Superconducting Ion Source Development in Berkeley

Superconducting Ion Source Development in Berkeley

Daniela Leitner, S. Caspi, P. Ferracin, C.M. Lyneis, S. Prestemon, G.L. Sabbi, D.S. Todd, F. Trillaud

HIAT 2009, Venice, Italy

• Motivation for developing superconducting ECR ion sources

• Key parameters for the performance of an ECR

• VENUS Source Project• Some results and status of the

VENUS ECR ion source • Future ECR ion source – Path to

56 GHz ECRIS

ECR ion sources have made remarkable improvements over the last few decades, but the demand for

increased intensities of highly charged heavy ions continues to grow

SC-ECRIS, RIKEN, Japan

P ost A cce le ra to r

Iso tope S eparato r

F ragm enta tion P roduction Targe t

F ragm enta tion S eparato r

D rive r L inac (400 M eV /nuc U , 900 M eV p)

R F Q ’s

E xperim en ta l A reas

“G as C a tcher”

N uclea r S tructu re

In F ligh t S epara tion

Iso topeR ecovery

E < 15 M eV /u E > 50 M eV /u

A pp lied P hys ics

A s tro P hys ics

E < 1 M eV /u

N o A cce le ra tion

VENUS, FRIBMSU, USA

SPIRAL 2, GANIL, France

525 eµA U35+ 1mA Ar12+270 eµA U33+ and 270 eµA U34+

Supermafios (Geller, 1974)15 eµA of O6+

VENUS (2007)2850 eµA of O6+

Factor 200

increase

The demonstrated VENUS source performance shows that these ion beam intensity requirements are possible

VENUS28GHz or 28+18 GHz

O6+ 2860 eμA

O7+ 850 eμA

O8+ 400 eμA

Ar12+ 860 eμA

Ar17+ 36 eμA

Xe35+ 28 eμA

Xe42+ .5 eμA

U34+ 200 eμA

U47+ 5 eμA

0

50

100

150

200

250

5 6 7 8 9 10 11

Ana

lyze

d C

urre

nt [

eµA

]

Mass to Charge Ratio

31

32

3334

35

36

37

38

39

29

28

2726

2524

23

O2+

O3+

4kW 28 GHz770 W 18 GHz

High Intensity Uranium Production

6kW 28 GHz770 W 18 GHz

Required temperature of 2100 C in a 4 T B field

ECR (1983)0.4 T, 0.6 kW, 6.4 GHz

AECR-U (1996)1.7 T, 2.6 kW, 10 + 14 GHz

VENUS (2001)4.0 T, 14 kW, 18 + 28 GHz

Higher magnetic fields and higher frequencies are the key to higher performance

Minimum-B field ConfinementSolenoid Coils

Sextupole

e- heatingµ wave

gas

ions

qopt log ω3

ne ωrf2

e = = rf e•Bm

Resonant electron heating

Higher magnetic fields and higher frequencies are the key to higher performance

Normal conducting

Super conducting

10-2

10-1

100

101

102

103

104

10 12 14 16 1810-2

10-1

100

101

102

103

104

10 12 14 16 18

ECR 6 GHz

AECR 14 GHz

VENUS 28 GHz

VENUS 56 GHz predicted

Ana

lyze

d be

am c

urre

nt [

eµA

]

Argon Charge States

ECR (1983)0.4 T, 0.6 kW, 6.4 GHz

AECR-U (1996)1.7 T, 2.6 kW, 10 + 14 GHz

VENUS (2001)4.0 T, 14 kW, 18 + 28 GHz

ne $

Challenges

• Superconducting Magnet

• Cryogenic Technology

• X-rays from the Plasma

• Ion Beam Transport

Binj ~ 4 ∙ Becr

Bmin ~ 0.8 Becr

Bext ~ Brad

Brad ≥ 2 Becr

Becr

4T

.5-.8 T

2T

2T

1T

28 GHz

8T

1-1.6 T

4T

4T

2T

56 GHz Magnetic Design 28 GHz 56 GHz

Max solenoid field

on the coil 6 T 12 T

on axis 4 T 8 T

Max sextupole field


on plasma wall 2.1 T 4.2 T

Superconductor NbTi Nb3Sn

Superconducting Magnets: ECR Design ‘Standard Model’

0

0.5

1

1.5

2

2.5

3

3.5

4

-60 -40 -20 0 20 40 60

B[T

esl

a],

sole

noid

s on

ly

Z (cm)

BECR

Bmin

Bext

Binj

Plasma

0

0.5

1

1.5

2

0 1 2 3 4 5 6 7

B[T

esla

], se

xtup

ole

only

R (cm)

