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THE LOS ALAMOS INTENSE NEUTRON SOURCE

R. A. Nebel, T-15 D. C. Barnes, T-15 R. Bollman, NIS-4 G. Eden, NIS-4 L. Morrison, NIS-4 M. M. Pickrell, NE-5 W. Reass, P-24

THE SECOND SYMPOSIUM ON CURRENT TRENDS IN INTERNATIONAL FUSION RESEARCH WASHINGTON, D.C. MARCH 10-14,1997

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product. process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, r a m - mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect thosc of the United States Government or any agency thereof.

L O 8 Alarno9 N A T I O N A L L A B O R A T O R Y Los Amos National Laboratory. an affirmative action/equai 0ppo:tunity employer, is operated by the irniveraity of Caiifornia for the US. Depart-nent of Ecergy under contract W-7405-ENG-38. By acceprance of :iris aeicle, !he ptblisher recogr.izes tha: the U.S. Government re?ains a nonexcIusive, royalty-free iiense IO publish of fepoduce the published brrn of this mntribution, ar to alfow oihers !o dcr so, for US. Gorernrnent purposes. Los Alamos National Laboratory requests that tho publisher identify this article as work performed under the auspices af the US. Department of Energy. The Los Alamos Nationai Laboratory strongly silpports academic freedom and a researcher’s right to publish; as an institution, hcwever, the Laboratory does not endarse ?he viewpoint of a pblication or guarantee It’s technical correctness.

Form No. 836 R5 ST2629 1W96

p

The Los Alamos Intense Neutron Source

R. A. Nebel, D. C. Barnes, R. Bollman, G. Eden, L Momson, M. M. Pickrell, W. Reass

Los Alamos National Laboratory Los Alamos, New Mexico 87545

Abstract

The Intense Neutron Source (INS) is an Inertial Electrostatic Confinement (IEC) fusion device presently under construction at Los Alamos National Laboratory. It is designed to produce 1011 neutrons per second steady-state using D-T fuel. Phase I operation of this device will be as a standard three grid IEC ion focus device. Expected performance has been predicted by scaling from a previous IEC device.1

Phase II operation of this device will utilize a new operating scheme, the Periodically Oscillating Plasma Sphere (POPS). This scheme is related to both the Spherical Reflex Diode2 and the Oscillating Penning Trap.3 With this type of operation we hope to improve plasma neutron production to about 101 3 neutrons/second.

Introduction

While electrostatic confinement has received interest for a number of years, it is only recently that sufficient insight has been gained to realize the important and unique capability of creating a high intensity fusion neutron source this way. The first suggestion to use electric fields for heating and confining fuel for nuclear fusion reactions was made by Salisbury 4 . Later and better concepts were proposed by Elmore, Tuck and Watson5 in 1959, and Farnsworth6 in 1956-66. Such concepts have come to be known as inertial-electrostatic confinement (IEC) schemes. They all envision that ions (Salisbury, Farnsworth) or electrons (Elmore, et al) projected radially inward will provide a negative potential well for the confinement of fusion-reactive ions in a core region. In the work of Elmore, et al., their original interest was piqued by their theoretical result that a 100 A electron current could maintain a potential well depth of 100 kV in a device size the order of one meter. Furth further explored this configuration in a theoretical study in 19637. In this study he concluded that an anisotropic electron distribution in a medium of neutralizing ions could lead to 8 pinch- type instability possessing a perturbation magnetic field, which could grow if the particle density maintained in the field was too large. Even so, later numerical calculations .suggested that net power might be achieved if the density distribution could be kept to at least an inverse-square dependence on the radius of the system.

Following this work additional experimental, theoretical and a few computational studies were completed.8-12 Perhaps the most intriguing of these was an experiment done by Hirschl, which demonstrated a much larger rate of neutron

production was occurring than could be accounted for by theoretical and computational models. In this work Hirsch, following the lines conceived by Farnsworths, modified the Elmore et al scheme by using ion beams injected into a sphere to bring in electrons which, in turn, set up the negative potential well structure desired to trap the ions internally.

