Observations of electrons in the Intense Pulse Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS)

J. C. Dooling, F. R. Brumwell, W. S. Czyz, K.C. Harkay, M. K. Lien, and

G. E. McMichael

Argonne National Laboratory, Argonne, IL 60439, USA

presented to the

Midwest Accelerator Physics Collaboration MeetingJune 11, 2003

Argonne National Laboratory

Introduction—Electron Sources

• Production of electrons in high intensity machines can have significant impact on beam stability and lifetime.

• Electrons come from several sources:– Stripping of H-

– Photo-ionization

– Background gas ionization (high pressure)

SE, Multipacting, may lead to E-P and other instabilities; also, plasma formation

Ionization and Neutralization

• Ave. pressure in IPNS RCS: 1-2 T N2

• RCS n~0.5 ms at injection; acceleration period: 14 ms

• At PSR, n~20 ms; store period: <1 ms

• At 4E12 injected, ng is 2 orders greater than nb

IPNS RCS Diagnostics

• Profile and Position System (PAPS) IPM

• Retarding Field Analyzer (RFA)

• Pie Electrodes

• Resistive Wall Monitor (RWM)

Beam electric field and potential

• Assuming a uniform beam, radius rb=1.5 cm and wall radius, rw=3.8 cm

• 3x1012 protons

• Bunching factor and frequency folded into peak current

0

0.5

1.0

1.5

V (

kV)

Er

(kV

/cm

)

0 0.5 1.0 1.5 2.0 2.5 3.0r (cm)

t=14 ms

0 ms

10 ms

5 ms

0 ms

5 ms

10 ms

14 ms

0

0.2

0.4

0.6

0.8

3.5 4.0

rb rw

rb rw

0 0.5 1.0 1.5 2.0 2.5 3.0r (cm)

3.5 4.0

Profile and Position System IPM

Vb = -400 V

fVb = -30 V

0 V

CL

10.80 cm

2.86 cm

5.72 cm

0.64 cm

H.V. plate

suppressor grid

0.38 cm x 10.16 cmon 0.64 cm pitch

beam

collector grid

PAPS IPM Data

• IPM profiles respond to pressure and bias

• Gaussian fit to data tracks horizontal position (Pie) electrode centroid

• Ionization bursts observed at injection and after phase modulation (PM)

• BW~5 kHz

a) pressure

b) bias

-5 0 5x (cm)

-0.2

0

0.2

0.4

0.6

0.8

1.0

0

0.5

1.0

1.5

-5 0 5x (cm)

S (

V)

S (

V)

-400 V

0 V

1.2 Torr

0.4 Torr

IPM responds to ionization bursts

• RWM Q and PM

0 2 4 6 8 10 12 14time (ms)

Q (

C)

0

.2

.4

.6

no PM

with PM

PMsignal(rad)

t

.1

Post-scrambler burst, 10 ms

-5 0 5

x (cm)

0

0.5

1.0

1.5

2.0

2.5

S (

V)

-400 V

0 V

-5 0 5

x (cm)

0

0.5

1.0

1.5

2.0

S (

V)

Injection burst, 0 ms

Retarding Field Analyzer—installed in the L5 straight section

Trans-impedance amplifier, 300 k

76 dB into 50

RFA data• Injection • Scrambler

injection begins at 344 s and ends near 420 s

b) t=528 s

RWM

sign

al (V

)

time ( s)532 533-.4

-.2

.2

.4

0

RFA

RWM

RFA

time ( s)

sign

al (V

)

452 453

a) t=448 s

-.4

-.2

.2

.4

0

-.2

0

.2

.4

12.0 12.2 12.4 12.6time (ms)

S (

V)

1 s

1 s

.3

0

.3

0

S (

V)

S (

V)

RFA issues

• Electron signals intermittent

• Power supply noise (switching—filter helps)

• Beam noise (di/dt)

• Magnetic shielding (not done with present amplifier)

• Radiation effects from probe at beam elevation

Pie electrodes

• Split can (short strip line yielding di/dt)

• Provides horizontal position information for tuning by operators

• Beam phase for fast phase feedback control of the rf amplifiers

• Vertical position

• Raw, single channel signal provides broadband beam information (motion)

Pie data—H position near PM. Noise increases significantly

• 9 ms • 10 ms

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0

t ( s)

-100

0

100

200

400

I (a

.u.)

300

.2

-.1

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0

t ( s)

H

.1

0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0

t ( s)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0

t ( s)

-100

0

100

-.2

300

400

-.1

0

.1

.2

.3

200

-.3

I (a

.u.)

H

Pie Electrode spectra from time data• Without scrambler • With scrambler

35 45 55 65f (MHz)

S (

dBm

)

-20

40

0

a) no scrambler, t = 10.88 ms

20

35 45 55 65S

(dB

m)

f (MHz)

-20

40

0

b) with scrambler, t = 10.88 ms

20

Lower hybrid waves? E perpendicular to B

Pie Spectra, con’t

• Spectra without scrambler, full cycle (80 s frames)

Pie Spectra, con’t

• With scrambler, full cycle (80 s frames)

Lower Hybrid Drift (LHD) Frequencies

• LHD frequencies for N+ and OH+

0 2 4 6 8 10 12 14time (ms)

N+

OH+

40

60

80

100

120

140

160

180

f L (

MH

z)

i

i e

L ce ci

1/ 2Z (t)m m

(t) (t) (t)

eB(t)

where,

dc ac mB(t) B B cos t

m

dc

ac

2

B 0.614 T

B 0.331 T

and

30 Hz

Higher Frequency Spectra500 MS/s, 20 s window, 10 kS

• No scrambler • With scrambler

180 190 200 210 22040

20

0

20

40

ttest

13.9 ms no scrambler

S (d

Bm

)

f (MHz) f (MHz)

180 190 200 210 22040

20

0

20

40

ttest

13.9 ms with scrambler

S (d

Bm

)

Conclusions• Electrons present, so are ions (plasma)• PM modifies plasma channel, adds stability• Large electron signals usually not observed

– Rarely at injection, more often after scrambler– Add magnetic shielding to RFA

Include background ions in simulations, especially when intense electron signals suggest significant neutralization.

Diagnostics to look directly at or near the beam regione.g., interferometry, spectroscopy, Langmuir probes, Rogowski coils

Acknowledgement

• This work would not be possible without the dedication and hard work of the IPNS Accelerator Operations Group, the IPNS and MSD Divisions, and DOE support.