PENNING FUSION EXPERIMENT – IONS(PFX-I)

Karl R. Umstadter*, Martin M. Schauer, Daniel C. Barnes

Los Alamos National LaboratoryLos Alamos, NM 87545

ICC Workshop

February 2000

Thanks to L. Chacón de la Rosa, M. H. Holzscheiter, F. L. Ribe,

L. S. Schrank, R. A. Nebel, J. M. Finn,

Outline

• Physics of Penning Fusion

• Experimental Approach

• Diagnostics

• Latest Results

• Summary

PF -- Motivation & Definition

• Pure electron plasmas may be well confined in cmsized systems ( = hours months)

• Density limited by Brillouin (B) and electrostaticstress (E)

• Use electron space charge to confine thermonuclearions

• Spherical ion focus to produce high power density

• Need 100 kV, cm radius, modest B

Small is Beautiful!

aV 2o /~n ><

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rc

ann

í

22 ~

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rc

a

a

f(V)~P

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Powergoesinverselywith size!

Ion Physics Theory Progress

•Multiple Wells -- Miley ...

•Maxwellian Component in Well --Chacón …

•POPS -- Barnes, ...

Two Convergence Modes

Ion Physics - POPS

•Uniform ne forms harmonic ion well

•Gaussian ion cloud stably oscillateswithout damping from ion-ioncollisions

•Periodic focus gives high powerdensity

•Deep well allows D-D operation

Electron Requirements

• Electron confinement must be excellent

– Thermonuclear electron energy ve ~ c

– Electron density ~ 1012 cm-3 for reactivity

• Goal:

– Produce uniform ne

– Electrostatic boundary conditions so that sc is spherical

• Conceptual Approach:

– Axial electrostatic electron well (end cathodes, central anode)

– Magnetic insulation of anode

– Shaping of B field near anode to give uniform ne

Electron Requirements (Cont.)

• Uniform density requires nonthermal electron distribution

– Consider density along axis of symmetry

– No magnetic contribution, only electrostatic

We

) W/exp(-e

Tknn

Tknn

eBao

eBao

>>?∪

=

– Electrostatic electron confinement requires appliedvoltage Vo >> kB Te /e

– Very inefficient use of Vo

100(s) kV in cm System

• Form electron beam in gun

• Transport beam along B to electrically chargeconfinement region

• Confinement region consists of high-stressdielectrics or (preferably) only conductors

• Electrons reflected from second cathode (usuallypassive cathode)

• Reflexing beam builds large space charge from lowto modest beam current

Experimental Approach

• Determine electron distribution in simple anode geometry

• Design field shaping with measured distribution

• Measure ne by space charge E diagnostic

No electrostatic shaping Electrostatic shaping onlyElectrostatic and magnetic shaping

Experimental Setup

SHOW SLIDES OF DIAGRAM (CAD) ANDPICTURES OF TRAP

Diagnostic Equipment

• Monochrometer & Photomultiplier Tube

• Optics in Trap

• Fiber Optic Delivery of Light

Light is split 50/50

50% : to spectrometer for Stark measurement ⇒ ne

50% : to PMT for photon counting ⇒ ni

• Silicon Barrier Detectors ⇒ ions/atoms lost from trap

• µChannel Plate ⇒ Destructive detection of Ne

PFX-I Low-Voltage Electron Operation

Anode

Collector

Lowerendcap

DT

UEC

Vb

Vo

Ia

Rr

Vr

ToScope

Coll

UEC

Anode

LEC

DT

Experimental Procedure

Load the trap(emitter ON)with reflectorat voltage…

Wait the delaytime with thereflector at

voltage

Turn emitterOFF: bias thefilament morepositive w.r.t.the end cap

Open relay,reflector

dischargesthrough R1 &

HV Probe

Electronsflow from the

trap to theMCP front

plate (scope)

Number of electronstrapped, need: #

electrons striking theMCP plate and noisebkgnd (from scope),

transparency ofreflector.

Record&

Restart

Electron Capture Example

Electron Capture - Exp 1

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035

Time (sec)

Vo

ltag

e (s

ign

al)

e- Signal

Vemitter/1x103

Vreflector /1x105

Electron Trapping in PFX-I

0 500 1000 1500 2000

\0.