Hirsch used six symmetrically-placed ion beams of 10 mA in a background of electrons emitted by a spherical cathode. No instabilities were observed. At least two virtual electrodes were implied (but not measured directly). Reproducible neutron output that increased linearly with current for fixed potential and pressure was produced. A higher neutron yield resulted from lowering the pressure, probably by reduction of beam degradation from neutral-ion and charge exchange reactions at high background pressures. Hirsch observed 2x1 01 0 n/s at densities between lx1012/cm3 and lx1014/cm3 with 10 keV and 100 keV ion beams in volumes between 0.1 cm3 and 10 cm3

The work decribed in this paper builds and expands upon Hirsch's basic ideas. Instead of using ion guns to supply the ions, we are building a three grid IEC. A three grid IEC device confines a plasma inside a potential well that is formed from a set of spherical concentric electrodes (see Figure 1). The center grid is charged to a high potential, relative to the outer grids, on the order of lo's of kilovolts. The center grid is charged negative, such that it takes on the role of a cathode. Any ions present in the region between the middle grid and the center grid will be attracted to the cathode. If the cathode is highly transparent, this spherical beam of ions will pass through the grid several times before being captured by the grid. The source of ions in a three grid system is from a background fill gas which is ionized between the two outermost grids. The middle electrode and the outermost electrode form a potential well which confines electrons. The outermost grid is held at about 600V while the middle grid is at ground (same as the wall). Electrons then oscillate around the outermost grid and ionize the background gas. Ions formed between the outermost grid and the middle grid will then be accelerated through the central grid and focussed at the center. As the ions focus into the center, they interact with one another through space charge effects. A typical potential profile is shown in Figure 2.

IEC has potential for immediate application as a portable neutron source. Applications for neutron sources in this range include safeguards and proliferation, assaying of nuclear and chemical waste, well logging, and detection of high explosives. These applications require a compact, pottabfe, intense and inexpensive neutron source. IEC devices have significant advantages over present neutron sources in terms of both cost and ES&H.

The goal of phase I of the Intense Neutron Source project is to build an IEC similar to Hirsch's that is capable of producing 1 O1 neutrons per second steady-state. The parameters for this device are scaled directly from Hirsch's data base. This device is described in the Phase I Device section. The goai of Phase II is to operate the IEC in a new regime, one that will hopefully greatly increase the gain and possibly ev0n reach breakeven. The physics of this device and neutron yield projections will be discussed in the Phase l l Device section.

Phase 1 Device

As stated above, the goal of Phase I of the INS project is to build a 101 1 neutrondsecond steady-state neutron source based on a three grid IEC design (see Figure 1). Source parameters have been scaled from Hirsch's datal. Pessimistic and optimistic scalings have been assumed in order to bracket the expected performance of the device. Experimental parameters have been chosen so that the desired plasma parameters can be achieved even under the pessimistic assumptions.

Figure 3 shows the projected current vs voltage curves required for a 1x1011 source asuuming that the neutron yield scales like the current (top curve) or as the current squared (bottom curve). We have sized our power supplies at 75 KV and 335 mA which should allow us to produce a 1x101 1 source even if the pessimistic assumptions prevail.

Table I is a summary of the device parameters.

Table I

Cham be r diameter Number of grids Inner grid radius Middle grid radius Outer grid radius Number of e-guns Maximum current per e-gun e-gun grid voltage Maximimum e-gun grid current Maximum grid voltage Maximum current rating Anticipated fill pressure Anticipated base pressure High voltage feedthroug hs

12 inches 3 1.5 inches 3.0 inches 5.5 inches 6 12 Ampere 100 v 15 Amperes 75 Kv 335 mA .1 mtorr 10-8 torr 2

Figure 4 shows the front view of the chamber layout, including all of the ports and their purposes. The chamber is disassembled along its equator. This is how grids are placed and accessed. The top and bottom halves of the chamber are identical. They are rotated 60 degrees with respect to each other so the e-guns s5t on the faces of a cube.

Since the device has up to 25 KW of continuous input power, active cooling is required. The shell is a double jacketed design with the coolant flowing between the chamber wall and the outer jacket. Note that the High voltage feedthrough port is cooled along with the chamber.

