What is the ISOLDE cooler RFQ CB - ISCOOL

H. Frånberg

Where at ISOLDE

Installation

Principle

• Reducing energy spread– Thermalization ions loose energy through

interaction with a gas– Confinement by electric field

220

4

RF

RF

mr

QVq

Optimization through the Mathieu equation:

mass

Inner radius

Angular frequency of RF voltage

Amplitude on RF voltage

Particle charge

Condition for stability:q < 0.908Optimum ISOLDE:q = 0.6

Ion motions

Ions are interacting with a gas.

Through the collisions they loose energy and change direction.

Buffer gas

Ion motions

Adding a oscillating RF field confines the ions in the gas, along the axis of the quadrupole.

VRF ~

Continuous mode

1. Ions are interacting with a gas.2. Adding a oscillating RF field confines the ions in

the gas.3. The electrodes have all a potential to “drag” the

ions through the RFQ.

VRF ~

80eV5-2V/cm

~1E-2 mbar 4He10-2mbar l/s

EXTRACTIONINJECTION

Bunching mode

1. Ions are interacting with a gas.2. Adding a oscillating RF field confines the ions in

the gas.3. Pulsing the last electrodes (HRS.AX(23,24,25)

(A/B))

VRF ~

80eV5-2V/cm

~1E-2 mbar 4He10-2mbar l/s

EXTRACTIONINJECTION

Trapping 50V

Summary

RFV

Three elements:RF quadrupolar fieldRadial confinement

DC potentialsExtracting ion in bunches or in continuos mode

Buffer gas Ion motion cooling

Results• Transmission:

– 50 % A>23 – 80% for A>40.

• Space charge limit of 1E8 ions/bunch.– The maximum limit of ions in the

RFQCB before the ions are lost.

• Cooling time < 1msec– At ISOLDE short lived isotopes with

half-lives of a few ms are explored.

• Bunch width ~ 30 µsec– For the time window of the experiments.

0.00E+000 1.00E+009 2.00E+009 3.00E+009 4.00E+009 5.00E+009

0.00E+000

2.00E+007

4.00E+007

6.00E+007

8.00E+007

1.00E+008

1.20E+008

1.40E+008

Ext

ract

ed

ion

s

Injected ions

-200 0 200 400 600 800 1000 1200 1400 1600 1800-1

0

1

2

3

4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

1.2x10-3

1.4x10-3

1.6x10-3

Bea

m g

ate

Time [s]

MC

P

-40 -20 0 20 40 60 80 100 120 140

-8x10-3

-7x10-3

-6x10-3

-5x10-3

-4x10-3

-3x10-3

-2x10-3

-1x10-3

0

1x10-3

2x10-3

MC

P s

igna

l

Time [s]

10 ms 100 ms 1000 ms

Why do ISOLDE need an ion cooler and buncher?

• Large beam emittance – Low transmission to

experiments– Low efficiency in mass

spectrometers

~35·mm·mrad 95% emittance

Why do ISOLDE need an ion cooler?

• Small beam emittance – High transmission to

experiments. During tests we had 100 % transmission between the RFQCB and LA2 beam line!

– High efficiency in mass spectrometers

• Mistral• ISOLTRAP

– Small beam spot• Decay/collection experiments.

~2·mm·mrad 95% emittance

Why do ISOLDE need an ion buncher?• Finite time definition of

beam pulses – Gating on the beam pulse

background suppression.– Decay experiment

• T1/2

• Charge particle decay spectran

• Background suppression factor~1E4– COLLAPS (less laser power) -7 -6 -5 -4 -3 -2 -1

0

20

40

60

80

100

120

140

160

180

200

Gat

ed c

ount

s

Tuning voltage

-7 -6 -5 -4 -3 -2 -1

490000

492000

494000

496000

498000

500000

502000

Tot

al c

ount

s

Tuning volts

Singles (continuous counting)

Bunched:12µs gated spectra