or 10 ns. In both time-integrated and time-gated LiF spectrometers, LiF convex crystals (radius of curvature 25.4 mm) were used. The time-integrated LiF spectrometer has low spatial resolution of 7.7 mm which was enough for estimation of position of the HXR emission region: near the cathode, at the center of the anode and cathode gap, or near the anode. The LiF time-integrated spectrometer was filtered with 110 mm mylar and 70 mm Be. Both time-gated and time-integrated spectrometers record mainly the ‘‘cold’’ characteristics Ka and La lines from wire material and the anode material in a spectral . range 1.28–1.54  A The imaging system consists of time-integrated and timegated x-ray pinhole cameras. Time-integrated x-ray pinhole images with a spatial resolution of 220 mm were recorded through two 70 mm diameter pinholes by three layers of the Kodak Biomax MS x-ray film. The first pinhole was filtered with 8 mm kapton, 3 mm mylar and 0.3 mm Al and the second pinhole consisted of additional 110 mm mylar and 20 or 60 mm copper foil. For the configuration with 60 mm copper foil, the first film registered the radiation with E1/10 > 1.2 keV and E1/10 > 8.6 keV, the second film at E1/10 > 3.4 keV and E1/10 > 8.7 keV and the third film at E1/10 > 4.3 keV and E1/10 > 8.9 keV, respectively. For the configuration with 20 mm copper foil, the first film registered radiation with E1/10 > 1.2 keV and E1/10 > 6 keV, the second film at E1/10 > 3.4 keV and E1/10 > 6.2 keV and the third film at E1/10 > 4.3 keV and E1/10 > 6.4 keV, respectively.

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The time-gated pinhole camera based on microchannel plate detector (MCP) has six time frames with adjustable frame duration from 1 to 9 ns with an inter-frame time up to10 ns. This pinhole camera with spatial resolution of 230 mm was composed of two identical parallel rows of Pt/Ir membranes (six in each row) with a 15 or 25 mm Be filter to protect the plasmas from plasma beams and debris. The one row formed images for E1/10 > 1.24 keV and one for E1/10 > 3.54 keV (or one E1/10 > 1 keV and another for E1/10 > 3.1 keV). The diameter of a pinhole was 70 mm. The direct measurement of electron beam was obtained by Faraday cup with the collector position above the anode and filtered by 50 mm iron foil [26] (the electron cut off energy >150 keV). 3. Results and discussion 3.1. Comparison of HXR yield for different material of loads Both the HXR yield and power were found to depend on wire material and geometry of loads and the HXR yield is of several Joules (Fig. 1 and Table 2). The HXR yield was found to rise with the increase of the atomic number Za of the wire material for all tested loads evidenced by Fig. 1. The HXR yield was maximum for tungsten (W) and minimum for Al in all tested wire arrays experiments as shown in Fig. 1 (for this figure and Fig. 2 the deviation of HXR yield in experimental results is about 10–25% for all load materials) and Table 2.The HXR yield was the highest for CCWA compared to DPWA and SPWA as shown in Fig. 1. No evidence was found that HXR yield was dependent on inter-wire gap. The total HXR yield depends on the diameter of CCWA. It is larger for 3 mm diameter compared with 6 mm diameter CCWA. HXR power also depends on wire material and geometry. It is maximum for W (in range 77–140 MW) and minimum for Al (in range 0.5–6 MW). Specifically, it is higher for CCWA than for SPWA and DPWA with the same wire material of load as shown in Table 2. Table 3 summarizes the comparison of the total HXR yield and total radiation yield (ET) for combined DPWA which consists of Al wires in one row and brass or Mo or W wires in another row. The presence of Al wires in combined DPWA like Mo/Al (Mo wires in one row and Al in another row), Brass/Al (brass in one row and Al in another row), and W/Al (W in one row and Al in another row) reduces both HXR yield and pulse amplitude. Similarly, both HXR yield and pulse amplitude are also bigger for uniform wire array with the main material like Cu, Mo or W and less for the wire array

115

Table 2 Experimental comparison of wire array hard x-ray sources on Zebra generator. Load

Total HXR yield ET (J)

Total HXR power PT (MW)

Al SPWA Cu SPWA Mo-SPWA W-SPWA Al CCWA Mo CCWA W-CCWA Mo-DPWA W DPWA Al DPWA

0.01–0.08 1.47–1.9 1.9–2.8 3–5.4 0.03–0.09 1.6–2.6 3–5.5 1.28–1.9 3.4–3.5 0.042–0.045

