NICA RF systems
Bunch formation in the injection complex
The NICA injection complex consists of the linear injector providing a 5 MeV/u ion beam at 8 mass to charge ratio, the booster synchrotron at maximum magnetic rigidity of 8 (25) Tm, stripping foil increasing the charge to mass ratio, the Nuclotron at 45 Tm maximum rigidity accelerating the bunch to the experiment energy.
The injection complex provides the ion bunch of required length and ion number during about each two seconds. This is realized in the following steps:
-a few consequent single turn injections (up to five) of the ions at 5 MeV/u are providing into booster at the table of magnetic field and switching on RF at small amplitude and fifth harmonics of the revolution frequency;
-adiabatic bunching and acceleration of the injected bunches up to the energy of about 70MeV/u;
-adiabatic debunching and rebunching at first harmonics of the revolution frequency with the help of electron cooling system, formation of the bunch longitudinal emittance and aspect ratio required for acceleration in the Nuclotron and further RF manipulations;
-stripping the ions in the transfer line from the booster to the Nuclotron;
-single turn injection of the bunch the into the Nuclotron and acceleration at the first harmonics of the revolution frequency up to the experiment energy;
-bunch rotation in the Nuclotron;
-transfer the bunch from the Nuclotron to one of the collider separatrix, if necessary – the final ion stripping in the transfer line from the Nuclotron to the collider.
The minimum RF voltage in the booster and Nuclotron is determined by required acceleration rate. The RF voltage has to be chosen to provide during the acceleration the bunch length acceptable from the tune shift requirements.
The bunch rotation in the Nuclotron is realized by synchronous phase manipulation without a voltage jump.
The harmonics number in the collider is determined by required RF bucket length and is chosen to be 105. The RF voltage in the collider at injection is determined by the matching conditions.
Requirements for RF frequency and voltage
The range of the frequency variation is given by the revolution frequency and harmonics number:
,
where C is the synchrotron circumference,h is the harmonics number, c is the ion velocity.
Minimum RF voltage in synchrotrons is determined by required acceleration rate. If the rate of magnetic field increase is defined, the RF voltage has to be larger than
,
where A is the ion atomic number, q is its charge number.
In the collider the RF voltage amplitude is determined by condition of a matching of the bunch at low length. If the longitudinal beam emittance || is given the rms momentum spread p is determined by
,
where s is the rms bunch length that is not longer than 30 cm from the luminosity requirements. m is the nucleon mass. Toavoidthebunchrotationafterinjectionintoseparatrixthe bunch aspectratio has to be equal to synchrotron function:
,
which is determined by the synchrotron tune Qs:
,
HereR = С/2isthemeanradiusofthering, the synchrotron tune is given by:
and the momentum slip factor is.
Booster
At the moment two versions of the booster design is under discussion:
- Maximum rigidity of 25 Tm. It permits to accelerate uranium ions at charge of +32 up to the energy of 400 MeV/u. At this energy one can obtain fully stripped uranium ions at 40% efficiency. This design presumes the booster circumference of about 200 m, correspondingly the elements of the booster magnetic system are located inside magnets of Synchrophasatron. This scheme allows working with all available ions and provides required luminosity up to uranium.
- Maximum rigidity of 8 Tm. It permits to accelerate gold ions at charge of 32+ up to the energy of 70 MeV/u. At this energy the efficiency of the gold stripping to 77+ is about 60% (RHIC data). Last two electrons are stripped at the exit of the Nuclotron at 100% efficiency. This design provides required luminosity in gold-gold collisions, for all other ions one needs to optimize total scenario and, more probably, the luminosity will be less. Circumference of this booster can be about 150 m, and it can be placed inside the Synchrophasatron.
Recently a third version of the booster design appeared: maximum rigidity of 4.8 Tm. It permits to accelerate gold 51+ ions up to 70 MeV/u. The expected intensity of the gold bunch is almost the same as in the version 2. Possibility to work with another ion species has to be discussed separately. For this version of the booster the linear injector has to accelerate the ions at 4 mass to charge ratio (instead of 8 in another versions). The booster can be build (?) using Amsterdammagnets; the circumference is below 150 m.
Depending on the circumference, charge state and final rigidity the RF frequency range and voltage will be different. In the table below the parameters for second version of the booster are listed.
Injection and acceleration are provided at 5 harmonics of the revolution frequency.
At maximum ion energy in the booster the harmonics number is changed from 5 to 1 and by means of electron cooling a formation of single bunch at required longitudinal emittance is realized.
Nuclotron
Provides the acceleration from 70 (400) MeV/u up to 3.5 (4.5) GeV/u.
The ion charge state is between 2.5 and 3 independently on the booster design.
First harmonics of the revolution frequency.
Maximum magnetic rigidity 45 Tm corresponds to the dipole field of 2 T.
At extraction energy the bunch rotation is provided. The RF phase is changed rapidly (in comparison with the synchrotron frequency) by 180 degrees. The bunch center is replaced to unstable point. After rotation of the bunch by some angle the phase is returned back to the stable point. When the bunch length reaches minimum value the bunch is transferred to one of the NICA rings.
NICA
Fixed frequency, 105 harmonics of the revolution frequency. The voltage depends on momentum compaction factor and, for instance, at transition gamma of 5, rms bunch length of 30 cm, rms momentum spread of 10-3 the voltage is less than 100 kV.
RF system parameters
Booster (150 m) / Nuclotron / NICA (250 m)Harmonics number / 5, 1 / 1 / 105
RF frequency range / 1.03 – 3.67 MHz,
735 kHz / 440 kHz – 1.17 MHz / 110 – 123 MHz
Rigidity, Tm / 1.98 - 8 / 8 - 36 / 14 - 36
, m / 4.5 / 22.5 / 7.5
Magnetic field, T / 0.44 - 1.8 / 0.09 – 1.6 / 1.87 - 4.8
A/q / 8 / 3 / 2.5
dB/dt, T/c / 0.75 / 0.75 / 0
Minimum RF voltage, kV / 4.5 / 12 / 100