Superconducting Magnet Structure 56 GHz: two options

Solenoid-in-SextupoleGeometry (SECRAL)

Sextupole-in-SolenoidGeometry (VENUS)

• Minimizes the influence of the solenoid on the sextupole field

• Significantly higher field required for the sextupole magnet surface due to the larger radius of the coils

• Strong forces on the solenoid coils

• Minimizes the peak fields in the sextupole coils

• Strong influence (forces) of the solenoid field on the sextupole ends

Superconducting Magnet 56 GHz: Magnetic Analyses

Critical line and magnet load lines: NbSn3

0

1000

2000

3000

4000

5000

8 10 12 14 16 18 20 22

J SC [A

/mm

2 ]

B [T]

4.2 K

5.7 K

6.7 K

Cur

rent

Den

sity

thr

ough

the

su

perc

ondu

ctor

Magnetic Field on the conductor

NbSn3

Superconducting Magnet Structure: Magnetic AnalysesC

urre

nt D

ensi

ty t

hrou

gh t

he

supe

rcon

duct

or


•Magnetic field and current density requirements exceed the capability of NbSn3

Goal: Achieve 4.2T on the plasma chamber wall radially and 8 T and 4 T on axis

0

1000

2000

3000

4000

5000

8 10 12 14 16 18 20 22

J SC [A

/mm

2 ]

B [T]

4.2 K

5.7 K

6.7 K

Injection Solenoid

Sextupole

Extraction Solenoid

This geometry can be ruled out as candidate for a 56 GHz ECR ion source

Solenoid-in-Sextupole

Superconducting Magnet Structure: Magnetic AnalysesC

urre

nt D

ensi

ty t

hrou

gh t

he

supe

rcon

duct

or


• 2.5 Kelvin temperature margin

for the Sextupole• Operates at 86% of current

limits

Goal: Achieve 4.2T on the plasma chamber wall radially and 8 T and 4 T on axis

This geometry is challenging but feasible with current NbSn3 technology

Sextupole-in-Solenoid

0

1000

2000

3000

4000

5000

8 10 12 14 16 18 20 22

J SC [A

/mm

2 ]

B [T]

4.2 K

5.7 K

6.7 K

Injection Solenoid

Sextupole

Sextupole-in-Solenoid: Clamping Structure

There are two limits to the maximum achievable field

with this design

To control these forces• In the end region each layer is subdivided in two blocks of conductors

separated by end-spacers. • The number of turns per block and the relative axial position of the end

spacers were optimized to reduce the peak field in the end region. • The coils are lengthen to reduce the peak field• Shell type support structure

Maximum peak field on the coil (15.1 T, 862 A/mm2 )

Maximum force on the end point (up to 175 MPa)

Sextupole-in-Solenoid: Sextupole Magnet

• 4-layer coils using cables (675 conductors/coil)• The same cable design is currently used by the

LARP program to develop high field

quadrupoles for future LHC luminosity

upgrades (peak fields 15 T)

Cable properties

Strand Dia 0.8 mm

Fill factor ~ 33%

No strands 35

Cable ~ 15.2x1.5 mm • The cable design requires high 8.2kA current

leads, the 56 GHz cryostat will most likely

require He filling during operation.

A practice coil winding for the LARP quadrupole (HQ)

Sextupole-in-Solenoid: Clamping Structure

• A shell-based structure using bladders and keys provides a mechanism for controlled room temperature pre-stress.

• Pre-stress is then amplified by the contraction of an aluminum shell during cool-down.

• The method was developed at Superconducting magnet group at LBNL and successfully applied to high field magnets.• 2D cross section structure

analyses has been conducted on the two critical regions

• Stress values are close to the maximum acceptable values

• Needs full 3D analyses

Quench protection

• Energy stored in the VENUS magnet is 800kJ

• VENUS coils do not require active quench protection

• Leads need protection for adequate cooling

• Energy stored in the 56 GHz Magnet 5.5MJ

• Active Quench protection with heaters at the coils (75% coverage, results in peak temperatures in the coil of 280K)

• Lead protection (Lesson from the VENUS quench failure)

LN

LHe

HTC leads

Passive Quench Protection

Sol 1 Sol 3 Sol 2

Sol 1 Sol 3 Sol 2

Sextupole Coil

Burned Lead

Spliced sextupole lead wire

Other Challenges

• Superconducting Magnet

• Cryogenic Technology

• X-rays from the Plasma

• Ion Beam Transport

Cold Mass

with Coils

Enclosed

Cold Mass

with Coils

EnclosedPlasma

HV Insulator

2mm TantalumX-ray Shield

Technical Solution VENUS Aluminum Plasma Chamber with 2mm Ta x-ray shield

Water Cooling Groovesat the plasma Flutes

A major challenge for high field SC ECR ion sources is the heat load from bremsstrahlung absorbed in the cryostat