0.5–4 8.5–20 30–66 77–140 1.5–6 34–68 116–204 54–69 101–124 2.5–3.2

with tracer Al wires for SPWA and CCWA. It also supports the idea on non-thermal HXR generation mechanism in wire array plasma. For comparison of HXR yield generated in described experiments with plasma thermal radiation, we shall estimate the HXR radiation power, assuming the thermal mechanism of photon emission and the Planckian black body radiation spectrum. In this case the density of a radiation power Jn in frequency range [n,nþdn] can be defined as

Jn dn ¼

2ph n3 dn ; c2 ehn=T  1

(1)

where T is the temperature of the black body radiation source, expressed in energy units. As it was discussed, for example, in Ref. [27] for stagnation stage in Z-pinch this temperature can be estimated as the Bennett equilibrium temperature TzTB.

TB ð1 þ ZÞ ¼

m0 2 AmA I ; 8p m L

(2)

where Z is the mean ion charge, I is the current through plasma, A is the atomic weight of plasma element, mA is the atomic mass unit, and mL is the pinch mass per unit length. It should be noted here that typically Z-pinch plasma is not in equilibrium with magnetic field. Z-pinch plasma pressure can be lower or higher than the magnetic field pressure if pinch is compressing or expanding respectively. Thus, the temperature of Zpinch plasma can be lower or higher than the equilibrium Bennett temperature. However, Bennett temperature is more appropriate to be used for the estimation of Z-pinch plasma temperature at the stagnation stage when the Z-pinch plasma is in quasi-equilibrium with the magnetic field. The estimations performed for Zebra Shots were based on analysis of shot #1014(Al) and shot#1025(W). Both loads have comparable masses. The mass per unit length is 100 mg/cm for shot

3

1 .8

HXR energy (J)

Hard X-ray yield(J)

4

2 1 0

--

SPW A Typ

--

DPWA

W 80 M o 50 40

TPW A

es o f lo a d

CCWA

60

C u 30 m nu A l 20 ic 10

At

70

r be

(Z

)

om

Fig. 1. The variation of total hard x-ray yield (in J) from different types of load and atomic number of wires material.

1 .6

1 .4 1 .2 1 .0 0 .8 0 .6 0 .4 0 .2 D P W A -M o /A l

Ma

te r

ia l

4 6

of

D P W A -M o

Lo

ad

8 10

In

te

o r-r

w

2

ga

p

(m

m

)

Fig. 2. Dependence of HXR yields from inter-array gap for Mo/Al-DPWA and Mo-DPWA.

I. Shrestha et al. / High Energy Density Physics 6 (2010) 113–120

Table 3 Parameters of combined DPWAs and characteristics of HXRs.

40 0.90

Material

Total mass (mg/cm)

Bolo max (KJ)

Total HXR yield (J)

Al (5056)/Mo Mo Brass Brass/Al W/Al W

98 80–160 62 62 82 87

13.7–22.5 18.7–22.7 20.3 15.8 18.6–21.3 16–22.4

0.19–0.6 2.17–3.2 0.97 0.31 0.68–1.34 3.5–3.5

30

Signal, V

20

0.50

10 0.10

0

Current, MA

116

-10 -0.30

Ps ¼

ZN

Jn dnz

nb

2ph c2

ZN nb

    hn 2p hn n3 exp  dnz 2 TB n3b exp  b : TB TB c

(3)

The total HXR power is defined by the relation

P ¼ Ps $S;

(4)

2

25

KV

E (KJ)

15

1

10

0.5

ET(KJ)

20

1.5

5

0 0

0.2

0.4 Hard x-ray yield (J)

0.6

-30

0 0.8

Fig. 3. Comparative plots of PCD (keV range output) and bolometer (total radiation output ET) yields vs. total hard x-ray energy for uniform Mo-DPWA.