The innermost grid is also actively cooled. This is the high voltage grid and shares the dominant heat load with the wall. Figure 5 shows a cutaway view of the innerrnos? grid along with the high voltage feedthroughs. The high voltage feedthroughs and the innermost grid are constructed of hollow tubing so they can carry

coolant (deionized water). Once again, the coolant flows from the bottom to the top of the device. Coolant system and heat transfer parameters are listed in Table It. The power split between the innermost grid and the wall depends on the secondary emission rate of electrons from the innermost grid. This number is somewhere between 1 and 10, depending on the ion impact energy and the grid material (Copper in our case). Consequently, we anticipate that somewhere between 50% and 90% of the energy will be deposited on the chamber wall. Both the chamber and the grid are designed to handle the full 25 Kw of power with substantial safety factors. (Heat transfer is not a major problem).

Table I1

Maximum coolant flow rate Maximum temperature rise Maximum head drop across grid Maximum head drop across chamber Maximum head available from pump Number of grid tubes Inner diameter of grid tubes Outer diameter of grid tubes Maximum Reynolds number in chamber flow Maximum Reynolds number in grid flow Chiller size Maximum grid temperature

4 gallonslminute 1000 F e 1 psi < 1 psi 55 psi 12 .096 inches .125 inches 670 200000 25 KW 3350 c

The outer two grids will be made of tungsten and will radiatively cool. Anticipated loads on these grids is less than 1KW per grid. The middle grid will be covered with a fine tungsten mesh (-1mm grid spacing) to isolate the electrons in the gun from the high voltage region.

It is anticipated that this device will operate in a hot cell at NIS-5. Initial operation will be with deuterium in a continuous feed pumped system. Optimal operational parameters wifl be determined during this phase. Final operation will be with tritium in a hermetically sealed gettered device. Full neutron production is anticipated in this phase.

Phase It

The goal of Phase I I is to increase the neutron yield by a factor of 100 and to provide these neutrons in 200 picosecond bursts at a repetition rate of about 107 Hertz. This source wilf be suitable for time-of-flight imaging schemes and also will test the physics required to scale IECs to net power producing systems.

For Phase II we are proposing to operate the INS device as an electron-based IEC rather than as a spherical ion focus device as was done in phase 1. We plan to

replace the ion beam character of the device with an oscillating thermal (Maxwellian) plasma. The scheme is called the Periodically Oscillating Plasma Sphere (POPS) and is a combination and extension of two previous ideas; the Spherical Reflex IDiode2 and the Oscillating Penning Trap?

For Phase 11, the three grid system will be replaced by a single grid. A conceptual design for this grid is in Figure 6. The grid is magnetically insulated in order to increase its transparency. Table 3 summarizes the grid parameters.

Table 111

Grid radius Maximum grid vottage Maximum current rating (Grid to wall) Current for magnetic insulation Maximum coolant flow rate Maximum temperature rise Maximum head drop across grid Number of grid tubes Inner diameter of grid tubes Outer diameter of grid tubes

4 inches 75 Kv 335 mA 6KA 4 gallondminute 1000 F < I psi 6 .096 inches .125 inches

Figure 7 shows the placement of the grid in the chamber. The grid is placed near the guns to maximize defocussing of the electrons. As in the case of the Spherical Reflex Diode, the goal is to spread the electrons as uniformly as possible over the volume enclosed by the grid. If the charge density is uniform, then the resulting potential well is a harmonic oscillator potential:

(1)

where rgfid is the grid radius and @ is the electrostatic potential. The ion response to the harmonic oscillator potential can be calculated using the fluid equations in 1-D:

(2)

(3)

along with the ideal gas law:

(4) pi/ni5’3 = constant

e = @O (1 - rjrgrid)

ani/& + a(niVr!/ar = 0

mjniaVr/at + miniV$Vr/ar = qiniEr - api/ar

and Poisson's equation:

where ni is the ion density, Vr is the ion radial velocity, mi is the ion mass, pi is the ion pressure, neis the thermal electron density, nb is the background electron density generated by the grid and the e-guns, and Er is the radial electric field where:

(6) E r = w r .

A set of self-similar soultions exist for these equations of the form:

where 6 = r/a(t) and a(t) is the moving radius of a sharp boundary plasma sphere. Substituting eqs (7)-(10) into eqs (1)-(6) yields:

(1 1) n(c) = 1.0 for 5 c 1.0 and n(Q = 0.0 for 4 > 1.0,

where no, po and a0 are the initial density, pressure and plasma radius respectively. These solutions reduce the problem to the following ordinary differential equation for the plasma radius:

Note that if the right hand side of equation (18) is ignored, the equation reduces to a simple harmonic oscillator equation. For now, we will assume that ene = qjnj and To = .025 ev.

