breakdown and ignition limits in lab6 hollow cathode ... · breakdown and ignition limits in lab 6...

1

Breakdown and Ignition Limits in LaB6 Hollow Cathode Discharges

Ricky Tang, Sonca Nguyen, Timothy B. Smith, and Alec D. GallimoreUniversity of Michigan, Ann Arbor, MI 48109

International Conference on Plasma ScienceTraverse City, MIJune 4-8, 2006

2

Contents

MotivationCathode AssemblyExperimental SetupResults

Breakdown Voltage CharacteristicsCurrent ProfileFrequency Spectrum

Conclusion & Future WorkReferences

3

Motivation

Two major failure modes for conventional hollow cathode – emitter poisoning and heater failureLaB6 is two orders of magnitude less sensitive to oxygen poisoning1

Erosion rate for LaB6 is lower for emission currents below 13 A/cm2

(Ref. 2)LaB6 operation temperature is higher to attain similar emission current densities1 due to higher low-temperature work function

Heater elimination increases robustness and simplicity

As a first attempt toward testing the feasibility of heaterless LaB6operation, we performed a series of experiments with a LaB6 hollow cathode to assess its breakdown and ignition characteristics

4

MAI Cathode Assembly

Orifice diameter: 4.25mm

Keeper-orifice distance: 9.0mm

5

Experimental Setup

1. All testing done in a 2-meter-long by 0.6-meter-diameter cathode test facility (CTF) at PEPL

2. Cathode conditioned by initiating a Xe flow of 10sccm and heating the LaB6pellet first at 6A for 5 mins followed by 12A for 10 mins

3. Keeper operates at a current limit of 1A

6

Breakdown Voltage

0

50

100

150

200

250

300

350

400

450

40 60 80 100 120 140

Heater Power (W)

BD

Vol

tage

(V)

10 sccm8 sccm6 sccm

7

Breakdown Voltage

0

50

100

150

200

250

300

350

400

450

40 60 80 100 120 140

Heater Power (W)

BD

Vol

tage

(V)

10 sccm8 sccm6 sccmPower (10 sccm)Power (8 sccm)Power (6 sccm)

8

Current Profile

9

Current Profile (cont.)

10

Current Profile (cont.)

11

Frequency Spectrum

12

Frequency Spectrum (cont.)

13

Frequency Spectrum (cont.)

14

Conclusion & Future Work

Breakdown voltage as a function of heater power follows the power law and increases with higher mass flow and lower heater powerInitial current growth rate does not vary with heater power and mass flowCurrent decay (to steady state level) shows interesting oscillation at ~104 Hz for higher heater currentsFrequency spectrum shows no noticeable peaks otherwiseIncorporate an anode to shift the discharge current from the keeper and leave the keeper floating to study steady-state characteristicsA full investigation of the LaB6 pre-ignition temperature via disappearing-wire pyrometryStudy the feasibility of heaterless breakdown

15

References

1) D.M. Goebel and R.M. Watkins, “LaB6 Hollow Cathodes for Ion and Hall Thrusters,” AIAA-2005-4239, 41st Joint Propulsion Conference, Tucson, AZ, Jul 2005.

2) K.N. Leung, P.A. Pincosy, and K.W. Ehlers, “Directly Heated Lanthanum Hexaboride Filaments,” Rev. Sci. Instrum., Vol. 55, pp. 1067-1068 (1984).

16

Breakdown Voltage

0

50

100

150

200

250

300

350

400

450

500

40 50 60 70 80 90 100 110 120 130 140

Heater Power (W)

BD

Vol

tage

(V)

10 sccm

17

Breakdown Voltage

0

50

100

150

200

250

300

350

400

450

500

40 50 60 70 80 90 100 110 120 130 140

Heater Power (W)

BD

Vol

tage

(V)

8 sccm

18

Breakdown Voltage

0

50

100

150

200

250

300

350

400

450

500

40 50 60 70 80 90 100 110 120 130 140

Heater Power (W)

BD

Vol

tage

(V)

6 sccm

19

Current Profile

20

Current Profile (cont.)

21

Current Profile (cont.)