-0.70 0

20

40

60

80

100

120

140

160

Time, ns

Fig. 4. W/Al (tracer) SPWA (Zebra shot# 1594), wire no NAl ¼ 2 and NMo ¼ 10, wire diameter FMo ¼ 7 mm and FAl ¼ 17.8 mm, inter-wire gap d ¼ 0.7 mm. HXR silicon diode signal >9 keV (gray line), and >25 keV (black faint line), Faraday cup signal (light black line), PCD (8 mm Be filter-black dark line), current wave form (top gray line) vs. time.

the estimated HXR radiation powers are much less than the experimental observations. This corroborates the hypothesis that HXR can be explained by non-thermal mechanism such as the electron beam interaction with Z-pinch plasma. Although, as it was mentioned above, the Z-pinch plasma temperature can be higher than estimated Bennett temperature, it still can not explain HXR radiation power measured in the experiments (since the difference between estimated and measured HXR radiation powers is several orders of magnitude). On the other hand, higher pinch temperature would significantly decrease plasma collisionality and, thus, significantly increases the electron beam intensity [29]. Hence, even at higher pinch temperatures, the HXR emission from the electron beam interaction with the anode surface or the material of plasma column will be much stronger compared to the thermal radiation from pinch plasma. The HXR yield are maximum for DPWA load with 1.5 mm interrow gap, and decreases with the rise of D in DPWA load with Mo and Mo/Al wires shown in Fig. 2. This is similar with the fact that the total radiation yield (ET) is also maximum for inter-row gap D ¼ 1.5 mm and minimum for 9 mm only in DPWA which was mentioned in Refs. [22,23]. From these facts, we concluded that both ET and HXR yield depend near linearly with inter-row gap. Fig. 3 shows strong correlation between HXR yield and observed keV radiation yield (E1/10 > 0.8 keV), and good correlation between the HXR yield and the total radiation output ET for shots like uniform Mo-DPWA. The same trends of correlations were found in

Signal, V

where S ¼ 2prplz is the area of the radiating pinch surface, rp is the pinch radius, and lz is the pinch length (gap between the electrodes). We can estimate pinch radius from time-integrated pinhole image as rp ¼ 0.5 mm. The black body radiation power depends on Bennett temperature exponentially shown in above Eq. (3). Yet, the Bennett temperature depends strongly on the load mass and less strongly on mean ion charge (see, for example, Refs. [27,28]). Thus we should see strong dependence of HXR power on the load mass and weak dependence on elements used. However, this is not what was observed in experiments. The experimental results in Table 2 and Fig. 1 show that HXR energy and power change near linearly with the atomic number of load material and weakly depend on the load mass. Thus, our assumption of thermal mechanism of HXR production predicts quite different trends for the dependency of HXR power on the atomic element number and mass of the load. This suggests another mechanism of HXR production different from thermal one, which should be linked with non-thermal mechanism like the electron beam generation. According to Eqs. (3) and (4), the calculated black body radiation power is w1022 MW for Al and w105 MW for W. In both cases,

-20

25

1.00

20

0.80

15

0.60

10

0.40

5

0.20

0

0.00

-5

-0.20

-10

0

50

100

150

Current, MA

#1014 and 91 mg/cm for shot#1025. The dependence of Z on Bennett temperature was calculated by using LTE (Local Thermodynamics Equilibrium) approximation. The estimated Bennett temperatures are 120 eV and 230 eV for Al and W respectively. As we can see, in both cases the Bennett temperature is well below the HXR range hnb >9 keV. Thus, the HXR radiation power estimations have to be performed at the high energy tail of Planckian spectrum hn >> T. Now, the black body radiation power per unit surface PS can be found by integrating Eq. (1) over the frequency range n  nb

-0.40

Time, s

Fig. 5. W/Al(tracer) DPWA (Zebra shot# 1599), wire no NAl ¼ 1 and NW ¼ 7 in each row, wire diameter FW ¼ 7 mm and FAl ¼ 17.8 mm, inter-wire gap d ¼ 0.7 mm, interrow gap D ¼ 3 mm HXR silicon diode signal >9 keV (gray line), and >25 keV (black faint line), Faraday cup signal (light black line), PCD (8 mm Be filter-black dark line), current wave form (top gray line) vs. time.

10

1.0

8

0.8

6

0.6

4

0.4

2

0.2

0

117

Current, A

Signal, V

I. Shrestha et al. / High Energy Density Physics 6 (2010) 113–120

0.0 0

50

100

150

200

Time, ns

Fig. 6. Uniform Mo-SPWA (Zebra shot# 794), N ¼ 10, wire diameter F ¼ 7.9 mm, interwire gap d ¼ 0.7 mm. HXR silicon diode signal >9 keV (gray line), >25 keV (black faint line), and >30 keV (gray faint line), PCD (8 mm Be-black dark line), current wave form (top gray line) vs. time. Each Hard x-ray signal is multiplied by 2 for clarity.