Assuming that the acceleration voltage is large (-75KV) compared to To, equation (18) reduces to a harmonic oscillator equation except in the region of a(t)/aocc 1 :

The pressure potential is very steep in the region of a(t) + 0 and its dominant effect is to cut the oscillation frequency in half. (This has been verified numerically.)

Note that nothing has been assumed about the time dependence of nb. If one makes nb sinusoidal then:

(20) nb = nbo + nblcos(wt)

and equation (19) becomes a Mathieu equationl3:

Phase locking is achieved by driving the system near one of the resonances. The lowest (and most efficient) resonance is:

It is well known that a harmonic oscillator driven at resonance (with a small amount of dissipation, here provided by collisions with either the electrons or the background gas) will phase lock to the driver. The oscillation in nb can be provided in one of two ways: either by oscillating the high voltage grid potential or by oscillating the extractor grid potentials (see Figure 7). It can also be shown that these oscillations are stable to multi-dimensional perturbations.1 4

Numerical solutions of equation (18) are shown in Figures 8 - 12. I'igure 8 shows a typical time dependence of the plasma radius. Note that the radius collapses to almost zero. An expansion of one of the collapses is shown in Figure 9. The minimal plasma radius is about 60 microns. The plasma density and temperature increase as the radius shrinks. This is shown in Figures 10 and 11 respectively. Maximum density is about lO22/cm3 and the maximum remperature is 75 KeV (same as the applied potential). Figure 12 shows the time averaged neutron production rate. These curves assume D-D fusion. The D-7 rate would be about 200 times larger. These curves asymptote to a production rate of about 6x1016 neutrons/second. Each spike up in the production rate is caused by a burst of neutrons produced by the plasma compression. These neutron bursts are about 200 picoseconds in width and occur at a rep rate of approximately 107 Hertz.

Figure 13 show the expected D-0 neutron yields vs. the normalized current. The normalized current is the total current which passes through the grid, including

that which intercepts the grid (labeled as prid). The vertical dashed iines indicate where the INS device should operate, given different assumptions for the grid transparency. These curves are drawn for both steady-state and pulsed, assuming for the pulsed case that there is 1 KJ of capacitor energy storage. Note that there is almost no yield unless the transparency is better than 99.9%. Consequently, a magnetically insulated grid is imperative to see an effect (see Figure 6).

The small numbers beside the curves are niohb. If this number is much greater than 1, that is an indication that the effective Debye length has become shorter than the device size and the validity of these curves is questionable. It is therefore quite possible that we will not be able to exceed 101 1 neutrondsecond in D-D or 101 3 neutronskecond in D-T with the Phase I I device. However, if we are able ts access the high density regions of parameter space, then it is easy to show that:

so

(24) Pfusion - igrid2

and

The uppermost point on Figure 13 is the breakeven point if the plasma were operating with D-T instead of D-D. Thus, breakeven is possible, but it isn't very likely.

Even if the high density plasmas are not obtainable, the POPS operating scheme stili has attractive reactor possibilities. Either the voltage can be increased, or the size can be shrunk down to a Penning Trap. If one decreases the size at constant voltage, then:

and

Penning Traps are gridless systems, so one will probably also gain an another order of magnitude in Qfusion due to the effective increase in the grid transparency.

Summary

The Intense Neutron Source (INS) is an Inertial Electrostatic Confinement (IEC) fusion device presently under construction at Los Alamos National Laboratory. It is designed to produce 1011 neutrons per second steady-state using D-T fuel. Phase I operation of this device will be as a standard three grid IEC ion focus device. The design parameters for the phase I device ndicate that expected yield should be

realized even under conservative assumptions. Expected performance has been predicted by scaling from a previous IEC device.1 Even if the presently utilized power supplies fall short of the desired yield, the thermal hydraulics and heat transfer are overdesigned by almost a factor of 10 so all that will be necessary to reach the desired goal would be to use a larger power supply and chiller.