2.E+08

4.E+08

Inve

nto

ry

Voltage

Electron inventory vs. time shows linear scaling with V

Anode Current vs. time shows linear scaling with V

⇒ τ independent of V

0

1.E-7

2.E-7

3.E-7

0 500 1000 1500 2000 2500

Voltage

An

ode

Cu

rren

t

Electron Trapping in PFX-I

0.0E+00

5.0E+07

1.0E+08

1.5E+08

2.0E+08

2.5E+08

0 200 400 600 800 1000

Time (msec)

Inve

nto

ry

If space-charge limit, then inventory should be independent of B

Early time (30 ms) data shows B dependence

1.14T

0.76T

0.38T

Electron Transport Models for PFX-I

• Decay rate independent of inventory

• Decay rate proportional to inventory

et

oNN(t) o−=

o

o

t1NN(t)

+=

Application to PFX-I

• Measure IA, N30, calculate No, τo

et

oNN(t) o−=

B(T) N30(108) No(108) τo(ms)

0.38 1.14 30.7 9.110.76 4.19 21.2 18.51.14 6.17 23.3 22.6

Phase II Experimental Setup

PFXI Electron Trapping

(P=4x10 -8 T o r r )

y = 1E+08e -1.094x

y = 7E+07e -1.4317x

0.0E+00

2.0E+07

4.0E+07

6.0E+07

8.0E+07

1.0E+08

1.2E+08

0.0 0.5 1.0 1.5 2.0 2.5

Time (msec)

Inve

ntor

y

Phase II Experimental Results

300

7001100

300

500

700900

110013

300

300

700

1100

700

1100

700

1100

0.E+00

1.E+08

2.E+08

3.E+08

4.E+08

5.E+08

6.E+08

7.E+08

8.E+08

9.E+08

1.E+09

0 200 400 600 800 1000 1200 1

Voltage

Inve

ntor

y

No (B=0.5T)

No (B=1T)

No (B=1.5T)

No (B=2.0T)

No (B=2.0T)b

Linear (No(B=2.0T)b)Linear (No(B=1.5T))Linear (No(B=1T))Linear (No(B=0.5T))

Transport Mysteries

• Adding any measurable amount of gas reducesconfinement

• Electron-neutral collision time is a few ms’s

• Thus, electrons must be lost in single collision. Howcan this happen with strong B?

• Some ideas are large field errors + collisions (bananatransport), collisions with trapped ions

Nondestructive Evaluation

• Determine Electron Density in Trap– In-Situ Measurement

– Small Access Size

⇒ Stark Splitting of H lines with Static Fill of Hydrogen

• Hydrogen Balmer Series• Hα : 3p -> 2s n=3

∀ λ = 6563 Å (Eγ = 1.892eV = 1.54104 cm-1)

• (Hβ : λ = 4861 Å)

• Some MS word slides go here

Electron Density Estimate (Photon Counting)

• Estimate electron density in anode volume by counting number ofphotons produced by electron impact ionization of neutral gas.

• Use excitation peak of N2+, 391.4nm

•S: Signal strength•K: Monochromator, PMT, and fiber efficiency•f: Optical fiber solid angle acceptance•V: Viewed volume•u: Electron velocity•n0: Neutral gas density•Q: Energy-dependent cross-section of process

• Uncertainties - reflection of photons on anode walls- electron energy distribution

QKfVunSn

0

=

Pressure (Torr) Signal (s-1) Density (cm-3

)

2.1x10-5

270 1.0x107

Obstacles/Difficulties

• High Voltage– 100 kV Goal

– 75kV/35kV Operation

• Alignment– Colinear

– Magnetic Field

• Size/Access– Magnet Bore

– HV Standoff

– Diagnostics

Massively Modular Approach

InflowOutflow

Coil

Penning Trap Fuel Rods (100s of cm sized sources)

Penning Trap Reactor Vessel

B

Plenum for electsources, gas conand High voltage

Pressure Vessel

Summary

•Ion focussing can be done with high Q (theory)

•PFX-I addresses electron physics required forforming ion well

•Scaling path is to higher voltages, small size

•Massively modular reactor concept avoids materialproblems, removes insulators from high neutron flux,provides heating power from direct conversion

Summary (cont.)

•PFX-I has operated with electrons at low pressureand with gas added•Diagnostics are electron dump and monochromater•Spectroscopic system proven to detect molecular andionic (molecular) lines•Understanding electron confinement and distributionallows designing spherical well (tools are in hand)•Our goal is to trap ions during the next few months