A major challenge for high field SC ECR ion sources is the heat load from bremsstrahlung absorbed in the cryostat

1.5 - 2 mm Ta shielding effectively attenuates the low energy bremsstrahlung, but becomes transparent for x-rays above 400keV

100

101

102

103

104

105

1

10

100

1000

104

105

-200 0 200 400 600 800 1000 1200 1400

1.5mm Ta xray xray

Co

un

ts

Energy [keV]

Integral 3.5 106

Integral 3.5 105

HV Insulator

2mm TantalumX-ray Shield

Water Cooling Groovesat the plasma Flutes

with shield

without shield

The high energy tail of the x-ray spectrum increases substantially at the higher microwave frequency(10s of ) watts of cooling power must be reserved for the cryostat.

0

0.05

0.1

0.15

0.2

0.25

21 22 23 24 25 26 27 28 29 30 31

28 GHz18 GHz

Nor

m. 1

rm

s-e

mitt

ance

[m

mm

rad]

Xenon Charge States

Beam transport is a challenge for high field SC ECR ion sources

Beam emittance grows with magnetic field at extraction (therefore with heating frequency)

Beam transport is a challenge for high field SC ECR ion sources

Todd et al., Rev. Sci. Inst. 79 02A316

Todd et al., Rev. Sci. Inst. 79 02A316

Experiment

Simulation

Experiment

10 cm

Simulation of oxygen beam extraction and transport

O7+O7+

Summary

• The requirements of the next generation heavy ion accelerator continue to drive ECR ion source development

• Higher magnetic fields and higher frequencies are the key to higher performance

• 200eµA of U33+ and U34+ have been produced, high temperature oven development is key for long term production

• 56 GHz ECR ion source magnet structures are feasible with current NbSn3 technology

• Development should start now to be ready for operation in 5-10 years

• Understanding of the plasma physics and the beam transport is important for the design of the next generation superconducting ECR ion sources

e = = rf e•Bm

Key parameters for an ECR ion source performance

Plasma is resonantly heated with microwaves

e

Magnetic flux line

Bq

vmr

rmBvq

2

f=28 GHz, B= 1T

rLamor=0.01…1 mm

Solenoids and Sextupole forma minimum-B field confinement structure

Plasma

e- heatingµ-wave

IONS

gas

Key parameters

Ion confinement times i ~ms

Plasma densities ne 109 - 1012 /cm3

Electron temperature Te eV to MeV

Charge exchange/neutral gas density ex

21276.217.1 10 cmIq p


28 GHz VENUS Tune

0

0.5

1

1.5

2

2.5

3

3.5

4

-60 -40 -20 0 20 40 60

B[T

esl

a],

sole

noid

s on

ly

Z (cm)

BECR

Bmin

Bext

Binj

Plasma

0

0.5

1

1.5

2

0 1 2 3 4 5 6 7

B[T

esla

], se

xtup

ole

only

R (cm)

Binj ~ 4 ∙ Becr

Bmin ~ 0.8 Becr

Bext ~ Brad

Brad ≥ 2 Becr

Binj ~ 4 ∙ Becr

Bmin ~ 0.8 Becr

Bext ~ Brad

Brad ≥ 2 Becr

4T

.5-.8 T

2T

2T

28 GHz

8T

1-1.6 T

4T

4T

56 GHz

BECR28 GHz BECR= 1 Tesla

56 GHz BECR= 2 Tesla

Magnetic Design 28 GHz 56 GHz

Max solenoid field


on axis 4 T 8 T

Max sextupole field


on plasma wall 2.1 T 4.2 T

Superconductor NbTi Nb3Sn


The high energy tail of the x-ray spectrum increases substantially at the higher microwave frequency

100

101

102

103

104

105

1

10

100

1000

104

105

-200 0 200 400 600 800 1000 1200 1400

1.5mm Ta xray xray

Co

un

ts

Energy [keV]

Integral 3.5 106

Integral 3.5 105

with shield

without shield

The scaling of the electron energy temperature with frequency has important consequences for 4th generation superconducting ECR ion source with frequencies of 37GHz, 56GHz. Several (10s of ) watts of cooling power must be reserved for the cryostat.

101

102

103

104

105

106

0 200 400 600 800 1000

Cou

nts

Energy [keV]

28 GHz

18 GHz

Superconducting Ion Source Development in Berkeley

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