combined asymmetric Mo/Al-DPWA, uniform Mo-SPWA, and combined Mo/Al (tracer)-SPWA. This observation differs from the experimental results from the Mo x-pinches study on 1 MA Zebra generator discussed in Ref. [8] where the reverse correlation between the total HXR yield and keV radiation yield (E1/10 > 0.8 keV) were found. These experimental facts show that the mechanism of generation of HXR in wire array plasma does not coincide with ones in X-pinches. From another side, the reverse correlation was observed between the HXR yield and the keV radiation yield (E1/10 > 0.8 keV) and the total radiation output ET only for W-SPWA. The reason of this fact is still not clear. 3.2. Time evolution and pulse shape of HXR and SXR signals and scaling of HXR yield with peak discharge current The maximum amplitude of HXR signal in all tested loads (current rise from 0 to 100% is 100 ns) was found in the range of 80– 110 ns from current start for all tested loads. In most of the shots, HXR bursts were located at the time closer to the position of SXR burst shown in Fig. 4. But in some shots, the front of the HXR burst was delayed by 1–6 ns with respect to that of SXR burst as shown in Figs. 4 and 5. The same result was obtained in the previous study of the time evolution of HXR emission which is consistent in Ref. [16]. The delay time also depends on material and geometry of loads. Using Faraday cup placed axially near the hole in the anode plate of load, an electron beam current with several kA was detected. The total energy of the electron beam (Faraday cup with the 50 mm Fe

Fig. 8. Uniform Mo CCWA (Zebra shot# 1260), N ¼ 24, wire diameter F ¼ 8.64 mm, diameter of array D ¼ 3 mm. HXR silicon diode signal >9 keV (gray solid line), >25 keV (black faint line), and >30 keV (gray faint line), PCD (8 mm Be filter-black dark line), current wave form (top gray line) vs. time. Each Hard x-ray signal is multiplied by 2 for clarity.

filter with electron cut off energy E > 150 keV) is up to 5 kJ. The energy of HXR (measured after 50 mm Fe filter with photon cut off energy E > 15 keV) is up to 5 J. So we expect no significant effect of HXR emission on electron beam measurements. The appearance of the electron beam was mostly correlated with the generation of HXR signals in SPWA and DPWA as shown in Figs. 4 and 5. But only in CCWA, the fronts of HXR pulses were delayed by 5–8 ns with respect to Faraday cup signal. Typically, strong amplitude of HXR signal was accompanied by a weak SXR signal and a weak HXR signal was accompanied by a strong SXR signal as shown in Figs. 6–8. This inverse correlation between amplitudes of HXR and SXR signals observed in the present study was also found in the previous studies of HXRs in the 1 MA X-pinches on the Zebra generator [10]. The HXR pulse duration for CCWA with W wires was about two times larger than for Al one. The HXR pulses in CCWA had relatively less number of peaks as compared to that in SPWA and DPWA (shown in Figs. 4 and 5).The hard x-ray pulses in CCWA with W and Mo have multi-peaks but in Al CCWA there were 2–3 peaks. The signals in TPWA consisted of short pulses (1–3 ns duration) as compared to that in SPWA, DPWA and CCWA. In most of CCWA shots, the amplitude of HXR peak was maximum near the middle of the whole HXR pulse shown in Fig. 8. The FWHM duration of HXR pulse varies from 5 to 50 ns depending on the material of load in CCWA. The FWHM duration of HXR pulse is about 23–25 ns range for W DPWA while that for W-CCWA is 16–20 ns shown in Figs. 7 and 8. The reason for the longest FWHM duration for W DPWA is

3

HXR yield ( J/cm)

2.5 2 1.5 1 0.5 0 Fig. 7. W DPWA (Zebra shot# 1038), wire no N ¼ 8, wire diameter F ¼ 6 mm, inter-wire gap d ¼ 0.7 mm, inter-row gap D ¼ 3 mm. HXR silicon diode signal >9 keV (gray line), >25 keV (black faint line), and >30 keV (gray faint line), PCD (8 mm Be filter-black dark line), current wave form (top gray line) vs. time.

0.6

0.8

1

1.2

1.4

1.6

Current(MA) Fig. 9. Dependence of HXR yield (J/cm) on peak load current for W/Al-SPWA.