Phase II operation of this device will utilize a new operating scheme, the Periodically Oscillating Plasma Sphere (POPS). With this type of operation we hope to improve plasma neutron production to about 1013 neutronskecond. It is estimated that this scheme also will give 200 picosecond bursts of neutrons at a repetition rate of about 107 Hertz. This source should have applications for imaging using time-of-flight techniques.

The Phase It device will also test the physics required to obtain high gain using the POPS operating scheme. How well this works should give an indication as to the possibilities of using this scheme for net power production.

References 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

R. L. Hirsch, J. Appl. Physics a 4522 (1967).

B. Edwards, Phd Thesis, University of Illinois (1980).

D. C. Barnes, Leaf Turner, Phys Fluids B 4,3890 (1992).

W. W. Salisbury, “Method and Apparatus for Producing Neutrons,” U. S. Patent No. 2,489,436, issued Nov. 29, 1949, filed Dec. 17, 1947.

W. Elmore, J. Tuck, K. Watson, Phys. Fluids 2 239 (1959).

P.T. Famsworth, “Electric Discharge Devide for Producing Interactions Between Nuclei,” U.S. Patent No. 3,358,402, issued June 28,1966, initially filed May 5, 1956, rev. Oct. 18, 1960, filed Jan. 11, 1962.

H. P. Furth, Phys. Fluids& 48 (1963).

T. J. Dolan, J. T. Verdeyen, D. J. Meeker, B. E. Cherrington, J. Appl. Physics && 1590 (1 972).

D. A. Swanson, B. E. Chemngton, J. T. Verdeyen, Appl. Phys. Lett. a 125 (1973).

D. A. Swanson, B. E. Chenington, J. T. Verdeyen, Phys. Fluids Ifi 1939 (1 973).

W. M. Black, J. W. Robinson, J. MI. Phys. a 2497 (1974).

D. C. Baxter, G. W. Stuart, J. MI. Phys. 4601 (1982).

see, for instance, M. Abramowitz, 1. Stegun, Handbook of Mathematical Functions, 721 (1964).

R. A Nebel, 0. C. Barnes, to be submitted to Physics of Plasmas.

Neutron Source Prototype 101lneutrons/second steady-state

(Phase I )

K2 (Emitter '2, 1 inches

XG2

G 1: 600 V Cathode) I

G 3 (Central Cathode) -75 KV

Fgure 1 : 3 Grid IEC Devim

0

=l 0

-20

> Y E

a C cp 0

- I

= -30

n CI

-40

6 0

Potential Profile

FiguG 2: Typical Vacuum Potential Profile -

1 10 A

100 mA

Current vs. Voltage (Scaled from Hirsch's data)

10 mA 30 40 50 60 70 80 90 100 110

Voltage in KV '. Figure 3: Empirical Data Scaling for Current Required for 1 el 1 Source

7 View port r H i g h Voltage port

iew port V iew port

instrument port Instrument port

e-gun port High Voltage por t

Figure 4. Front View

Figure 5: Cutaway view showing grids

Coolant and Current

Figure 6: Magnetically insulated grid

She1 1 (Grounded)

5.5 inches

Grid

Figure 7: POPS grid placement

0 d A

I

00

E .\o 0 c CCI .I

0

A

0.0 0.5 1 .o Time in Microseconds

1.5 2.0

fgure 8: Plasma radius vs. time

* 0

m 0

0 0

-6 -4 -2 0 2 4 6 -1 1

Time in Seconds ~ 1 0 + 8.78335e-07

figure 9: Plasma radius vs. time

e4 e4 0 . x

t c

e-

0.0 0.5 1 .o 1.5 Time in Microseconds

figure 10: Plasma density vs. time

2.0

I

I

L

P

9 S P € Z I 0

0 Pi

vs 1_

0 1

vs 0

0 0

.- i!! c

c m a

i! i=

.. c t-

E 3 CR G

e 0 x H

0.0 0.5 1 .o Time in Microseconds

1.5 2.0

figure 12: Average neutron yield

I d 10

10 lS

1014 d Y

10 lo

109

lo8

io7

rteady-state rteady-state puked pulmd 500

4 - 4 I I I I I I I I 1 I I I I I I I I I I I

4 I I I I I I I I I I

I I I I I I I I I I I I I I

10 100 1000 loo00 1OOOOO 1000000 1oooOOOO

Normalized Current I = Plgrid((1-T)

figure 13: Neutron yield vs. normalized current