LLNL RACE SoftXRay 1989

UCID- 21643

J. H. Hammer, C. W. Hartman, and A. W. Molvik

INITIAL SOFT X-RAY PRODUCTION EXPERIMENTS ON RACE

April 4, 1989

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INITIAL SOFT X-RAY PRODUCTION EXPERIMENTS ON RACE

J. H. HAMMER, C. N. HARTMAN AND A. W. MOLVIK

Lawrence Livermore National Laboratory

Initial ring stagnation experiments on the Ring Acceleration Experiment (RACE)

show photon fluences consistent with modeling (approximately a few kilojoules

with hv > 10 eV) and an output time scale of the order of the shock heating

time (5-10 iiS).

ABSTRACT

Livermore, CA 94550

DE89 011341

UCID—21643

Large-scale accelerators (~ 10’s of MJ) could provide MJ’s of x-ray flux in

the 10 keV spectral region which is of interest for simulation of nuclear

weapons effects. The method of converting compact torus kinetic energy into

x-rays is closely analogous to the process in z-pinches . The plasma ring,

after acceleration to high velocity, impinges on a surface (a stagnation

screen). This launches a strong shock wave back through the compact torus

plasma. The shock wave heats the ions to high temperatures, which in turn

heat the electrons through collisional equilibration. The electrons partially

strip the ions and excite line radiation with characteristic photon energies

of the order of the electron temperature. The efficiency of the process can

be high and is predicted to be strongly dependent on the compact torus plasma

properties: density; velocity; scale length and ion species.

In this report we describe the first experiments on soft x-ray production in

the RACE device at LLNL. The RACE experiment^ shown in Fig. 1 is a coaxial

accelerator of magnetically confined plasma rings (compact toroids) that

produces plasma velocities in the range 10 - 3 x 10 cm/s with directed

kinetic energies from 4 to 40 kj. RACE is a proof-of-principie scale device

with the goal of demonstrating the feasibility of the compact-torus

accelerator as a high-power density driver for many applications, including

x-ray production.

Gun center electrode.

Gun outer electrode.

8 pulse gas valves

Outer solenoid

To accelerator capacitor bank

Inner solenoid

80-X-0389-0121-3 DM/MS

Compact torus

Accelerator outer electrode

Accelerator center electrode

To solenoid capacitor bank

Turbomolecular pump

To gun capacitor bank

0.03 |ifd, 5 kV, 250 kJ

Cryopump

Fig. 1. The RACE experimental apparatus.

111 ^ifd,60 kV, 200 kJ

Focusing cone

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Vacuum vessel

Simulations predict that the mean photon energy for high conversion efficiency

increases with plasma density and is highest when the compact torus is focused

to small dimensions (~ a few cm). The preliminary tests of shock heating

and radiation production in RACE have been conducted with unfocused rings.

For these experiments, we have observed partial confirmation of the radiation

process in that the onset of the radiation pulse is coincident with the

arrival of the compact torus at the stagnation screen, the observed fluences

are consistent with modeling (~ few kilojoules with hv > 10 eV) and

ouput time scale is of order of the shock heating time (5-10 ps).

-3

the

The geometry of the stagnation region on RACE is shown in Fig. 2. The

stagnation plate is 519 cm downstream from the gun muzzle/accelerator breech.

The outer electrode and stagnation plate are composed of 53% transparent

copper screen with 0.75 inch mesh spacing to allow diagnostic access. Two

x-ray diodes (XRD’s) on loan from the Air Force Weapons Lab at Albuquerque, NM

were deployed so as to view the plasma near the impact plate as shown in Fig. 2.

Note that the impact plate itself was not in the field of view of the diodes.

Both diodes viewed the same volume of plasma, contained polished aluminum

photocathodes, and were biased at -3000 volts. One of the diodes was

unfiltered while the other was covered by a Kimfoil filter that effectively

eliminated all photons of energy less than 200 eV.

For these experiments, the gun and accelerator were operated in a mode such

that comparatively heavy and slow but well localized compact toroids were

produced (M ~ 10 g, v ~ 10 cm/s). Fast rings (v > 10 cm/s, M ~

10 g) produced no detectable signals on the XRD’s as expected from the

modeling for unfocused rings. Ring mass and speed are controlled largely

through the timing and amount of gas inlet by the pulsed gas valves, (see Ref.