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I. Shrestha et al. / High Energy Density Physics 6 (2010) 113–120

Fig. 10. W-CCWA (Zebra shot#1626), a) the images of time-gated spectra recorded by 4th, 5th and 6th, frames. b) HXR silicon diode signal >9 keV (gray line), and >25 keV (black faint line), Faraday cup signal (light black line), PCD (8 mm Be filter-black dark line), current wave form (top gray line) vs. time. The correlation between intensity of lines in TGSP (with LiF crystal) x-ray spectra and corresponding amplitude of peaks HXR pulses.

still unclear. The FWHM duration of HXR pulse is maximum for DPWA and minimum for CCWA. In addition, FWHM duration for HXR pulse is maximum for W and minimum for Al for all tested loads. In W experiment, the FWHM of HXR signals is minimum for CCWA whereas maximum for conventional cylindrical wire arrays. The results show HXR yield depends near linearly on a peak load current (in 0.8–1.4 MA region) for all kinds of multi-planar wire

arrays and CCWAs. Fig. 9 depicts the HXR yield is found to increase with the rise of peak load current. A scaling is proposed which relates the HXR yield to the peak load current. Specifically, the HXR yield varies with load peak current approximately following a scale law:

EwI 0:94 ;

(5)

 ) with spatial resolution 7.7 mm Fig. 11. Characteristic spectral lines in by hard x-ray spatially resolved spectra recorded by a time-integrated LiF crystal spectrometer (2d ¼ 4.027  A on the Zebra generator a) Brass SPWA (shot#811). (F ¼ 10.9 mm, d ¼ 1 mm, N ¼ 10). The line spectra shows the localization of and intensity electron beams b) W-CCWA (F ¼ 5 mm, D ¼ 6 mm, N ¼ 24) shows La lines. The dotted line shows the position of anode and cathode. Anode was made from stainless steel.

I. Shrestha et al. / High Energy Density Physics 6 (2010) 113–120

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Fig. 12. W/Al CCWA (shot#1052), a) Time-gated EUV pinhole images (frame 1–4), the frame duration is 3 ns and inter-frame time is 10 ns b) HXR silicon diode signal >9 keV (gray solid line, PCD (8 mm Be filter-thick black dark line), Faraday cup signal (thin dark line), EUV pinhole camera images (black spot on current wave form), current wave form (top thick gray line) vs. time.

where E is HXR yield in Joule and I is a peak load current in mega Ampere. 3.3. Spectroscopic measurements The direct correlation between intensity of lines in x-ray timegated spectra and corresponding amplitudes of HXR peaks with timing recorded in W-CCWA experiment is shown in Fig. 10a and b. Specifically, images of time-gated spectra recorded by 4th, 5th and 6th MCP frames are shown in Fig. 10a. These spectra recorded by the LiF crystal cover the spectral region from 1 to 1.7 Å and include ‘‘cold’’ characteristic L-shell W lines. The experimental observation showed that the amplitude of HXR signals correlated with intensity of time-gated x-ray spectral lines and blackening on the MCP frame (due to bremsstrahlung radiation). Although the Faraday cup signal is clipped, the timing of the onset of e-beam detection is clear and corresponds to that of generation of HXR in Fig. 10b. This direct correlation between intensity of lines in time-gated x-ray spectra and corresponding amplitude of peaks HXR pulses gives another evidence of generation of HXR in plasma by interaction of electron beams with wire material plasma. In brass SPWA experiment, time-integrated ‘‘cold’’ characteristic K-shell Cu and Zn lines recorded with spatial resolution demonstrated that the HXRs are emitted along the whole anode–cathode gap. Both ‘‘cold’’ K-shell Cu and Zn spectral lines are from wire material plasma whereas ‘‘cold’’ K-shell Fe line is from the anode material (stainless steel) plasma in Fig. 11a. The presence of these lines indicates that the energy of electrons in the beam was at least 8.1 keV [26]. In W-CCWA shots, ‘‘cold’’ L-shell W lines are emitted

from the wire material plasma and K-shell Fe and Cr lines are from plasmas from the anode material (stainless steel) as shown in Fig. 11b. These spectral lines indicate that the energy of electrons in the beam was at least 9.6 keV. Fig. 12a illustrates the process of implosion of wires and can be used for the explanation of the possible mechanism of generation of HXRs due to interaction of electron beam with Z-pinch trailing masses. Fig. 12a and b show correlation of the HXR signals and Faraday cup signal with the frames of time-gated EUV pinhole images in W/Al CCWA experiments on 1 MA Cobra generator with current rise time 100 ns [30]. Due to axial non-uniformity of ablation in the plasma implosion, a fraction of initial array mass is being behind by the imploding current sheath and central plasma column [31]. This remaining mass is called trailing mass. Frames 3 and 4 show the dynamics of the trailing mass. The timing of appearance of the HXR radiation correlates with the arrival of the trailing mass to the pinch region, when the electron beam starts to interact with the cold dense plasma as shown in Fig. 12a and b. This fact evidences that the HXRs can be generated due to interaction of e-beam with the trailing mass. 4. Conclusion The experimental results produced on Zebra generator presented in this paper show the following specific features of HXR generation. The HXR yield is of the order of several Joules. A good correlation between the total HXR yield and the intensity of keV xray radiation was observed for Mo-DPWA and SPWA loads. The estimated HXR power and energy are found to depend on the