2 for a description of the RACE apparatus) although in the heavy ring case,

much of the plasma in the rings is apparently derived from electrode surface

Outer electrode (Cu screen)

Inner electrode (opaque Cu)

Cu impact plate

Emitting volume viewed by XRD

XRDs

Gun & accelerator feed

Vacuum vessel wall

Fig. 2. Compact Torus impact region on the RACE experiment

contaminants — mainly carbon and oxygen. The modeling of x-ray production

indicates that carbon and oxygen are (fortuitously) about the right Z for good

coupling of compact torus energy to photons for this ring parameter regime.

Compact torus properties were determined by magnetic probes, a HeNe laser

interferometer, a visible light monochrometer and VUV detectors. The latter

are similar in operation to the XRD’s, with aluminum photo-cathodes and a bias

voltage of -250 volts.

Many shots showed large signals (<10 volts) on the bare XRD although no

signals were observed on the filtered XRD. The threshold for detection (2 mV)

would require photon fluences with hv > 200 eV that are of the same order as

the total fluence observed on the bare XRD, so it is consistent with the

expected photon energies (few tens of eV) that no signals were observed on the

filtered XRD. Table 1 shows the ring parameters for shot #3520.

v = 10 cm/iiS

Q. = 50 cm

B = 5 kG

where:

Table 1 - Shot 3520

M = ring mass

UK = 10 kJ

Um = 8 kJ

a = full-width-half-maximum ring length from magnetic probes

v = ring velocity at impact plate

B = peak ring magnetic field

Um, Up, Uk = ring magnetic, thermal, and directed kinetic energy

ne - 1.4 x 10^5 cm-3

M = 2 mg (carbon, oxygen)

Figure 3 shows the signal observed on the unfiltered XRD. The output power of

the compact torus is approximately proportional to the XRD voltage with a peak

output power of 280 MW for a XRD signal of 8 volts. There are factor of ~2

uncertainties in relating the XRD voltage to the total radiated power from the

ring. Since we did not have spectral data, we cannot account for the varying

response of the XRD with photon energy, so we have simply used 0.5* peak

response (from the tabulated response data) which is reasonably accurate if

most of the emitted radiation is in the range 10-40 eV. If large amounts of

energy are at photon energies > 40 eV then the scale factor can substantially

underestimate the radiated power. The other largest uncertainty is the total

ring volume (>> volume sampled by the detector). For the calculated scale

factor the total radiating volume of ring plasma is taken to be one half the

volume before stagnation as expected from shock wave theory.

Table 2 shows the total observed fluence (given the uncertainties stated

above) and compares it with the predicted output for an analytic theory of

shock-heating, electron-ion coupling and radiation (see Ref. 2), and a LASNEX

simulation performed by Maggie Gee of the LLNL Physics Department. For the ID

LASNEX run, the parameters were chosen to model the ring properities given in

Table 1 with the assumption of a pure Carbon plasma.

Note the “foot” on the radiation pulse at > 70 MW is roughly the magnitude

expected from the balance of Ohmic heating with radiation in the torus before

impact, and is consistent with the magnitude of the radiated power observed on

the VUV detectors upstream of the impact plate.

Fig. 3. Bare XRD signal on shot # 3520

Time (tis)

Observation Shot #3520

Urad = 2.7 kJ (Q. = 25 cm assumed)

XRAD = 7 F15

hv > 10 eV

trad = -? = 5 f15

^-shocked = 25 cm

= 5 kJ

pshocked

Urad = 1-8 kJ

hv =: Te = 40 eV

Analytic X-ray Output Theory

LASNEX

Table 2

XRAD = 3

Urad = 4-1 kJ

hvpeak ’ 35 eV

^shocked = 21 cm

spectrograph to characterize the spectrum.

The spectrum predicted by LASNEX is shown in Fig. 4. The observations while

somewhat preliminary, are thus consistent with the expectations from a simple

theoretical model as well as the LASNEX code. We intend to repeat these

experiments in the near future with a larger complement of x-ray/VUV

diagnostics in the stagnation region, including a grazing incidence

parameters of shot #3520

Photon energy (eV)

Fig. 4. LASNEX predicted spectrum for the compact torus

J. H. Hammer, C. W. Hartman and J. L. Eddleman, Phys. Rev. Lett. 6J_, 2843

J. H. Hammer, C. W. Hartman and J. F. Holzrichter, “The Compact Torus

Accelerator, A New X-ray source for High-Fidelity Nuclear Weapons Effects

Simulation,” LLL-PROP-00212, July 5, 1988.

N. R. Pereira and J. Davis, J. App. Phys. 64, 3 (1988).

(1988).

0517b,lb

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REFERENCES