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atomic number of wire material and geometry of the load, and to depend weakly on the load mass. The HXR power is minimum for Al and maximum for W. The presence of aluminum wires in the load with the main material such as Cu, Mo, and W in combined wire arrays reduces the total HXR yield. An assumption of thermal mechanism of HXR generation does not explain the dependency of HXR power on the atomic number correctly. Some of the ‘‘cold’’ spectral lines in HXR region were observed that appeared only during the interaction with e-beam with load material. These facts suggest that the mechanism of HXR emission should be associated with non-thermal mechanism like the result of interaction of the electron beam with the load material. Some of the evidences were found that HXRs were generated during the interaction of electron beam with plasma trailing mass and anode material. The HXR was generated along the whole anode–cathode gap as well as near the anode surface. For multiplanar wire arrays and CCWA, a strong HXR pulse was accompanied by a weak SXR pulse and vice versa. This inverse correlation between the intensity of HXR pulse and SXR pulse observed in the present study was also found in the previous Mo X-pinch experiments on the 1 MA the Zebra generator [10]. The maximum intensity of HXR pulse was observed from 80 to 200 ns from current start for all tested wire arrays with minimum pulse duration 5–15 ns. The scaling of current with HXR yield shows that HXR yield depends near linearly on the load peak current. The mechanism of generation of HXR emission during the implosion of wire arrays and its theoretical justification will be discussed in future work. Acknowledgments The authors thank NTF Director Dr. J. Kindel and technical team for the support of the experimental campaign. The authors also thank D. Hammer, R. McBride, P. Knapp, K. Bell, P. Wilhelm and D. Chalenski for the support of the experimental campaign at Cornell University. This work was supported by the DOE/NNSA under Cooperative Agreements DE-FC52-06NA27586, DE-FC5206NA27588, in part by DE-FC52-06NV27616 and DE-FC0302NA00057. References [1] W.A. Cilliers, R.U. Datla, Hans R. Griem, Spectroscopic measurements on vacuum spark plasmas. Phys. Rev. A 12 (1975) 1408–1418. [2] C.R. Negust, N.J. Peacock, Local regions of high-pressure plasma in vacuum spark. J. Phys. D Appl. Phys. 12 (1979) 91–111. [3] R. Beier, C. Bachmann, R. Burhenn, Investigation of polarization of nonthermal bremsstrahlung from a vacuum spark plasma. J. Phys. D Appl. Phys. 14 (1981) 643–648. [4] A. Serban, A. Patran, S. Lee, M.S. Rafique, Time–resolved electron beam and xray emission from a Neon plasma focus, in: 27th EPS Conference on Contr. Fusion and Plasma Phys. ECA 24B, 2000, pp. 480–483. [5] N.K. Neog, S.R. Mohanty, Study on electron beam emission from a low energy plasma focus device. Phys. Lett. A 361 (4–5) (2007) 377–381. [6] D.R. Kania, L.A. Jones, Observation of an electron beam in an annular gas-puff Z-pinch plasma device. Phys. Rev. Lett. 53 (2) (1984) 166–169. [7] P. Choi, A.E. Danger, C. Deeney, C.D. Challis, Temporal development of hard and soft x-ray emission from a gas-puffs Z-pinch. Rev. Sci. Instrum. 57 (8) (1986) 2162–2164. [8] V.L. Kantsyrev, D.A. Fedin, A.S. Shlyaptseva, S. Hensen, et al., Energetic electron beam generation and anisotropy of hard x-ray emission from 0.9 to 1.0 MA high-Z X-pinches. Phys. Plasma. 10 (6) (2003) 2519. [9] A.S. Shlyaptseva, S. Hensen, V.L. Kantsyrev, D.A. Fedin, et al., Advanced spectroscopic analysis of 0.8–1.0 MA Mo X-pinches and influence of plasma electron beams on L-shell spectra of Mo ions. Phys. Rev. E 67 (2003) 026409.

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